Sunday, 10 January 2021

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.

This was a little project I dreamed up myself and I designed the circuit too. This circuit outputs a Control Voltage and the height of that voltage is dependent on the brightness of the light falling on the LDR. It's quite a simple build. It consists of two opamps. The first one has the LDR (Light Dependent Resistor) 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 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.

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. 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, the brighter the light that shines on the LDR 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 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.

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. Here is the link to that item: 
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.

Schematic drawing of the circuit:

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):

Print Only:

Bill of Materials:

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 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 socket 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.

If you like what you see and you would like to support these projects and the upkeep of this website you can buy me a coffee. There's a button for that underneath the menu if you're on a PC or Mac. Otherwise you can donate a few bob using this Paypal link. All donations go to the purchase of new components for projects. Thank you very much!

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.


  1. Great website. Lots of great ideas that I will apply to my modular build. I like that you have provided stripboard circuits. One problem though. Could you make your website narrower? It is difficult to read as I have to scroll left and right to view it. I use a Chromebook to view the web.

    1. Hi James. Glad you like the site. I'm sorry I can not change the width of the website. I'm using a template hosted by Blogger. And it works fine on a PC or Mac and on mobile devices.

    2. Can you not zoom out of the page to make it fit the screen? It should be possible in the Chrome settings.

    3. Thanks for your reply, Eddy. I run blogs on Blogger too. You might try looking at Layout page in the Blogger menu and the Theme Designer link inside that page. That gives the option to narrow the width of your chosen theme.

    4. I did use zoom but for this old man's eyes I'd need a magnifying lens to read it.