Monday, 30 October 2017

LED Oscilloscope with 100 LEDs.

Hello everyone,

One word before we start, don't build this project if you're in need of an oscilloscope for measurements or checking waveforms. This scope has no trigger and therefor no stable waveform unless you exactly match the frequency of the waveform with that of the timebase. Furthermore, 10 by 10 LEDs is way too low a resolution to check waveforms with. If you need an oscilloscope for audio waves you can start with a cheap one from eBay for $20.
This project is just a fun thing to build with LEDs.

Following on from my 81 LED chaser with 2 NE555s I now set out to build a LED oscilloscope using the same type of LED matrix I used in the last project, only this one has 100 LEDs instead of 81.  First of all I'll show you the circuit schematics I used for this project. You can easily find this on Google and it's a very simple design. Actually easier to build than the 81 LED chaser.



I made some changes to the way the NE555 was configured. To test the schematics I build this pulse generator on a breadboard and took some measurements with my oscilloscope and the pulses that came off only had a duty cycle of 6%. Maybe that was meant to be and actually works better, but I changed it to a design that gave a 50% duty cycle. I wanted to be able to extend the range of frequencies by adding the possibility of switching between capacitors on the 555 and I wasn't sure how this short duty cycle worked on the higher frequencies I intended to put in, and I also didn't have a 500K potmeter. I only had a 100K so I needed a design that gave me a good frequency range with a 100K potmeter. Here's what I came up with:


When set to the highest capacitor value (330 nF) this gives a range of 17,5 Hz to 6,2 kHz. Then, by choosing the lowest value capacitor, it goes up to about 650 kHz. That's a nice range for a timeline I thought. The ranges overlap a lot and you only really need the first and last setting but I liked having some choise and it adds yet an other switch to the front panel which always looks cool :)
After I had build this circuit I came to the conclusion that the higher frequencies for the timeline don't look good at all because this scope doesn't have a trigger-mode. So with high speed signals it just looks asif the LEDs are on all the time. So you don't have to bother with the alteration to the NE555 and just keep to the original schematic. I just thought I'd include it in this article in case you had the same idea ;)
Do not forget to put in the 470µF electrolytic capacitor (even if the circuit is fed from a 9 volt battery). This prevents oscillations on the positive voltage rail caused by the NE555. (I had the same problem with the 81 LED chaser. This is a well known issue with the normal NE555 chips but if you use a LM7555 cmos version of the 555, this problem won't occur). The capacitor makes sure you get a nice ripple free supply voltage, which needs to be 9 Volts btw. I also put a Schottky diode in the positive voltage rail to prevent damage from accidental polarity reversals. This circuit draws between 24 and 34 milli-amperes (depending on the frequency of the timeline) so it can easily be fed from a 9 volt battery.

I proceeded to build the LED matrix first and I wanted to make a better job of it than I had done with the LED chaser. So I again had to grind down 100 LEDs on 4 sides to make them fit tight together on the perforated circuitboard. I had to glue on an extra bit of circuitboard because I could only fit 9 rows of LEDs on there and I needed room for ten rows. After I had soldered them all in place I took my Dremel tool and shortened all the negative leads so I could fit the positive rails over the negative rails without them touching and creating a short circuit. Here's a picture of the backside of the LED matrix which came out very well. (You can see the extra strip of circuitboard I glued on at the left side):


After that I proceeded to solder together the rest of the electronics which was quite straight forward really. I did end up with a mess of wires and knobs etc. but that was unavoidable. But it was going to be build into a nice case anyway. Here's the finished product mounted in its case but still very accessible because only the display is glued into place so changes and repairs can easily be made.



Here's a closer look at the switch with the different capacitors on it, to change the frequency range:



And here it is in full working order:


Problem solving:
I did encounter a little problem after I had assembled the scope in its case. I had made a BNC connector on the front panel to attach a probe to but it turned out that the ground wire caused the 3rd column of LEDs to turn off so I cut the ground wire for now. I need to mount the BNC connector in such a way that it is insulated from the case completely.
I also build in the microphone with the little amplifier which you can see in the schematic on the lower right. This works very well and I put in a switch to choose between the microphone or the probe. But I wanted an amplifier that is a bit more powerful and has a volume control button that I can put on the front panel aswel. In the next paragraph I explain how I did that.

The Amplifier:
Like I mentioned above, I wanted to build a more powerful microphone amp with a volume control to put in this scope. Well, recently I did just that. It took me just over an hour to build it and put it in and it works very well. The mike is much more sensitive now and reacts even to random noise it picks up. I build it with a 2N3904 transistor as a pre-amp for the electret microphone and then a LM 386 to amplify the signal. Here's a little sketch of the circuit:


(Last revised: 08-Feb-2020 Changed 10µF cap on pin7 for 100nF.)

Here's a picture of the scope with the new volume control added to the front panel:



Here's a little video of the scope working (with a bit low battery) with the synthesizer I build. There are no knobs on the scope this time because I needed them all for the synthesizer, LOL :)  :



I can really recommend using this design over the microphone amp in the main circuit schematic. This one works much better.

Okay, that's it for this project. I hope you enjoyed this read and if you did please consider supporting me by subscribing to my YouTube channel EdEditz or by following this blog or clicking on the adds.

If anyone has any idea how to incorporate a trigger section into this scope I would love to hear from you!!
If you have any questions or remarks, feel free to post them beneath in the comments or on my YouTube channel. I always love to hear from you!!!