i am building a circuit for a quiz show lockout buzzer application ( the type were the first light to turn on is the only light that turns on). I have imagined a circuit that could do the job except i require a part that would effectively do the opposite job that a triac would do. That is, it would conduct until a current is applied to its gate at which point it would stop conducting. It must be a device that operates on low current (order of milliamperes) and DC. So far, i have learned of the device called the gate turnoff thyristor which is close to what I am looking for except it's for high current applications. Does anyone know of a device that would do such a job?

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    \$\begingroup\$ Any sane person would use a 20 cent microcontroller for this. \$\endgroup\$
    – pipe
    Commented Apr 13, 2017 at 2:01
  • \$\begingroup\$ It's usually best to imagine circuits that use parts you understand well and can readily purchase. \$\endgroup\$ Commented Apr 13, 2017 at 2:02
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    \$\begingroup\$ If you don't want to use a microcontroller for some reason, this kind of logic can be implemented with a few basic transistors. Could you explain why you have been looking at exotic thyristors? Is there some unusual requirement you haven't mentioned? \$\endgroup\$
    – user133493
    Commented Apr 13, 2017 at 2:02
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    \$\begingroup\$ @pipe Not entirely fact. As replete said that could be done with simple discrete components. A microprocessor is a bit over dimensioned for the job (well you can increment more things so it starts making more sense). This is a design choice, sometimes is not only price that maters. \$\endgroup\$ Commented Apr 13, 2017 at 4:48
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    \$\begingroup\$ A device like a GTO thyristor can be made from two transistors and a resistor, if you use small transistors it can be sensitive to small currents. but the discrete approach seems better solved using gates than thyristor-based logic, and the problem is easier to solve using a microcontroller. \$\endgroup\$ Commented Apr 13, 2017 at 5:04

1 Answer 1


Does anyone know of a device that would do such a job?

You can use a low resource micro-controller, and if you decide to add functions, on some cases you could simple rewrite the firmware.

You can do this with logic circuits too. Where is an example:

first button to press indicator Copyright http://www.next.gr

There are more simple circuits, the idea is to block the others inputs when one is pressed.

In this specific circuit, when one input goes low (a key is pressed), the output goes low to, lighting the led. Also the outputs are connected via NAND ports to the Enable pin of the latch. So when one output goes low, the latch is not more enabled until reset is pressed. The resistors (except for the ones with the leds that are current limiters) are pull-ups for the pushbuttons.


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