R2-D2

Normally an optoisolator is between 2 non-connected circuits and I understand how that works. How is the optoisolator being used here? Does the transistor in U1 only start conducting when the LED in U1 is on by having Q1 conducting? But it looks like Q1 needs the transistor in U1 to conduct before Q1 conducts.
 
This is a variation on a Fuzz Face. In the classic Fuzz Face, there is negative feedback from the 2nd transistor's emitter thru a resistor to the first transistor's base. Instead of doing the negative feedback that way, I'm using photons. In the circuit above, Q1's emitter current flows thru the opto's LED. The higher the current, the more photons. Those photons land on the opto's transistor and bias it on. That lowers Q2's base voltage which in turn lowers Q1's emitter current. We have a DC-coupled negative feedback loop, just like in a Fuzz Face. The opto is being operated in the linear region in this circuit. I'm not using the opto for isolation.
 
It's pretty similar. I need to hook it up again and play it. I've made other opto-based distortion circuits that exploit the non-linearity inherent in a photo-transistor opto.
 
You must log in or register to reply here.
Back
Top