using transistor to activate membrane switch

Blake

hi,
I am new at this but when I try to use a transistor to activate a small membrane switch (connecting collector and emitter to the switch's two contact points) just touching it to the contacts completes the circuit. I was under the impression this would only happen if current was supplied to the base.·I·thought the best way to control the switch with a basic stamp would be via transistor, but is there another way? Am I doing something wrong, or need a different transistor. I have TIP32A.

Thanks,
Blake·

Dave-W

Blake,
Anytime I need to hook the Stamp to the outside world where static may shock the stamp I always use an optoisolator like the 4N36. The transistor side would hook to the switch and the LED through a current limit resistor would hook to the stamp port.
Dave

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D. A. Wreski

Mike Green

Blake,
What are you trying to accomplish? What do you mean by "activate a small membrane switch"? Are you trying to substitute some signal from a Stamp for a mechanical keypress? Very often, these membrane switches are at places in the device's circuit where a non-isolated transistor may not work. You need a relay (like a small reed relay - switched with +5V from the Stamp) or an optoisolator. Sometimes the optoisolators won't work because they (the optoisolator) may drop a small amount of voltage across it (like 0.3V) or the resistance is too high across the phototransistor in the optoisolator.

Chris Savage

TIP32A is a 3 amp PNP Power Transistor. I would think if this was a membrane switch and a transistor would work you'd use a 2N3094 or similar NPN Switching transistor.

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Chris Savage
Parallax Tech Support

Dave-W

Blake,
Since it is a membrane switch you will not have a problem with the very small amount of leakage with the optoislator. Just make sure the collector of the darlingtion transistor side is more positive than the emitter. A simple voltmeter check will tell you.

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D. A. Wreski

Blake

Thanks everyone for the replies. It will probably take me a few days to sort through all of those options.

Blake

Blake

Part no. 275-232, spst reed relay with 5 volt coil from Radio Shack·did the trick. I forgot that i used one of these on another project. thanks for the suggestions everyone.



Blake