Let me start by declaring that I have absolutely no idea what I'm doing so please try to keep it simple if possible. Thanks in advance.

I have a project requirement that uses simultaneous input from two momentary switches (safety feature) to activate or deactivate a SPDT relay. Subsequent simultaneous inputs from the same pair of momentary switches should toggle the state of the relay.

I understand (I think) that the behavior I require is referred to as an impulse or bistable latching relay. What I'm struggling with is that I also have a requirement that the relay should reset to "off" on power loss so that when power is restored, the relay remains "off" until triggered again.

I know this is the opposite of how true latching relays work, so I'm thinking that what I probably want is a non-latching relay that's implemented such that it behaves like an impulse relay only as long as it has power. But I don't know how to do that.

Do you think this can be done with some type of relay logic or off the shelf module? or do I need to build it from scratch?

Any input is appreciated. Thank you.

  • \$\begingroup\$ "Subsequent simultaneous inputs from the same pair of momentary switches should toggle the state of the relay." This sounds dodgy for a safety application. Safety should require continuous detection of the monitored lines. Loss of signal on the monitored lines should cause the system to go into a safe state. \$\endgroup\$
    – Transistor
    Commented Jan 13, 2021 at 11:23
  • \$\begingroup\$ Thanks. In this case there are other momentary inputs occurring while the relay is "latched." Requiring two-handed input to deactivate the relay guarantees other functions have halted. \$\endgroup\$
    – Jim
    Commented Jan 14, 2021 at 0:00

2 Answers 2


A generic solution would be to use a PLC.if it is safety related, the you’d want a safety rated PLC. You could probably do it ‘old skool’ by using a handful of relays, but this is not the place to ask for a design.


You are on the right track! You need to define requirements for the circuit, now that you have already determined you need a NON-LATCHING, Normally Open (Form A) type relay.

  • What is the load voltage?
  • What is the load Amperage?
  • What is the microcontrollers voltage?
  • Which relay would be best suited for the job? Electromechanical or a Solid State Relay.

based on the answers above, and sourcing a relay that suits your need you would have to decide whether/if you are going/need to do a

  1. high Side switching
  2. Low side switching

to control the power going into the relay.

and rest is relays' job!


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