Synthesizer Build part-54: JOYSTICK CV Controller (Eurorack).

 An easy to build joystick module that outputs 2 CV voltages to control anything from pitch to filter cutoff and anything else that can be changed with a control voltage.

The finished Joystick module installed in a Nifty Case.

Before I started building my modular synthesizer I had a brief try at flying FPV drones. I bought all the gear and some cool drones but the damn things were way too fast for me to control. This was before the stabilized DJI FPV drones were on the market. Anyway.... the gear landed in the coupboard for a few years. I have now successfully taken the hobby up again and now I can fly FPV drones but my transmitter/controller was now outdated so I used one of its gimbals for this project. I found a good schematic on the Mod Wiggler forum.

Close-up of the circuit:

The image below shows the circuitry for one axis. You need two of these circuits to work both axis of the joystick, left and right [X-axis] and up and down [Y-axis].

HOW THE CIRCUIT WORKS:

It's a very simple circuit. Each of the 2 axis of the joystick is assigned two opamps. The voltage coming of the joystick potmeter goes into the inverting input of an opamp and added to that is voltage from the Zero Point trimmer to make sure the voltage is at zero when the joystick is in the rest position (middle). The gain of the opamp is adjustable with the 1M potmeter marked Range. This determines the maximum voltage you get when you push the joystick fully to one position. This goes from 0 to 10V max. when used with a 12V powersupply.

The CV voltage then goes into a second opamp which has an offset potmeter so we can turn the signal into a unipolar one if we want (all positive or all negative voltage) or just give it some offset or even just to make sure the voltage is zero when the joystick is in the middle position. 

This module is meant for Eurorack (dual 12V powersupply) but it will run just as well on a dual 15V powersupply and if you build it for a Kosmo or 5U synthesizer you have more space on the faceplate to accomodate some extra features.

The joystick I used came out of a Taranis QX7 RC controller/transmitter and it has the following resistance values:

When in the middle position (rest) the resistance is 1,31kΩ. Fully right is 2,15 kΩ and fully left is 550 Ω. Same for the up-down potmeter.

The circuit will take a wide range of joystick resistance values so practically any joystick can be used.

I left the springs installed so the joystick always returns to the middle position when let loose.

Ideas for extra features:

The circuit is very bare bones but you can extend it with, for instance, a momentary switch that cuts the CV voltage if you push it, or one that makes contact if you push it and so outputs an extra gate signal.

An other idea that was suggested to me is to have two input sockets with the voltage connected to the socket switches (normalized) but then you can input an audio signal that cuts the voltage and then the joystick controls the amplitude of the audio thus creating a Manually Controlled Amplifier (MCA).

I'll leave all that up to your imaginations. I didn't have room for extra functions on my panel so I left it as presented here.

I did put in two bi-coloured LEDs to give a visual representation of the voltages on the outputs. It glows red for positive and blue for negative voltages. I connected them straight to the output sockets but with a big 10K current limiting resistor so they only glow at their brightest with the full voltage applied and don't pull down the CV outputs. Also to keep the number of components to a minimum. It works like a charm and looks very cool. Their brightness is a good indicator for the amount of voltage present at the output sockets.  They start glowing at around 2V and then get brighter with higher voltages. I mounted the LEDs above the joystick so they are in full view.

You can use any type of quad opamp for this circuit. I used one of my fake LM324 chips from China and because there are no high frequencies involved it works just fine. You can use a TL074, TL084 etc. They all work fine as long as the pin-outs are the same. It's a good idea to use miniature potmeters for the offset and range controls to save some space on the faceplate. The offset potmeters don't have to be 10K, I used 100K potmeters myself. The range potmeters do need to be 1M otherwise the range of the range will be different ^___^

The trimmers can also be different values. I used 200K trimmers. Afterall they are just voltage dividers in this circuit, so the value is not that important.

CALIBRATING:

The zero point is the point at with the joystick is at rest, right in the middle and in this position the CV outputs must be at zero Volts. You set the zero point with the two multiturn-trimmers.  

The best way to set the zero points for both axis is to have both the Offset and the Range potmeters at the 12 o'clock positions and then connect the CV output to an oscilloscope or volt meter and turn until the voltage is zero. 

