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Momentary Switch Teamed With Latching Relay

This circuit allows an SPST momentary pushbutton to act as a push-on push-off switch, using a DPDT latching (bi-stable) relay. It was originally intended to allow a single pushbutton switch on the dash of a vintage car to provide a latched function. The relay only draws current when it is being switched. At other times, the only current drain on the 12V supply is the leakage current of one 22µF capacitor, which is very low. It works as follows.

Assume that initially the latching relay is in the reset state, with pins 4 and 6 connected together. In this state, C2 charges up to +12V via 2.2kO resistor R2 while capacitor C1 remains discharged as it is not connected to the 12V supply. If S1 is pressed, C2 discharges via the relay’s “set” coil, diode D2 and S1. This switches the relay into its set position, connecting pins 4 and 8. C1 then begins to charge via R1. While S1 is being held down, the relay does not return to the reset position because the current supplied via R1 is insufficient for the coil to latch the armature. As soon as S1 is released, current no longer flows though the coil so C1 can finish charging, ready for the next button press.

Momentary Switch Circuit Diagram Teamed With Latching Relay

Momentary Switch Circuit Diagram Teamed With Latching Relay

Once the relay has switched and C1 has finished charging, pressing S1 again causes the relay to switch back to the reset state via the same process. The unused set of relay contacts can be used as an SPST or SPDT switch. The circuit as shown has been tested with the Jaycar SY4060 relay. It will work with other DPDT twin-coil latching relays but the resistor and capacitor values may need to be adjusted to suit. Relays with lower resistance coils will need larger value capacitors and smaller value resistors.

Author: Merv Thomas – Copyright: Silicon Chip Electronics Magazine

Tags: relay, switch gear, latch,

Comments on this post:

Chris Morgan
By Chris Morgan (0) 0Score: 

1 year ago:  Good Morning Merv,

We need to use a latching relay in a job we are doing and it is giving us some trouble.

We are using Omron relay MK2KP-UA-DC24. This is a latching relay with 24VDC coil. I have a 2200uF capacitor and a 10 OHM resistor on both sides, using your setup above.

On the bench, the Set and Reset coils both worked fine. We have installed the circuit in the machine and the Set side work just as before. However, we have been unable to get the Reset side to work at all.

I have learned that the Rest coil has a much higher resistance than the Set coil, so I may need to adjust the capacitor/resistor values. I may have just lucked into values that work for the Set side.

I know that T=C*R. Could you tell me please, how you calculated your resistor/capacitor values?

Thank you for your time,
Chris Morgan

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