# PNP transistor with audio signal output on collector is used to drive opto-isolator. How does it work?

I have tried assembling the circuit but it doesn't work. Now I need to understand how it works in order to debug it. The part that confuses me is:

Channel 1 is a standard audio output from a smartphone or PC. I have read that LED D1 should flicker depending on a music. But is it even possible? I don't think so. It is always on because there is a constant electrical connection between LEDs' pins and power supply.

Now more difficult part of this circuit. I guess that Q1 is used to amplify the voltage of audio input in order to power optoisolators' infrared diode. Am I right? I have search through common transistor configurations and never seen even one configuration like this. I would like to know how does it work.

• Are you sure that the opto isn't supposed to be a 6N135 or 6N136?
– CL.
Mar 9, 2018 at 18:17
• @CL. I am 100% sure.
– 3Qax
Mar 9, 2018 at 21:32

Very odd circuit.

It looks to me like the LED with Q1 and R1 is acting like a current source of ~10-15mA to bias the opto-isolator LED into conduction .

$I_D = (V_{F} - V_{be})/R1$

simulate this circuit – Schematic created using CircuitLab

Any signal coming down CH1 will either add of subtract current from that bias, and, consequently, change the drive on the phototransistor.

Attaching the signal directly to your smart-phone output would be a bad thing though. You should at least have a DC blocking capacitor in that line.

• The effective impedance of Q1's collector is very high, so the load seen by the audio signal is determined entirely by the dynamic impedance of the optocoupler's LED -- which is actually probably pretty close to what something designed to drive 32-ohm headphones expects to see. But I agree, a blocking capacitor is required. Mar 9, 2018 at 15:49
• Everbody's assuming old-school analog source. How likely is it to work with a H-bridge Class-D signal source meant to see a coil-magnet headphone load? Mar 9, 2018 at 16:05
• @glen_geek depends on your definition of "work" :D It's not a great circuit to start out with. Mar 9, 2018 at 16:08

Ignoring the input connection, the circuit is a constant current generator attempting to put about 14 mA into the opto-coupler's LED. So, in order to debug it you need to establish the current; you could measure the voltage across R1 (100 ohm) and establish that it is around 1.4 volts. If not 1.4 volts then does D1 (the red LED) illuminate?

Once you have that part working you need to apply your audio connection to the collector via a coupling capacitor. The capacitor is there to ensure that your audio source doesn't short out the DC voltages at that point. Then you should be in business. Here's a simpler circuit to consider if you want to know how it works: -

Picture source

• +1 Current and voltages will depend on the LED Vf though.. I assumed 1.6V in my answer, but who knows. Not sure it is critical. Mar 9, 2018 at 15:47
• Red LEDs are normally about 2 volts. Mar 9, 2018 at 15:51
• Yup..whatever "Normal" is ;D Mar 9, 2018 at 15:54
• I have connected 1kHz sine waveform played from smartphone to CH1. The voltage across R1 is 2,96V. I have used OSR5JA39B1B as D1. According to a datasheet Vf=2,1V (typical). When I plug in 5V to the unit the D1 light up.
– 3Qax
Mar 9, 2018 at 16:15
• @3Qax set it up with no audio connection - you should see about 1.4 volts across R1. If more than about 1.6 volts then something is wrong. Have you breadboarded this? If so show a picture. Mar 9, 2018 at 16:36