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I need a basic swtich driven by an audio signal

So I have to drive a coil with about 9 to 12 volts. I have a trigger signal which is basically a sine wave coming out of a cell phone's headphone jack, which I believe should be around 1/2 a volt.


The question is can't I just drive this with a Transistor used as a swtich with a diod attached to the latch pin to cut out the reverse voltage from 1/2 the sine wave to get pulsed DC at high Voltage?

If so, anyone know some transistors I can use? Obviously they need to be rated for the voltage/amperage I'd be working at.


Second. How badly is the pulsed DC going to kill me with the coil due to impedance/back EMF?

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omadawn's picture

Oh, I get it the stacked 2N2222s are the amplifier. I was still thinking of them as a switch and wondering why they weren't being triggered by the +12v source in your design. They are they are just amplifying the signal that they get from the AC source and THAT is what triggers the 2N3906 and 2SA968 combination.


I'll have to order a 2SA968/2SC2238 for this. I haven't had a chance to touch it all week.Is there any difference between the two?

/me heads over to digkey right now.

omadawn's picture

I'm not seeing either of those transistors @ digikey. Where do you get your parts? Where's this polida that you mentioned?

omadawn's picture

I just remembered some caps are not biased. I thought I was supposed to infer which way to connect them. I just need to use ceramic disk capacitors.

omadawn's picture

That's, of course, what I meant by kill me. I was wondering if it was likely to fry my circuits and I would need a flyback diode in the circuit but at the time confused about how to rig it up. Obviously you have them in your schematics and they've been recommended by others.

omadawn's picture

Yay, I might actually have all the parts for that second one. I had just posted "Where do I put the second wire from my heaphone out" and then when it refreshed the page I see the second circuit.

I was just banging my head against trying to figure out what changes I would have needed to make to replace the 741 with a 386. (no 741 at tha shack today)

Thank you very much for this. These are both awesome. It seems that about every two or three years I forget everything I've ever learned about electronics and have to start over. I'm there now and it's frustrating as hell. Couple that with the fact that I've never worked with coils before and this whole exprience has been kind of depressingly difficult. Too much stuff that I just don't understand. I appreciate the help and the patience. I'll let you know when I have made some progress.

Dan M's picture

Oh, I should ask, did you figure out about coils or do you still need help there?


omadawn's picture

All my questions about the coils have been answered. It was more that I didn't know what the implications of working with them were. Like, Odds are it's going to be kind of a high current circuit. That was a surprise to me. And the question my friend raised about perhaps being closer to 60hz would be better (which you debunked) but never having worked with them I just didn't have a clue.

Dan M's picture

Using the 386 is perhaps overkill powerwise. Where the 741 is in the circuit it is working with low currents. I would suggest that at that point, it would be better to use one specifically called an op-amp (operational amplifier), where the 386 is called an Audio amp. What's the difference? In this case basically the power and the wiring is 'slightly' different.

Any op-amp that has + and - inputs, an output and positive and negative battery connections should be able to drop in as a replacement for the 741. I know, I am generalizing, but I did say, "should"  <g>.

omadawn's picture

I have everything but the 2SA968/2SC2238. It looks like I might have to make a purchase online but I was hoping to get working on this. I picked up a TIP120 at the shack yesterday. Can I use that? I'm having trouble finding a datasheet for the 2SA968/2SC2238 in english (finding it in Japanese is easy.)

Dan M's picture

You cannot use the TIP120 directly as I drew it because it is an NPN. You need a TIP 125, 126 or 127 which are PNPs.

Now the TIP120 could be used, but the circuit would need redrawn.

If you use a TIP 125/126/127 it is a darlington and so will replace both the 2SA968/2SC2238 and the 2N3906, leaving out the 180 ohm resistor as well.


I found a data sheet for both the 2SA968 and the 2SC2238 online, so if you decide to get one, I can find that again for you.

(Years ago, I had a course in spoken Japanese, but never did learn to read or write it. I found some of the data sheets in Chinese. I don't even speak that , let alone read it.)