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.


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

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


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.

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.

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