0
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\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
    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
    Jan 14, 2021 at 0:00

2 Answers 2

0
\$\begingroup\$

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.

\$\endgroup\$
0
\$\begingroup\$

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!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.