Then set the scope or meter to a more sensitive setting and again correct until it reads zero Volts. Try to get it as accurate as you can. After you're done calibrating both channels you don't have to touch the trimmers again. 

Here is the schematic I used for the layouts:

I made a Falstad simulation of the circuit which you can see by clicking here.

Here's one observation I made about this circuit. The voltages from the wipers of the joystick potmeters go through a 51K resistor into an opamp, the gain of which is determined by the 1M potmeter (Range). I noticed that the Range potmeter reaches its maximum at about 1/3rd before the full clockwise position is reached. I think this is due to the 51K resistor. I think it will be better to put in a 91K or even a 100K to get the gain in step with the throw of the potmeter. 

The gain of this stage is determined by the formula: Av = (-Rfeedback/Rin) = (-1M/51K) = -19,6 (the minus simply means the output is inverted). This is too much and that's why the potmeter reaches full gain way before it's turned fully clockwise. With a 100K the gain would be -10 and that would result in the full throw of the potmeter being used. To play it save and make sure you get all the gain you can before you reach the fully clockwise position of the Range potmeter I would suggest using a 91K resistor instead of the 51K on the layout. I've changed the Bill of Materials to include two 91K resistors. However I have not made this change in my own module because I can't access those resistors easily anymore, so I can not guarantee it will fully solve the potmeter throw issue but I can't see why it wouldn't work because the mathematics says it will.

The Falstad simulation doesn't really show this discrepancy so do not rely on it for component values. 

LAYOUTS:

Here is the layout I made for this circuit. It is verified, I used it to build my project. It is small enough to fit flat behind a 14hp Eurorack panel. Beware there are two copper strips underneath the IC that are not cut. They connect the grounded pins together. Pins 3 and 12 and pins 5 and 10. 

There are three 100nF caps visible in the layout but I also put a 100nF cap over pins 6 and 7 of the IC. This is to suppress any voltage spikes or noise. This cap is not visible on the layout and because I had no room for it on the component side I soldered it straight to the pins on the copper side. So there are 4 caps in the Bill of Materials. (I didn't use any bypass caps myself but they are in the layout and B.O.M.).

Here is the stripboard only view. 

Here is the layout for just the cuts and wirebridges. 

As ever mark the cuts at the component side and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut them with a sharp hand held 6 or 7mm dril bit.

And finally here's the bill of materials. It's quite a cheap project if you already have a joystick in stock and anyway, joysticks aren't that expensive if you know where to look. The resistance value of the joystick potmeters isn't that critical. The circuit just uses them as voltage dividers so any value will work. They usually don't go down to zero Ohms. The one I used goes from 550 Ω to 1K3 to 2K15 in the lowest, middle and highest positions.

You can find joysticks on AliExpress for under $20,- for a pair. Just Google: "Radio Rocker Joystick 5K." Those should work just fine.

How to determin which wire is for up and which for down, left or right with a joystick.

Connect an Ohm meter to the middle wire and one of the outer wires of one of the potmeters on the joystick. Say for instance we're looking at the potmeter for the Y-axis (up and down). Now we measure the resistance while moving the joystick up. If the resistance goes down you have the correct wire for the up position. If the resistance goes up that wire should go to the down position on the stripboard, for the Y axis. So if you have the correct wire for a specific direction the resistance between the middle wire and that wire should go down when moving the joystick in that direction, because the wiper of the potmeter moves closer to it. I hope that makes sense.

Here's a screenshot from my oscilloscope. Yellow = X-axis, Blue = Y-axis. In this picture I moved the stick to the outer most positions and you can see both voltages land on exactly 10V maximum with Range turned fully clockwise and no offset applied.

PICTURES:

Here are some pictures I took during the building process:

This is the faceplate I made. Notice the two square holes. I tried fitting two push switches for extra Gate outputs but I came back on that idea because I didn't have enough room to accomodate that.

I made the big round hole with a hand held jig saw.

The finished face-plate with everything installed but without the stripboard. As you can see the knobs are very close together which isn't ideal so when you design your own faceplate for this module take some time to find out the best places to put these potmeters. If you use miniature potmeters you have more room to move them about to find the best placement.

