Hi all! I'm trying to design an LED current modulator circuit but I'm stuck. I've scoured the interwebs and have found plenty of circuits but none that will work.
The specification is simple. The circuit idles with the LED at a set current, say 1.7 mA. A full wave rectified control voltage (audio) is the modulator. This control voltage increases the current based on amplitude up to say 5 mA. The problem I'm having is that the control signal modulates the current around the idle current (so from 0 to 5 mA) instead of only adding to the idle current (from 1.7 to 5 mA). I can get either the correct idle current, or the correct range with max control voltage, but never both in the same circuit.
The only caveat is that I must use a single supply, preferably positive. Opamps (preferably one section) and/or transistors would be on the table.
Seems like this should be super simple but it's been 3 days of simulation and nothing. Any clue how to do this? Thank you in advance!
The specification is simple. The circuit idles with the LED at a set current, say 1.7 mA. A full wave rectified control voltage (audio) is the modulator. This control voltage increases the current based on amplitude up to say 5 mA. The problem I'm having is that the control signal modulates the current around the idle current (so from 0 to 5 mA) instead of only adding to the idle current (from 1.7 to 5 mA). I can get either the correct idle current, or the correct range with max control voltage, but never both in the same circuit.
The only caveat is that I must use a single supply, preferably positive. Opamps (preferably one section) and/or transistors would be on the table.
Seems like this should be super simple but it's been 3 days of simulation and nothing. Any clue how to do this? Thank you in advance!