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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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Your coil needs to be between the collector and + with the emitter going directly to ground. You also need a spike suppression diode accross the coil. I didn't catch what frequency audio you're using, but you may need to put a small capacitor, (couple microfared), from base to ground to prevent chatter.

Ah yes, was reading the transistor diagram backwards. I thought I had it connected to + in that diagram. my bad.

What nature of diode do I use for spike surpression and how do I connect it?

 

I have a cap connected between base and ground but through the trim pot. Though I put it there to smooth the pulsed DC into straight DC.

Can you define 'chatter?' You're talking about spurrious signal leaked from one portion of the circuit to another?

 

The frequency is completely flexible. Right now I'm sending a 2600hz signal but someone said I might have better luck at 60. Advice?

better luck at 60 hz? I doubt it.  With 60 Hz being used in the power grid, (50 in some countries) it would make a lot of extra electrical noise and cause false triggering.  In my opinion it is best to stay away from that frequency.

 

Ah yes, was reading the transistor diagram backwards. I thought I had it connected to + in that diagram. my bad.

What nature of diode do I use for spike surpression and how do I connect it?

 

I have a cap connected between base and ground but through the trim pot. Though I put it there to smooth the pulsed DC into straight DC.

Can you define 'chatter?' You're talking about spurrious signal leaked from one portion of the circuit to another?

 

The frequency is completely flexible. Right now I'm sending a 2600hz signal but someone said I might have better luck at 60. Advice?

What about this.

I don't know how to make DipTrace angle the diodes. That's supposed to be a bridge rectifier there.

I started thinking about your circuit again and the input to it. Would you really need a bridge rectifier? You are feeding your circuit a wave that never breaks the ground plane. A bridge rectifier forces the half of a wave form that is negative to become positive. You don't need that. I would think a cap on the input should smooth out your wave.

PS - I think a lot. I am not always right. :)

Using the bridge rectifier allows for more current to be delivered from the signal line to the transistor, but in this case you'll still end up with pulsed current through the coil, since the rectifier output will swing through 0V twice for each sine wave period.

You could always combine this with your previous circuit (adding the capacitor to the transistor's base) to keep the transistor switched on whenever the sine wave is being transmitted.

You might run into a problem with the diode's forward voltage drop however - compared to the other circuit, the sine wave signal needs to overcome the minimum forward voltage of two diodes instead of just one. Normally this wouldn't be an issue, but the audio signal is very small, so you'll have to find/build a bridge rectifier with a low enough forward voltage drop.

Are you going to use a mono (2 contact) or stereo (3 contact) headphone plug? The ring contact nearest the wire is the ground, the other ring and/or tip contacts are for the audio signal itself.

Thanks for the tips. You just answered several of the questions I was going to ask.

I figured I was going to need a cap in there to smooth it from pulsed DC into solid DC but I wanted to look it up first so I left it out.

I realized just after I created this diagram that I had just designed an AC to DC converter. Now that's a phrase I can search the googles for and see a number of reference designs. So I figured I would check out some to make sure where I would be putting the cap would be correct. I'll have a new schematic soon.

The tip on the germainium diodes is great also TY.

New questions.

A friend of mine is speculating that 2600hz might be too fast. That I would have more success at something like 60 specifically because it would give the cap more time to charge. What do you think?

 

For the hardware I am going to use a stereo jack. I'm actually planning on just cutting the speakers off an old crummy pair of earbuds. I suppose it might be nice to the device if I soldered a resistor across the leads of the one that I'm not using? I know some electronics don't like generating a charge that doesn't have anywhere to go.

Are you saying he should look for Germanium diodes to make his bridge then? I have read they have only a .2 to .3v drop vs the .6 to .7v drop of a standard diode.

Yep, germanium diodes have the lowest forward voltage drop of anything widely available that I can think of. At low current the voltage drop will be even less =)