So, here is a simplified schematic for the Solar Lifeforce. Basically I have removed the expression/CV out stuff, removed the toggle for the buffer so the buffer is always on, and made it as simple as possible. It could maybe do with some small value caps between both R5 and ground and R9 and ground, but they aren’t essential. The LDR is a pretty unreliable thing for any consistent readings so maybe buy a bunch and try them out until you find one that suits you, I have tried the circuit with ones of varying specs and they all have behaved OK. The best I found was on doctor tweeks website which can be found here. It has a dark resistance of 20M which is plenty, 10 lux (‘room’ light) resistance is between 50k-100k and it has a rise time of 20ms and a decay time of 30ms.
Before I go into the circuit I will warn you – LDR’s aren’t perfect for cutting volume alone as they roll off treble, so your tone will be altered. This isn’t a problem so much when you are playing in really well-lit conditions but it does become noticeable when you are in stage type conditions. Of course this may be what you want, but if you are after a ‘transparent’ volume cut a LDR is not the way to go.
Anyway, here’s the circuit. I havent included the LED/switch wiring etc in this one, and I haven’t put in reverse polarity protection. It is up to you if you think it is needed. If you think it is you could just use a diode such as a 1N4001 in series with the 9v rail.
R1 is a pull down resistor to help prevent popping when the circuit is bypassed/engaged. R2, R3 and C1 form the network which establishes the reference voltage for the op-amp. The first side of the op-amp acts as a buffer, and the second side acts as a gain allowing you to increase the volume to compensate for poor lighting conditions. VR1 controls the range of the LDR between being covered and uncovered and R5 sets the minimum for this. R9 and R10 set the gain of the b side of the op-amp, which is around 5 (G = 1 + (R10/R9) = 5.545), which is then attenuated by VR2 just before the output. C4 stops any DC leaking from the circuit into your amp. You could use a lot of different op-amps for this circuit. I used a TL072 because I like them, have a few kicking around and is a dual op-amp so takes up less room than two signals. They are also pretty cheap and quiet so are good for this kind of work. On the Barefoot which was really the predecessor to the Solar Lifeforce I used DPDT toggle switch to change between if the signal kills when the LDR is covered or is restored there, you could probably mod this circuit to include it. You could also use a DPDT toggle to cut the buffer out of the circuit if you wanted too easily enough. I wired the original so the buffer was independent of the pedals bypass, so the signal hit the buffer, then the bypass, then the rest of the circuit, but you could wire it after the bypass so that you have a switchable buffer aswell. Just to clarify on this diagram the buffer is always in the circuit.
Here’s the part list:
VR1 500k Lin
VR2 10k Log
I found the best way of mounting those odd shaped LDR’s that are specified as 5mm diameter but aren’t actually round is to use a 5mm bezel mount. And be sure no light can get to it from inside the enclosure from the LED!
The circuit above is a little crude in places, it is a barebones version of the hand drawn circuit I drew when experimenting with it. Perhaps a nicer version would be to have a gain control directly controlling the gain of the b side of the op amp (ala the MXR Distortion+) and a separate volume control as the gain pot is wired in this circuit in this circuit. That way you are not boosting the signal only to attenuate it. The volume control is then completely optional as in many ways the gain will act as a kind of volume control anyway. Here is my revised circuit with the aforementioned mod, a few additional caps and a resistor to help cut out the really high frequencies before the buffer. Probably still a fair bit of room for improvement, but here we go! I haven’t built this version yet, but it should be OK.
I’ve modded this circuit again (haven’t tested it but thought I would post anyway, so beware!) to have some basic filtering on the power supply, set the output impedance at 1k and also adjusted the values of a few components to make them a bit more sensible. If you build this and can verify it works well please let me know!