Below is the stripboard with all components mounted except the power connector. The bottom two strips I later cut away go have some more space for the gimbal to move because when I tried to mount the board behind the panel I needed a bit more space. The bottom two copper strips are not used so I could just cut them off.

Here's how I mounted the stripboard behind the panel. I used some plastic tube as a stand-off. If you do the same, drill a few small holes in the sides very near both ends so the glue can run into those and provide a good grip. Then I hot glued that to the back of the panel, making sure the glue flowed around some of the mounting screws for the joystick, for extra grip. Then I hot-glued the stripboard to that stand-off after the wiring up was all done. I had to be careful not to disrupt the movement of the joystick gimbal, keep that in mind when mounting the stripboard behind the panel. There's almost no place to drill a hole through the stripboard for a normal M3 threaded stand-off so this seemed like the best solution. Works fine.

And here's the finished product. Front view:

Back side. The depth of the module is just under 4 centimeters. It's 14hp wide (7CM):

Finally a little demo video of the module in action in my 'Nifty Case'. This is just a simple patch I put together in 5 minutes. The X-axis CV is controlling the cutoff of the filter in the Doepfer A-111-6 synthesizer voice and the Y-axis CV is controlling the reverb amount from the FX-Aid.

Okay, that's it for this one. Quite a simple build. The only thing I did wrong was that I forgot that the wipers of the offset potmeters connect to the inverting inputs of the opamps so I had the offset potmeters wired the wrong way around. An easy fix. This is a very easy to build module and, I think, a very useful one especially for live performing. It's in fact the equivalent of a synthesizers modulation- and pitch-bend wheels all in one.

If you have any questions or remarks please put them in the comments below or in the special Facebook group for this website.

Synthesizer Build part-52: 4 CHANNEL FEEDBACK EQ/DISTORTION (Monotropa) Eurorack.

A 4 channel feedback equalizer / distortion module that will fit a Eurorack system. 

I came across this circuit in a post on the LookMumNoComputer forum. Bpbby posted a Falstad simulation of this circuit and it intrigued me because I never heard of it before. He found the circuit on this website: www.reverselandfill.org

It's a pretty cool circuit. Simple too. We have 4 filters, each covering a part of the audio range, and then there's a feedback loop that connects the output back to the input. The circuit is called the Monotropa, which is the name of a plant. Don't ask me why. I don't see any logic in that. ^___^

Here's the schematic of this circuit:

Here is the Falstad simulation of this circuit:

--- CLICK HERE ---

LAYOUTS:

Here are the layouts I made for this project. They are verified as always. I built it for my Eurorack case but you can just as easy make this for a Kosmo sized synthesizer. I that case you could even build the 7 channel version because you'd have more space on the panel for the extra potmeters. Yes there is a 7 channel version of this circuit but you'd have to Google that. This article deals with the 4 channel version. This circuit is designed for +/-12V but I can't see why it wouldn't run equally well on a +/-15V powersupply.

Here's the wiring diagram. For the first time in the history of this website I show the potmeters from the back side! I should have done that all along because it's easier with wiring up the panel but there it is. I started out showing potmeters from the front in my layouts and for the sake of consistency I stuck with that, upto now. I had to connect some components straight to the potmeters and audio jacks to save space on the stripboard:

Below is the stripboard only view. The stripboard is small enough to mount parallel with the front panel behind the potmeters and sockets. You could drill a hole through the lower two strips which are not in use and use a standoff to mount it to the front panel. The wiring will also act as a stabilizing feature. I just used some plastic tube and hot-glued them to the back of the potmeters and to the copper side of the stripboard. That's secure enough. I soldered the powerconnector straight to the stripboard without using pinheaders and sockets. That way you only have 3 thin wires coming from the board with a Eurorack connector (female) on the other side to plug it in. If you want to use bypass/de-coupling caps there's room enough to solder those in over the powerrails and add some 10µF electrolytic caps if you want extra stabilization of the power supply voltage. These components are not in the layouts and are not listed in the Bill of Materials!

The Cuts and wirebridges as seen from the COMPONENT SIDE!!

