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working with a bistabel relais


New Member
Bistable (Latching) Relay

If I read your circuit correctly you have a normal relay with a contact that 'makes' when the relay is energised (a normally open contact).
A pushbutton (your makeswitch - a push to test or push to make button) will energise the relay, the normally open contact of the relay is wired in parallel with this pushbutton and provides a 'sustaining' or 'latch' action, maintaining the power to the relay coil after the pushbutton is released.

The only way to cause the relay to drop out is to kill the power to it somewhere in the circuit; where this 'kill' is depends on your priorities :-

Option 1 is to place a normally closed pushbutton in series with the relay's 'latching' contact - this will allow the relay to drop out UNLESS the other button is still pressed forcing the relay to stay energised. This gives the 'ON' button priority.

Option 2 is to place a normally closed pushbutton in the positive power rail before the push to test and relay's 'latching' contact thus killing the whole circuit regardless of the push to test button's state. This gives the 'OFF' button priority.

If the relay has another contact available (normally open) you could consider using this to control the lamp (wired independantly of all other switches - lamp and contact directly across the supply). This reduces the load on your pushbuttons as they then only switch the relay.


New Member
bistabel/monostabel relais

Is there no different in bistabel and monostabel relais?
I thought the makeswitch is pulsing the relais on
and off.
And so using one normaly closed pushbutton less.


New Member
Bistable & Monostable

Bistable means "two stable states", energised and not energised. It will be stable in either state, the input signal swapping its state between the two.
I guess a normal light switch is a simple example of a bistable device.

Monostable means "one stable state", a signal to it will cause its output to change state for a period of time and then revert back to its stable state awaiting the next input 'pulse'.
A 'push-to-test' button will do as an example - its output only maintains its 'unstable' state while the input (your finger) is applied; a monostable usually has a timing function such that its output will have a minimum 'set' time before reverting to its stable state.

Proper bistable relays can be bought, they have mechanical latching and a small solenoid coil to unlatch them. The two coils (set and reset) have seperate terminals to allow individual control of the two coils. They used to be used in telephone exchange systems (aah the good old days!).

I have never seen a circuit with a single pushbutton to 'toggle' a latching relay between states, though it must be possible - you basically need to construct a 'D-Type Flip-Flop' in relay logic, this could be complicated, maybe using five or more relays. It could be simpler to use electronics (a D-type flip-flop is a single IC).

Another idea (too stupid ?) is to use a latching pushbutton as used on some anglepoise lamps?, first press switches the lamp on, another press switches it off ???

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