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

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


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.


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.


Does the following switch actually work? It's a little confusing