Here's the Bill of Materials:

PICTURES:

Here's a look at the finished product:

Here are some screenshots from the oscilloscope showing the influence of the feedback on the output signal: 

And finally a little DEMO video I made. I built my version with 100K potmeters because that's all I had and consequently it doesn't sound as good as it could be with 10K pots. I assure you though, it is worth building but keep to the component values in the layout and schematics. Some potmeters are more effective than others depending on the frequencies that are put through this circuit because this is of course an equalizer. So a Low Frequency potmeter isn't going to have much effect on a high frequency bit of audio that's put through it. In the video I have it connected to a 555 VCO that is fed by the Sample and Hold of the previous project.

There's a useful tip in the comments below suggesting to use this EQ with a squarewave and then play with the Pulse Width Modulation of the squarewave in combination with the feedback of the EQ. That should sound pretty awesome!

Okay that's it for now. Not much of a write up I admit but real life issues got in the way. I might revisit this article later and expand on it. I hope you understand and don't mind. For now I just wanted to give you all the necessary layouts etc. to build this Feedback Equalizer. I already heard from one person who built it and he's very happy with it. If you have any questions please put them in the comments below or on the special Facebook Group for this website.

Synthesizer Build part-46: 808 KICK for EURORACK. (Juanito Moore circuit).

The kickdrum from the famous Roland 808 drummachine. With four controls and a stripboard small enough for Eurorack (although a bit deep). Naturally you can just as well build it in a Kosmo size if that's the size you're using to build your modular synth. 

Now that I've started to play the modular synth I built more and more, I felt the need for some percussive action so I started out with this famous kick drum sound using a schematic from Juanito Moore who is famous for building his modular system without using any circuitboards at all just 'dead bug' soldering and he's really good at it too. A real inspiration for DIY synth builders like me. 

The layout I made worked flawlessly right from the get go. This is quite a straight forward build. It requires 4 panel potmeters of different values and you must keep to these values too. You need a 5K, 10K, 100K and 500K panel potmeter. (470K instead of 500K and 4K7 instead of 5K will be fine too.)  

You can modify the T-filter by changing the 15nF caps. Smaller values will give you higher tones but it will disrupt the balance of the filter and cause the Decay function to stop working correctly. Also there's no CV control for this module because it's not practical to implement. Here's what Juanito himself had to say about that:

"The decay not working right with different cap values is due to the properties of a bridged-T filter that oscillates with a ping of voltage. I gave up on voltage-controlling an 808 kick because the decay, dictated by the laws of physics, changes with pitch. Also, if you use a fancy voltage-controlled resistor (LM13700 datasheet) when you change the CV, the kick will trigger. A Vactrol was the best I managed to get."

So you can get away with putting a Vactrol over the 'Pitch' potmeter but that's about it. I personally didn't bother with CV control.

LAYOUTS:

Here are the verified layouts I made for this module. I marked two screwholes on the layout but I didn't use them. I just hot-glued the board straight to the back of the potmeters once I had the panel ready and this works just fine. I glued the topside of the board with the eurorack power connector pointing downwards (see pictures below). Beware that this does make the overall depth of this panel 7.5cm which won't fit some eurorack cases!

Wiring Diagram:

Stripboard only.

Pay extra attention to the connection of the transistor in the upper left. The emitter leg skips one copper strip and is soldered directly to the ground strip of the power rails. Strips B,C and D are all ground and I connected them together on the print by putting extra solder under the power connector so it bridged the middle three ground pins, shorting them together. Make sure to use polystyrene, polyester or silver mica type capacitors for all but the de-coupling caps (if you choose to include de-coupling caps. They're not on this layout). It's important not to use ceramic caps in the filter section because of various reasons. If you want to include de-coupling caps then solder some small ceramic 100nF caps over pins 4 and 5 and pins 10 and 11 of the TL074:

Here's an overview of the cuts and the wirebridges seen from the component side. As always; mark the cuts on the component side with a sharpy marker pen, then stick a pin through the marked holes and mark them again on the copper side and then cut the copper at the marked holes. Do this and the wirebridges first and check the cuts and wirebridges by measuring with your multimeter for continuïty. Then solder in the rest of the components.

Cuts and Wirebridges seen from the component side:

Schematic:

Here's the schematic I used to make the layouts, drawn by Juanito Moore in his distinctive cool style. There are two versions of this schematic in circulation and one of them has the clipper section wrongly connected but this is the correct schematic:

Here's a Falstad simulation of this circuit which allows you to test different component values and see what the result is on the output. I put an audio output at the end that allows you to play the result and listen to it. It comes really close to the real thing.

