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 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

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