Disclaimer: I am not an electrical engineer. I understand the logic of what I want, but I do not know which components are available to solve my problem. Please be gentle.

This is what I have:

  1. I have a device A that puts ground (-31) on a wire when in rest. I can deactivate and reactivate this externally, so I can use it like an on/off switch.
  2. I have a device B that puts ground (-31) on a wire when it is active and 'nothing' when it is inactive.
  3. I have a relay C that connects when it receives a ground (-31), allowing the mechanism behind it to function, like an on/off switch. The relay also receives a +15 as general power supply to do its magic.

This is what I want: I want to hook up device A and B to the relay C in such a way that:

  1. When device B activates, the relay disconnects forever until device A overrides it by being activated (or deactivated and then reactivated, if need be), regardless of the current state of device A.
  2. If device B deactivates, the relay is still disconnected. If device B activates again, the relay is still disconnected.
  3. When device A deactivates, the relay disconnects until device A reactivates.

Simply put: device A controls the relay and acts as manually operated master override, but device B can interject regardless of the state of device A.

What component(s) do I need between A, B and C to get the behavior I am looking for, and am I making any sense? :)

Timing diagram (edit: now shows system in rest as active as described): Timing Diagram

  • \$\begingroup\$ Might be related to this question, but I honestly can't tell... Sorry. \$\endgroup\$
    – Beijerinc
    Nov 19, 2020 at 15:58
  • 1
    \$\begingroup\$ I tried to read your question. Sorry. A timing diagram is better than many words. See if you can edit the one I've added to your post. Try to cover every condition that you care about. \$\endgroup\$
    – Transistor
    Nov 19, 2020 at 16:08
  • 4
    \$\begingroup\$ What does "ground (-31)" signify? \$\endgroup\$
    – Andy aka
    Nov 19, 2020 at 16:15
  • \$\begingroup\$ When you say "nothing" do you mean disconnected/open, or 0V? \$\endgroup\$
    – Reinderien
    Nov 19, 2020 at 16:18
  • \$\begingroup\$ Ground (-31) is a ground signal like the wire is grounded somewhere, I suppose. Nothing means disconnected/open. Ah I see what you're doing with the timing diagram, I will have a look at it and correct it if necessary. \$\endgroup\$
    – Beijerinc
    Nov 20, 2020 at 7:34

2 Answers 2



simulate this circuit – Schematic created using CircuitLab

Figure 1. A possible solution.

The trick here is to provide a pulse or "one-shot" when A turns on. You then use this to latch relay C and use B to unlatch relay C. Now due to the pulse action A remaining on will not re-enable C.

  • When A turns on the DELAY relay does not energise immediately due to the R1 C2 time delay.
  • Current flows through A (NO), DELAY (NC) and RLY-B (NC) and relay C is energised. RLY-C then latches itself on and bypasses the DELAY contact.
  • The DELAY relay energises and opens its NC contact but RLY-C is providing the latch.
  • C remains on until RLY-B is energised. C then drops out. The latch contact opens and even if B is turned off the C will remain off until the next OFF-ON transition of A.

Be aware that C2 will prevent C being retriggered if A is cycled too quickly.

  • \$\begingroup\$ Thanks for your answer! I'm sure it will work but there are too many components for me to use it practically. I will have to think of something else... \$\endgroup\$
    – Beijerinc
    Nov 25, 2020 at 9:32

Here's the timing diagram.

enter image description here

And the relay logic based circuit.

enter image description here

enter image description here

A, B and K1 to K6 are standard relays.

C is a magnetic latch relay.

Freewheeling diodes are not shown.

  • \$\begingroup\$ The timing diagram looks wrong. When A turns the relay into the on position and B triggers, it shouldn't 'flip' the relay back. It should stay active regardless of B's signal. And relay C should be triggered when device B is activated, even if A isn't active. \$\endgroup\$
    – Beijerinc
    Nov 20, 2020 at 8:06
  • \$\begingroup\$ According to your description, Relay C is to turn on when Device A is activated and turn off when Device A is deactivated. Relay C is to turn off even when Device B is activated. However your timing diagram contradicts your description by showing Relay C turning on when Device B is activated. \$\endgroup\$
    – vu2nan
    Nov 20, 2020 at 9:41
  • \$\begingroup\$ I'm getting a bit confused (by myself), because the sheer state of device A being hooked up to the system already 'activates' relay C. If device A is detached, relay C closes. So it's like one half is inverted. I will fix the time diagram. \$\endgroup\$
    – Beijerinc
    Nov 20, 2020 at 10:01
  • \$\begingroup\$ I fixed the time diagram, sorry for my confusion. Would you mind having another look? \$\endgroup\$
    – Beijerinc
    Nov 20, 2020 at 15:59
  • \$\begingroup\$ It tallies with your description one hundred percent and so does my logic! \$\endgroup\$
    – vu2nan
    Nov 20, 2020 at 16:57

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