--- CLICK HERE ---

The 'Clipper' switch increases the amplitude of the drum sound extra (when the switch is open) and provides a little bit of distortion which makes the sound more audible. The low frequency of this kick drum can be so low that you can hardly hear it but through a good PA system you will feel it in your stomach because of the extreme low frequencies. It shakes the windows in my attic and makes the dust fall from the beams LOL. It really is an exact replica of the original 808 kick drum sound. I have the Behringer version, the RD-8 drummachine in my little studio and the kick sounds just the same.

I soldered the 33µF electrolytic cap for the Decay straight to the potmeter to save space. There was one opamp left over as you can see in the schematic. I used that opamp to drive a little LED connected to the output so we have a visual reference of the output without pulling any current from the output to drive the LED. It's always handy to have a visual indicator to see if the circuit is triggered correctly. Plus a LED always looks cool in a module.  The 1K current limiting resistor for that LED is soldered directly to one of the legs and reinforced with some heat-shrink tubing. 

Here's the Bill of Materials. Make sure not to use ceramic caps, except for the 100nF de-coupling caps for the chip if you want to include those, but they are not included in the layout or this BOM. 100nF de-coupling caps can be soldered directly on the copper side from the plus pin of the TL074 to ground and from the minus pin of the TL074 to ground. Make sure the legs have some heat-shrink tubing on them so they don't cause short circuits:

Pictures:

Here are some pictures from the build proces and the finished panel:

  

 

 

Like I mentioned before, the depth of the module as you see it here is 75mm (7.5cm) so it might not fit in some Eurorack cases. Keep that in mind. The width of the module is 6hp. (3 cm). You could save some depth by rearranging the potmeters to be directly underneath eachother and then hot-glueing the print straight to the back of the panel instead of on top of the potmeters. 

DEMO:

Instead of making my own demo I thought I'd embed Juanito's own video here. This video is over two hours long because he shows the complete build process but this link will start the video at the end where he demonstrates the functions. It should start at 2:23:36 If not, then just jump to that time manually.

If you can't see the video on your mobile device then CLICK HERE to view on YouTube directly.

Here's a link to Juanito's YouTube channel. Subscribe to his channel while you're there :)

Juanito also sells some modules on his 'Modular for the Masses' website. Here's the link to his webshop:

----- CLICK HERE for MftM webshop ----

Okay that's if for this one, with grateful thanks to Juanito Moore for his reactions and for just being awesome :). 

If you have any questions or remarks please put them in the comments below of post them on the Eddy Bergman Facebook group where we have an awesome little community willing to help you with any problems you may encounter.

Synthesizer Build part-42: 8 RANDOM GATES by Yusynth.

 Creates 8 random gate outputs from one gate input signal which can be as high in frequency as an audio signal. Lots of creative possibilities with this module.

There is an other random gates project on my website already. That one is included in the Noise Module article and it creates random pulses on one output. With this module we have 8 different outputs which trigger in a completely random order. It needs a squarewave on the input that can come from an LFO, the gate out from a sequencer, the clock pulse from a sample and hold or even the pulse wave output from a VCO. To quote the YuSynth website: "If feeding the GATE IN with a high frequency pulse coming from a VCO, each GATE output becomes an individual coloured digital noise source usable for sound effects. The colour of the noise will directly depend on the frequency from the VCO. White noise is obtained for frequencies above 30kHz".. 

ABOUT THE CIRCUIT:

The module is fed with only positive voltage so no dual powersource needed. It works fine on both +15V or +12V. You can feed the gate-in with signals that have a negative cycle to them. It will simply ground the negative part of the cycle through diode D1. The output gate signals have an amplitude of 8 Volt when powered from a +15V powersupply.

This build consists mainly of wirebridges. My layout has 37 of them. All the output stages are made on separate pieces of stripboard with just 4 strips of 10 holes. They are soldered straight to the output sockets. I did this to save space otherwise I would have had to make a separate board with all the outputs on them. This way saves space and also hookup wire. The three 100nF capacitors you can see on the layout are meant to be de-coupling caps but where they are positioned is really too far away from the chips to be effective. So instead of putting them where the layout shows them, solder them straight over the plus and ground pins of the IC's (top right and bottom left of each chip).

