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I am trying to implement a simple latching circuit that will turn an LED on if the signal from an AND gate is high (LED latches ON if high, and only way to reset is to remove power). I was thinking of using an SR latch, where I reset to the initial state (resulting in an off LED) then when the signal comes in, I switch the state. However, I don't know if 1) this is the right way to do this (seems a bit too complicated, since I don't need 2 outputs or 2 inputs) or 2) how exactly this would even be implemented in a circuit (especially the initial reset, or the power removal feature).

I hope my question is clear, since I am a beginner and am not sure if my descriptions are technically informative. I appreciate any help!

EDIT1: Attempt 1 Using SCR and Transistor:

enter image description here

EENoob
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  • Do you require it to be digital logic? You can also do this with an SCR or TRIAC. – Aaron Jan 28 '21 at 17:13
  • Yes, the logic comes from 2 comparator inputs. I pass through an AND gate and if both are high, a timer cirucit is triggered. If these timer reaches a given time, then the circuit should latch – EENoob Jan 28 '21 at 17:21
  • So it sounds like it is the output of the **timers** that you need to latch, not the input to the timers? – Aaron Jan 28 '21 at 17:51
  • Yes, that;s correct – EENoob Jan 28 '21 at 18:32

1 Answers1

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You don't need to complicate it with digital logic, just use an SCR.

When power is first applied no current flows through the LED.
After the button has been pressed, then the SCR is latched on and the LED is lit.
The only way to turn it off is to stop current from flowing through the SCR ie. remove power.

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT with AND gate control

schematic

simulate this circuit

Aaron
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  • The AND gate uses 2 comparator input, so if both the comparator inputs are high, then the LED turns on. So I think I would have to replace the switch with something. – EENoob Jan 28 '21 at 17:20
  • Should I use a transistor here? – EENoob Jan 28 '21 at 17:26
  • @EENoob see the update with an AND gate to control it. – Aaron Jan 28 '21 at 17:49
  • So if I understand correctly, if the voltage from AND1 is high, then SCR1 latches ON? – EENoob Jan 28 '21 at 18:32
  • You might find purchasing a small SCR isn't as easy as you think. Here a link on how to make your own. _Can a thyristor be built from two transistors?_ https://electronics.stackexchange.com/questions/344321/can-a-thyristor-be-built-from-two-transistors – D Duck Jan 28 '21 at 19:48
  • @EENoob yes that is correct. – Aaron Jan 28 '21 at 19:48
  • @DDuck 851 active and in stock: https://www.digikey.com/en/products/filter/thyristors-scrs/281?s=N4IgjCBcpgnAHLKoDGUBmBDANgZwKYA0IA9lANogAMIAusQA4AuUIAykwE4CWAdgOYgAvsQC0AJmQg0kLgFcipCiACsdRi0ghhIkJMiVcKTnSFA – Aaron Jan 28 '21 at 23:41