I came across a simple circuit in which when a reed switch (which is normally closed) is activated then a LED is being turned on (the LED turns ON when the switch in closed as it is depicted in the schematic). Obviously the led is ON only when the magnet is away from the switch.
My question is how can the LED stay ON even when the magnet comes close to the switch after the first activation? I would like a solution without the use of a microcontroller.
In order to make things clear: If the magnet is next to the switch the LED is OFF. If the magnet goes away the LED is ON. As long as the magnet is away the LED is ON but I want the LED to remain ON even if the magnet comes back next to the switch as long as the switch has been activated once.
simulate this circuit – Schematic created using CircuitLab
Best Answer
You might be able to use a thyristor for this. Once triggered they stay on until the current is interrupted by another switch. e.g.,
simulate this circuit – Schematic created using CircuitLab
You'll need to find one that will stay on with 10 - 20 mA going through it. Alternatively, you can make a thyristor with a PNP and NPN transistor. A web search should get you plenty of examples.