Here's the layout I made. First the wiring diagram:

(Last revised: 14-April-2021: Added missing 1K resistor to output prints.)

In the box, on the wiring diagram above, you can see the schematic drawing of the output stripboards. I left out the 1K resistors in series with the output in my original design. I had simply forgotten it but I have now updated everything and the 1K resistor is now included. It helps to protect the transistor against short circuits, smooths the output voltage a bit and also determins the output impedance.

The 270 Ohm and the LED together with the 1K resistor to ground form a voltage divider that determins the voltage of the outputed Gate signal. That voltage is normally 8 Volt but if you want it to be higher you can make the 1K to ground a higher value like 1K5 or lower for a lower output Gate voltage.

Here's the main stripboard. It's only 24 by 48 holes but you could try to redesign it and make it even more compact so it would fit in a Eurorack system. For instance, if you connected the outputs straight to the correct pins of the chip instead of using the wirebridges you can save about 8 or 9 holes in width. Certainly enough room to make it fit a Eurorack system. And because it's a "Random" gates generator, the correct order doesn't really matter does it?

And here's a close-up of the little output stripboard that is soldered straight to the output socket: (If you print this one, choose the A6 format to save some printer ink.)

You need to make 8 of these output prints. Some cuts are a bit hard to see but the top two strips are cut at position 5 and there's an other cut at position C8.

(Forgive my use of the word 'print' when I mean stripboard. Print is the Dutch for printed circuit board so that's why it sometimes sneaks into the text.)

(Last revised: 14-April-2021: Added missing 1K resistor to output prints.)

Here's the Bill of Materials:

Here's the schematic by Yusynth. You can find the original YuSynth article by clicking HERE.

As you can see it's actually quite a simple circuit. It mostly consists of connections between the three IC's. It is mentioned on the YuSynth website that this module needs a bit of time before it starts behaving correctly. When you first start it up it will probably not fire on all cilinders and display a repeating pattern with only about 4 or 5 LEDs lighting up and after at least ten cycles this will change into a random pattern using all the outputs. However, since I changed the new CD4070 I had in there for a used vintage CD4070 from the 1980's that I had lying around, the module works good right from the start. 

The module that I built was at first prone to hanging. It would suddenly stop being random and get stuck in a 4 or 5 LED pattern. Only by changing the input gate frequency or pulling the Gate-In cable in and out a few times would I get it working again. It turned out this was also due to the IC's I was using. I don't know if it was a fake chip or if it was damaged but I changed IC-3 for an old stock CD4070 that I once de-soldered out of an organ circuit board and the problem was solved immediately.

So make sure the chips you're using come from a reputable source!

MODE SWITCH:

There's a mode switch that lets you choose between two settings. In the ON position the output stays high until it detects the next pulse, so the pulses don't have any dead time between them. In the OFF position the output pulse stops on the negative slope of the input gate pulse, so the output pulses will have the same length as the input gate pulses.

There's also an option to advance the pulses manually with a momentary switch (normally off). This switch is connected to the internal switch of the Gate input socket so it will only work when there is no cable connected to the gate input socket.

Some screenshots from the oscilloscope. The first one shows how extremely fast the risetime of the output gate signals is. Just over 123 nanno seconds! That's 0 to 8 Volt in 0.000000123 seconds. This means theoretically that it could handle signals upto 40MHz! (Agreed, this knowledge is of no use in the synthesizer world but it fascinates me personally because I also have a background in radio technology and transmitters ^___^).

Here's what the output sequence of one of the random gates looks like. A non-repeating sequence of pulses with an amplitude of 8V. 

Here are some pictures of the build proces:

Wirebridges. In this picture there's a little wirebridge missing connecting pins 7 and 8 of IC2 (CD4051).

Here's the finished print. Like I mentioned earlier, the de-coupling caps are much to far away from the chips to be effective so get some small ceramic 100nF caps and carefully solder them straight over the plus and minus connections of the chips on the copper side. I myself left it like this and it works just fine because I don't use a switchmode powersupply but a linear one, with a big transformer. 

