I am trying to set up a circuit where a "switch" is locked into closed from "NO" using 12 vdc trigger. Once the "switch" is closed, it would take another 12 vdc to return to the "NO" position. The return could also be accomplished with a manual button.

I have been searching and found self latching relays and that looks like what I want, but they seem to either have wireless or buttons for activation.

There are also "bistable" and others that do not list as "bistable". I have no idea what that means.

I am trying to latch a small circuit (5 vdc 200ma) closed and then later reset to the normally open when I am done with my process. I want to trigger the relay with 12 vdc. It stays in the latched position (closed) without power applied. Power interruptions would have no effect on the circuit.

The small circuit (5 vdc 200ma) is part of a logic control in a PLC where it is an Exit Condition if "On". Basically, I have several Steps in a Process and have been manually turning a Toggle Switch "On" and close the circuit when part of the Process is complete. If that part of the Process is complete, a Power interruption would cause the Process to re start automatically at Step 1. If step 1 is complete and the logic Toggle is closed, the Process Jumps to State 2. I have several steps and need a board that has at least 6 latching relays. My quandary is that I cannot find instructions on how these boards work. It seems that many have buttons for activation, but that would do me no good as I already can manually turn on the Toggle. Some talk about adding a trigger switch but give no additional information. I want it automated and can logically apply a 12 vdc trigger when the Step is complete. Can someone point me to a relay board that would do what I want?

  • \$\begingroup\$ Can you not do all this logic in non-volatile memory in the PLC? What's the make and mode? \$\endgroup\$
    – Transistor
    Commented Mar 20, 2016 at 15:21
  • \$\begingroup\$ If you need to keep the memorised state between power interruptions, bistable relays are a good solution. They have two stable positions : something like an electrically actuated rocker switch. \$\endgroup\$
    – Grabul
    Commented Mar 20, 2016 at 18:29
  • \$\begingroup\$ It is a BCS 462 and I cannot do it in non volitile memory. I am hoping to find a ready made latching relay board that will do the job. \$\endgroup\$ Commented Mar 21, 2016 at 14:53

1 Answer 1


A very simple way to do this involves two diodes and a regular DPDT relay. The relay could also be a DPST relay with normally open contacts.

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The idea is that the input 12V pulse comes and powers the relay coil for the duration of the pulse. During that time the relay closes and supplies power to your load. At the same time the other pole closes and gates power through the second diode to maintain the relay coil in the energized state as long as the NC unlatch switch is closed. A benefit of the circuit is that if the 12V supply goes off the circuit automatically resets itself.

This topology can be made somewhat simpler if the whole thing is powered from the same voltages and suitable relay coil voltage is selected.


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