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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Momentary switch teamed
with latching relay

Click for larger image

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 2.2kΩ 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.

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.

Merv Thomas,
Mutarnee, Qld.

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