Here's the main board with the 8 output prints. My output prints are missing the 1K resistor in series with the output sockets (I had forgotten those) but they are included in the layouts. That 1K resistor helps to make the output waves smoother. I could see that on the oscilloscope images. It also protects the transistors by limiting the current going through them should the output be shorted. (Although damage will be very unlikely even without the 1K resistors because the pulses are so short).

Finished panel backside wiring:

Frontal view of the mounted panel:

And here's a little test video showing the module firing randomly on all cilinders :)

TIP:

If you want a fast pulse train with random gaps in it, then connect 4 outputs from this module to the 4 inputs of a mixer, like the mixer/passive attenuator module on this website. At the output of the mixer you will get a pulse train with random gaps in them. It's cool to use this on the cut-off of a filter to add some random spice to the sound.

If you then set the switch on the random gates module to 'Stay high until the next gate pulse' you have sort of a random voltage generator, although there will still be random 0V gaps in the output but that makes it unique :)

You could even make a little TL072 mixer print and include it in this module. Choose how many inputs you want (less than 8 of course) and connect those mixer-inputs to whichever outputs you choose and then make an extra output socket on the panel that carries the output from the mixer and label it "Pulse Train". It's just a thought but there are many ways to adapt this design to your own needs.

Okay, that's an other one done. If you have any questions or remarks please put them in the comments below or post on the special Facebook Group for this website where we have a great community of synth enthousiasts willing to help you.

If you successfully built this module and you're using it in a cool way that others might enjoy, please make a video, put it on YouTube and contact me with the link. I'll add it to the article with full credit given.

Synthesizer Build part-38: SIMPLE LIGHT THEREMIN.

 A fun little project to make your modular synthesizer react to light. It has offset and level controls and CV smoothing plus an external LDR input.

Let me start by saying that this module needs to be connected to a VCO to work so it's not a stand-alone thing. This circuit outputs a Control Voltage and the height of that voltage is dependent on the brightness of the light falling on the LDR, the Light Dependent Resistor. 

This was a little project I dreamed up myself and I designed the circuit too. It's quite a simple build. It consists of two opamps. The first one has the LDR input and the Offset control but it inverts the CV voltage. Then the second opamp inverts the CV voltage back to normal and feeds it to the output socket via a level potmeter. There are two 3mm LEDs on the output to indicate if there's a positive or negative voltage present and the brightness indicates the voltage level.

The Smooth switch is there to suppress the 50Hz or 100Hz hum you get from LED (or other) light fittings. They flicker so fast you can't see it but the LDR reacts to it. The switch simply puts a 10µF electrolytic capacitor over the LDR smoothing out the control voltage.

This circuit works on +/-12V but it will work equally well on +/-15V. It consumes very little current. On 12V the maximum current I measured was 5.6mA.  1.6mA of that is consumed by the LEDs if one of them is on. So you see, it hardly draws any current at all. It will also work on 9V although the CV voltage will be less high obviously. The offset feature needs a dual powersupply in order to transpose the CV voltage down so you'd need two 9V batteries to create a dual 9V powersource. I haven't tried it on 9V but I get questions about it but I don't see why it wouldn't work. You might need to adjust some of the resistor values or experiment a bit to get the best out of it. That's up to you.

There's a link to a 3D printed front panel for this project in the comments below.

HOW TO USE IT:

This circuit doesn't generate any sound itself. It just outputs a Control Voltage. You can connect the output Control Voltage to the CV input of a Voltage Controlled Oscillator (VCO) to get the Theremin effect. You can also put it through a Sample and Hold first and then into a VCO. In that way you will get stepped tones. An other option is to lead the CV through a Quantizer first, to get it to output true notes that adhere to the 1V/Oct standard and so try to make melodies with light. Of course you can also use the CV to affect the Cut-Off frequency of a filter. Your imagination is the limit :)

The CV voltage and therefore the tone will get higher as the light that shines on the LDR gets brighter, and lower when it gets darker. The exact way it reacts can be set very accurately using the offset control, without limiting the dynamic range of the notes. (The same idea as with version 2 of the 8 step sequencer).

