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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ do you need isolation between the source and the control? \$\endgroup\$ – JonRB Nov 8 '15 at 23:50
  • \$\begingroup\$ @JonRB This comes from a simple circuit I stumbled across online which was used in a door as a diy alarm so I believe that isolation is not necessary. \$\endgroup\$ – Adam Nov 8 '15 at 23:51
  • \$\begingroup\$ Please clarify your question: does the LED turn ON when the switch is Opened or Closed? \$\endgroup\$ – Dwayne Reid Nov 9 '15 at 1:12
  • \$\begingroup\$ @DwayneReid the LED turns ON when the switch in closed as it is depicted in the schematic. I will also edit the question. Thanks for the remark. \$\endgroup\$ – Adam Nov 9 '15 at 3:42
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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.,

schematic

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.

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  • \$\begingroup\$ I will try your circuit and I will let you know. Thanks! \$\endgroup\$ – Adam Nov 9 '15 at 17:18
  • \$\begingroup\$ @Transistor would a latching switch circuit work just the same? \$\endgroup\$ – kit Oct 23 '18 at 9:22
  • \$\begingroup\$ It is a latching circuit and only uses one solid-state component. There are plenty of other options such as relay latch, transistor, flip-flop, etc. \$\endgroup\$ – Transistor Oct 23 '18 at 10:38
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One solution would be to create an electromagnet in series with the LED

schematic

simulate this circuit – Schematic created using CircuitLab

By taking the return wire of the LED via a "home made" electromag (iron nail & winding a few turns) you should be able to establish the required field strength to keep the REED engaged once you take the magnet away

NOTE: to disengage you will need to turn the main source of power off, but I am guessing you are aware of this

NOTE: the electromagnetic will need a freewheel diode

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  • \$\begingroup\$ I think his switch is normally closed type. The magnet is normally in position keeping the switch open. He wants to light the LED if the magnet is moved away from the switch and keep it on. \$\endgroup\$ – Transistor Nov 9 '15 at 0:45
  • \$\begingroup\$ I put that extra switch in as there needs to be a way of turning it off. His reed switch is the one on the right \$\endgroup\$ – JonRB Nov 9 '15 at 6:54
  • \$\begingroup\$ I understand that, but if his switch is normally closed type. It will turn on when the magnet is moved away. He wants to keep it on when the magnet is moved back. See my answer. \$\endgroup\$ – Transistor Nov 9 '15 at 7:32
  • \$\begingroup\$ @transistor I have edited my question so I hope it helps both of you to understand exactly what I ask. \$\endgroup\$ – Adam Nov 9 '15 at 7:53
  • \$\begingroup\$ rechec the question. the magent is normally present, exactly cancelling the magnet without activating the reed switch is unlikely to be practical. \$\endgroup\$ – Jasen Nov 9 '15 at 8:12
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schematic

simulate this circuit – Schematic created using CircuitLab

This circuit may help you. When the switch is closed (magnet away) the transistor Q1 is in saturation and the LED is ON. When the magnet is close the switch is off and Q1 is open, nocurrent on the LED. Please Calculate R2 and R3 to have apropper volatge level at the basis of the transistor. Calculate R1 to limit the current on the LED.

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  • \$\begingroup\$ Your circuit gives the same result as the OP's circuit. It lights when magnet is away but turns off when magnet returns. OP wants it to stay on when magnet returns. \$\endgroup\$ – Transistor Nov 9 '15 at 14:04

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