So I'm building a laser maze for a science project for high schools, where we have lots of mirrors and the laser bounces around the room, when it hits the detector I want music from a source to switch on and off when the beam is broken. Since the audio signal will be analog, google told me I would need a triac to switch an AC signal.

Since the signal will have an amplitude of < 2V from the iPod, I want it to work for all voltages less than that, the signal doesn't have to be perfect, but I want it to be reasonable for all frequencies < 20 kHz for audio (see my schematic below, but even if it works for frequencies < 10 kHz I can still play some dubstep that will sound good for the kiddies).


simulate this circuit – Schematic created using CircuitLab

Firstly I have literally no experience with a triac, but reading online it says that both halves of the waveform will not fire simultaneously, is 'not simultaneously' still fine for audio purposes? If so, I bought this photodiode off ebay see the data sheet here, is the short circuit current out of the photodiode the relevent figure I should look at? Data sheet says 0.16 uA output current but the triacs gates I found on ebay were mostly 3-5mA to turn them on. Just checking that means I need to put an amp between the photodiode and the R2 resistor, then limit the current by selecting the correct value of R2 to be in the range of the 3mA triac.

It seems that would require a power supply when I would prefer to do it without a power adapter. But if I need that, would something like this photodiode and amp suffice? Output current is 15mA, then I could just put a resistor to get the value to 5mA or just buy a triac with a gate current of 15mA and it should be all fine and dandy right?

Anyway, just wanted to check what I was doing was okay before I buy all the components, since I don't have much experience with this sort of stuff.


Good question, but at the same time this circuit will probably not work. Things are a bit more complex than you foresaw. Three reasons:

  1. The photo diode will not be able to drive a TRIAC directly. As you mention yourself, you need something amplifier/driver stage.
  2. A TRIAC will drop at least 1.2V, leaving very little signal at the output.
  3. The audio signal will not be able to source/sink the hold current for the TRIAC.

What would be simplest is to use a relay, as that is easy to drive and your audio signal won't be influenced much. You also need a detector, relay driver and probably a one-shot timer to leave the music playing for a brief moment after the detector is hit. Designing such a circuit is -in my opinion- beyond the scope of this question.

  • \$\begingroup\$ Thanks for your comment! Exactly the sort of thing I was looking for, didn't really think about voltage drop across the triac (ipod signal output voltage is only about 1.5V to begin with anyway), I'm sure I can handle designing a simple relay circuit to do the job. :) \$\endgroup\$ – alder Apr 17 '13 at 6:44

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