Building a Uni-Vibe type guitar effect pedal

I have always wanted one of these pedals so I thought it would be fun to try and build one;   this will be my first pedal build so i'm expecting to make a few mistakes along the way.

After a bit of research I thought I'd go for the Easy-Vibe circuit that John Hollis published on his website, the website itself can be found here, easy-vibe.

My initial thoughts are I like the op-amp approach to the circuit rather than using the original transistor based approach as it makes for a smaller component count.

The Easy-Vibe seems to use LEDs to drive the LDRs;  now, I'm not sure if that makes any significant difference in the sound when compared to using a light bulb - i'm guessing it does, but it might be interesting to try one against the other.

A few weeks ago I ordered most of the main parts so now I can actually get started with step 1 and that is to try and build up a working circuit on a breadboard.

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So here is some progress I made last night,  this part of the circuit is the power supply / pre-amp circuit.

 My breadboard is powered by my bench power supply set at 9V.

This is my simulated input signal from my signal generator, the frequency is set at ~600Hz @ 424 mVrms

And this is the preamp output sine wave, as viewed on my scope.











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I started work on the low frequency oscillator part of the circuit, or LFO as it's commonly referred to.  And after a bit of tweaking,  the LEDs are lighting up but they are not pulsating which is I believe what they should be doing.  As far as I understand in the circuit there are 3 opamps to the LFO ; one is power to the LEDs based on the position of the DEPTH pot, and the other two are for the LFO .  So i'm thinking I'm going to have to check my circuit in more detail and see if I've missed anything.

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ok, so I fixed that; turned out one of the salvaged leds wasn't pushed into the breadboard properly.
so now I get a nice triangle waveform on my scope, and the depth and rate seem to work as they should.

So, now all I need to do is build the 4 stage (phase shift?) part of the circuit.

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ok, so this the final part of the circuit;  the only tricky part here is coupling the LEDs with the LDRs and wrapping tape around each one;  the Easyvibe build instructions suggest using heat-shrink tubing as a light shield, but I didn't have any to hand, so I just used black electrical tape.

I've given the 4 stages a quick look over and there were a couple of mistakes I missed, so I'll come back to this with fresh eyes later and make sure I haven't missed anything else before hooking it up for a proper sound test

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After giving it another look over I did in fact miss a couple of wires;  so after hooking it up to my guitar it worked :)  the depth isn't quite there, prob about 45% of what it should be, by my ears anyway;   but i'm sure that's just tweaks to the bias / photcells and leds to get that working right;  which is what I plan to look at next.

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Well, I tweaked the circuit slightly so I could use super bright white LEDs and then realised I had the pedal on the Chorus setting!  so after I moved a wire across, it then burst into Univibe mode! lol    Anyway - because I tweaked the circuit I need to re-add a LED bias part of the circuit back in.

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21st July.   So, I added another little change to the LED driver circuit,  I think it sounds better - well to my ears anyway;  I don't own or have ever used a Univibe so don't hold me to that!   What I have tried to do is smooth the LED pulsating as much as possible to give more smoothness to the "whoshing" at slow speeds;  this has to happen both with the ON and OFF parts of the cycle.

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22rd July.  I have been researching pedal cases and looking at designing a prototype PCB;  the case I think I will go for is the Hammond series as they are really popular with pedal builders and it seems a no-brainer.  The PCB is a bit more involved and will take a little more time to get right;  I'd much prefer the jacks,  leds and and pot on the pcb to make it less work less with soldering and trimming wires etc.

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4th Aug.  ok.  so i've been playing around with using tiny bulbs in an arrangement like this..

And I like how that sounds compared to the LEDs.    I have also been looking at a different LFO circuit;  so what I have done is I've used a microcontroller and a 12bit dac to generate a nice triangle wave which I hope to use to feed into the lamp driver circuit in order to drive the lamps; and that little circuit looks like this..

Pretty nice looking triangle wave.





So, now i'm going to add some POTs to this circuit so I can vary the Rate / Depth and Offset of the lamp driver waveform.

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30th Aug.  I have some more bulbs to try which were delivered recently;  these are a slightly higher voltage than the mini bulbs so i'll need to work something out in order to power those.