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What transistor should I use for a touch-sensitive switch?

I would like to run a 5v green laser diode off 4v (because it is easy to blow at 5v) using a touch-sensitive switch. I hope to use the switch with a thin layer of plastic in front but if the current from a human cannot pass through to activate the switch then I'll leave that feature. Anyway, I would like to know what Darlington transistor or regular transistors (to make a Darlington pair) to use that will operate the laser diode momentarily and safely.

Thanks,
Jim

The link to the laser diode: http://www.dealextreme.com/details.dx/sku.10094


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markee26 years ago
actually you can  use any general purpose type transistor 

you can refer to this circuit  copied from
http://www.simple-electronics.com/2010/11/simple-touch-switch-using-transistor.html




simple touch switch using transistor.png
swagatam7 years ago
Learn how to make a simple, accurate and faithful touch sensitive switch here:

http://www.brighthub.com/engineering/electrical/articles/63305.aspx


contact me : hitman2008@live.in
If its that sensitive to over voltage, feed it from a 5V regulator !

Touch switch ?  Look for suitable circuits on line A darlington won't do it. You need to use a capacitance switched circuit.

Steve
pyro-jim (author)  steveastrouk7 years ago
I thought that a Darlington gives such a high current gain that putting a finger across contacts attached to the base and a voltage supply will switch the Darlington. I would use a capacitance switch but I thought they needed IC's and are quite complicated, I'll try to look up a simple one or cheap ready made switch.
Sorry Jim,

You won't have enough leakage through the dielectric that a bipolar transistor can handle through your touch contact. And the laser diode needs such a lot of current, that even <very high> x <base current> won't be enough.

Try looking for simple touch circuits, but companies like Atmel/Qprox do single chip, cheap capacitance switching solutions that take all the headaches away.

Steve
pyro-jim (author)  steveastrouk7 years ago
Does the following switch actually work? It's a little confusing