Momentary Switch Teamed With Latching Relay

This circuit enables an SPST momentary pushbutton to function as a push-on push-off switch by utilizing a DPDT latching (bi-stable) relay. It was designed to allow a single pushbutton switch on the dashboard of a vintage car to provide a latched function. The relay draws current only during switching, minimizing power consumption. The only current drain on the 12V supply during idle is the leakage current from a 22 µF capacitor, which is very low. The operation is as follows: Initially, the latching relay is in the reset state, with pins 4 and 6 connected. In this state, capacitor C2 charges to +12V through a 2.2 kΩ resistor R2, while capacitor C1 remains discharged as it is not connected to the 12V supply. When the pushbutton S1 is pressed, C2 discharges through the relay's set coil, diode D2, and S1. This action switches the relay into its set position, connecting pins 4 and 8. Capacitor C1 then begins to charge through resistor R1. While S1 is held down, the relay does not revert to the reset position because the current supplied through R1 is insufficient to latch the armature. Upon releasing S1, current ceases to flow through the coil, allowing C1 to complete its charging cycle, preparing for the next button press. Once the relay has switched and C1 is fully charged, pressing S1 again causes the relay to return to the reset state through the same process. The unused set of relay contacts can be configured as an SPST or SPDT switch. The circuit has been tested with the Jaycar SY4060 relay but is compatible with other DPDT twin-coil latching relays, although adjustments to resistor and capacitor values may be necessary. Relays with lower resistance coils will require larger capacitors and smaller resistors.

The circuit operates with a straightforward mechanism that allows for efficient control of the relay using minimal components. The SPST momentary pushbutton serves as the primary input, initiating the switching action of the relay. The DPDT relay is crucial for achieving the latching functionality, maintaining the last state until the pushbutton is pressed again. The use of capacitors C1 and C2 is essential for timing and energy storage, with C2 facilitating the initial switching and C1 ensuring the relay remains in the desired state until the next activation.

Resistor R1 and R2 play pivotal roles in controlling the charging rates of capacitors C1 and C2, respectively. The values of these resistors can be fine-tuned to optimize the circuit's response time and power consumption characteristics. The diode D2 is included to prevent back EMF generated by the relay coil from damaging other components in the circuit, ensuring reliable operation over time.

This design is particularly suited for applications where a momentary switch is preferred for toggling power to devices or systems, such as in automotive or industrial settings. The ability to adapt the circuit for different relay specifications allows for versatility in implementation, accommodating various power requirements and operational conditions. Overall, this circuit exemplifies an efficient and effective solution for push-on push-off switching applications.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 t he 12V supply is the leakage current of one 22 µF 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. 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. 🔗 External reference