Control relay with two devices, where one can override the other

Question

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):

Answer 1

^{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.

Answer 2

Here's the timing diagram.

And the relay logic based circuit.

A, B and K1 to K6 are standard relays.

C is a magnetic latch relay.

Freewheeling diodes are not shown.