I am trying to replace the 50k pot on here (which is acting like a floating resistor from 0-50k)
and make it voltage controllable.
I tried using Vactrols but they do not go low enough in on-resistance and are very non linear to drive.
Lately I tried this analog fet opto coupler H11F1M https://www.fairchildsemi.com/pf/H1/H11F1M.html but I am having trouble with the control circuit. I am using a 2n3904 with the led of the opto coupler connected to the collector with a resistor for current limiting. When I vary the current to the base of the transitor I seem to only get a switch like response to the opto coupler (it is either on with a resistance of about 200 ohms or it is off with an infinite resistance). I need to get a linear response.... e.g. if I attache a potentiometer to the base of the transitor (with a 10k resistor in series with the wiper) I would like to be able to sweep the pot to get a range of resistance on the fet of approximately 200ohm to 50k. Right away I see I need to sum an offset voltage at the base so that the transistor is always on (e.g. sum about 0.6V)
Here is the circuit:
**broken link removed**
How would you make this CV controllable?
and make it voltage controllable.
I tried using Vactrols but they do not go low enough in on-resistance and are very non linear to drive.
Lately I tried this analog fet opto coupler H11F1M https://www.fairchildsemi.com/pf/H1/H11F1M.html but I am having trouble with the control circuit. I am using a 2n3904 with the led of the opto coupler connected to the collector with a resistor for current limiting. When I vary the current to the base of the transitor I seem to only get a switch like response to the opto coupler (it is either on with a resistance of about 200 ohms or it is off with an infinite resistance). I need to get a linear response.... e.g. if I attache a potentiometer to the base of the transitor (with a 10k resistor in series with the wiper) I would like to be able to sweep the pot to get a range of resistance on the fet of approximately 200ohm to 50k. Right away I see I need to sum an offset voltage at the base so that the transistor is always on (e.g. sum about 0.6V)
Here is the circuit:
**broken link removed**
How would you make this CV controllable?
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