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I am a complete novice when it comes to electronics but I am trying to open a transistor when there is audio coming out of my iPod, completing the circuit for my dslr and releasing the shutter.

I have control of the audio so i am outputting a 20hz tone because it has the most voltage somewhere about 1 volt and 5ma AC of course. At the other end my dslr outputs a voltage of 2.7 volts and 0.06ma DC.

I tried this circuit without the capacitor or diode (does the transistor need it?) with a resistor about 2k and a bc548 transistor (only npn one I had) and the signal did trigger a response out of the dslr but it seemed to be interference I guess is coming from the other terminal of the audio line.

Would the capacitor and diode make my circuit work by rectifying the signal and do I need a diode on the other audio source as well.

Circuit Diagram

Bruno Ferreira
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Cadell Christo
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1 Answers1

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The capacitor and diode are important elements of the circuit. They allow a steady base current when audio is present, even though the audio swings both positive and negative. Without the diode and capacitor, the transistor will be off for part of every audio cycle. This is not likely to be what the camera wants to see.

By the way, you want to close the transistor when audio is present, not open it. A closed switch conducts (is on), a open switch is a open circuit (is off).

Olin Lathrop
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  • I think people should use on/off for the switches, but that can create misunderstandings, too, depending on the mindset I guess. I hate it when I struggle understanding the switch position. – abdullah kahraman Jul 13 '12 at 20:00
  • okay thanks for clarifying that for me if the audio signal is 20hz what size capacitor should i be using? – Cadell Christo Jul 14 '12 at 01:18
  • @Cadell: The diode and cap are forming a half-wave rectified power supply to drive the base. You need the cap large enough to continue driving the base in between positive peaks of the audio waveform. What you need exactly depends on several parameters you haven't given, but I'd probably use at least 100 uF. More would be better as long as the audio source is OK with that. – Olin Lathrop Jul 14 '12 at 12:15
  • @OlinLathrop Thanks for your help my circuit is currently working very well and I dropped the resistor from the base of the transistor and introduced one parallel to the capacitor so it discharges quicker, letting go of the shutter sooner. I used your suggested 100uf capacitor and the smallest parallel resistor I could use was 2k ohms but I also tried a 10uf capacitor with the smallest resistor being 8k ohm. Sorry if it's irrelevant but which setup should I use and should I consider a larger capacitor? The audio input is 20hz if that helps. cheers. – Cadell Christo Jul 15 '12 at 05:32