The idea behind the external LDR input is to make it possible to bring the LDR to the light-source instead of having to shine lights on the panel itself (with the built in LDR). This makes it much more flexible. You can just take the LDR in your hand and point it at things and use it as an instrument. The whole circuit works so much better when you use this option. Try it!! 

If you feel you need more output voltage then change the 100K feedback resistor over pins 6 and 7 of the IC. Double the value to double the gain. 147K should be enough to go really high but normally it shouldn't be necessary to change it. Obviously you are limited by the height of the power supply voltage but with 1V/Octave you really don't want your CV output to exceed 8 Volts.

You can experiment with light sources and where you put the external LDR and let your imagination run wild. You could build a whole light operated synthesizer with this project as a starting point. =)

LDR:

I can not give you a part number for the LDR to use in this circuit. I ordered a set of 5 different LDRs with 10 pieces of each, from eBay. The link to that has expired but here's an other one to a set of 70 with 7 different values. That will last you a long time. Handy for making Vactrols too to have this in stock:

- CLICK HERE -

I used the one marked 5537. That's a very fast reacting LDR. But to make sure, test them with the resistance meter on you multimeter and choose one that reacts fast to light changes.

Here's how you can make an external LDR that you can move around and point at light sources. Simply take a female jack socket like the ones you mount in panels and solder an LDR over the audio and ground contacts. Use some hot glue to make it nice and stirdy. Now you can connect it to a patch cable and connect the other end of the cable to the external LDR input. Flick the switch to 'Ext.' and you're in business. =)

External LDR. Just put some hot glue around the LDR leads to stiffen it up a bit. The external LDR worked really well when I tested it. Of course you can shorten the leads of the LDR or bend them at 90° so it can lay on a table and catch the light from above. It's all up to you.

Schematic drawing of the circuit:

LAYOUTS:

Here is the verified layout. As you can see you can build this on a very small piece of stripboard. Don't be fooled by the wiring of the Offset potmeter. It looks like it's wired the wrong way around but remember that the opamps are wired as inverters so the offset voltage is turned the right way up in the second stage. (All potmeters are viewed from the front with shaft facing you):

Stripboard only:

Bill of Materials:

TAKE NOTE OF THIS:

There are a few things you need to beware of when building this project. I built in the option to connect an external LDR to the panel instead of using the built-in LDR with a switch to choose between them. The socket for this External LDR input must be mounted in such a way that it is completely isolated from the panel if you are using a metal panel. The socket is connected directly to +12V, if it touched the panel it won't short out because there is still a 10K resistor between it and ground but it won't work as an input anymore. So connect the socket to some plastic and make a big enough hole in the panel to glue or screw the socket behind the opening without it touching the panel itself. (See pictures below to see how I mounted it to the panel.)

DO NOT CONNECT ANYTHING ELSE THAN AN LDR TO THE EXTERNAL LDR INPUT! It has +12V on it and may damage sensitive electronics. So make sure you label that input clearly! Maybe it's wise to use a different kind of connector for the Ext. LDR. That way you can't put in a patch cable by mistake.

The same caution must be taken with the LDR that is mounted in the panel itself. Make sure it makes no electrical contact with the panel when you glue it in place with hot-glue. Check it with a continuity meter after glueing it in place.

Here's a short video I made of the very first test I did. This thing can really make a VCO squeel!!:

Here are some pictures of the build proces and the end product:

In the picture below you can see how I mounted the 'External LDR' input socket. I used some white plastic and mounted the socket in that and then I drilled a hole in the panel, wide enough to take the socket with plenty of room around it, so it wouldn't touch the metal of the panel. Then I hot-glued it all in place and it works very well like this:

Here are some oscilloscope screenshots showing the maximum amplitude of the light pulses I got with testing. It was more than 20Vpp when I ran the module on +/-15V but you can turn that down with the level control. 

Here's an image which shows the 100Hz hum you can get from flickering lights (the small ripple in the lower parts of the waves):

Here's the effect the 'Smooth' switch has on that. It not only gets rid of the ripple but also dampens the pulses. You can lower the value of the smooth capacitor a bit to 4,7µF but no lower than that otherwise the ripple won't be surpressed. 

Okay, that's it for now. If you have any questions please put them in the comments below or visit the Facebook Group that was setup especially for this website.

DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.