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Transistor as a switch

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bryan

Member
Hello:

How would I use a transistor to substitute as a switch. In the schematic I would like to replace the switch by using a transistor. The output to drive the switch is 12V. The two circuits are isolated from one another, but share the same power supply. I planned on using a relay, but would prefer a semiconductor solution. I have on hand some 2n2222 and 2n2904 transistors.

Maybe using a optoisolator?
 

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Roff

Well-Known Member
I don't believe that two circuits that share the same power supply can qualify as "isolated".
What is the stimulus for your switch? In other words, what events will cause the switch to close and open? How much current does your switch have to carry? Is the load truly resistive, or does it have capacitive or inductive components?

Ron
 

bryan

Member
Ron H said:
I don't believe that two circuits that share the same power supply can qualify as "isolated".
What is the stimulus for your switch? In other words, what events will cause the switch to close and open? How much current does your switch have to carry? Is the load truly resistive, or does it have capacitive or inductive components?

Ron
The current draw in only about 20 ma and resistive. Basically I want to be able to "close" the switch using the power from the 12 volts on the left side from my poorly drawn schematic. The left side of the circuit is the output from a temp/fan controller. When the temp reaches a preset point the output goes to 12 volts. I want to be able to use this output to a activate a switch to a sperate timer module I have that uses the same power supply.
 

bryan

Member
Sebi said:
Do You think about this circuit....
I did not have any luck, the timer module( part of a old security system) accepts a trigger. Basically the user shorts the positive + terminal of the timer module to the trigger terminal to start the timer. I tried to use the transistor to do this, it would start the timer, but the timer would stay on continuously as if the the transistor was still switching, even though the output from the fan controller driving the base of the transistor was at 0 volts. Maybe some feedback was causing the problems, or leakage from the transistor??

When I used a multimeter between the trigger and the positive terminal the current required to trigger was only a couple of milliamps.

I ended up using a mini relay to turn on the timing module, would still like to figure out how to switch the timer module on from a transistor though.
 

truenoteno

New Member
I have a similar problem..
I want to control a digital device from my computer.
How could i simulate the push button ?
Could i hookup the GND from the parallel port to the GND of the device and 5v fro parallel port to one of the terminals of the switch ?
 

Sebi

Active Member
An optocoupler give always solution:just measure the polarity on pushbutton pins, and connect opto-collector to positive.But here is a picture only with transistors....
 

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