A pulse-operated relay operates by being either set (ON) or reset (OFF) through the application of a pulse voltage. Once triggered, this relay maintains its established state even if the input voltage is interrupted. It only reverts to its initial state upon receiving the next inverting input. This type of relay is commonly referred to as a keep relay due to its ability to sustain its set or reset condition.
Modifying a Standard Relay with Basic Components
With the incorporation of a few essential components, a standard relay can be engineered to change its state whenever a pulse is applied or a switch is pressed. In the inactive state, transistor T1 remains off because its base and collector share the same potential through R2. During this period, capacitor C1 charges through R4, R3, and the relay coil. When a brief pulse, signifying a logic zero and lasting less than 0.5 seconds, is introduced through the input or when S1 is pressed, T1 conducts. As a result, the relay is energized, leading to a change in the relay contact’s state. Simultaneously, T2 provides current to the relay coil. Capacitor C1 discharges until its potential matches the base-emitter voltage of T2.
Maintaining Relay Energization After Pulse Termination
When the pulse ends or the switch is released, T1 returns to its off state. However, T2 ensures that the relay remains energized even after T1 is off.
Setting the Duration for Relay Deactivation
To deactivate the relay, a slightly longer input pulse is necessary. This extended pulse duration allows C1 to discharge almost completely via R3. Once the switch is released or the input returns high, T1 ceases conduction. Concurrently, C1 starts charging again. Consequently, the base current of T2 diminishes rapidly, approaching zero. T2 turns off, leading to the de-energization of the relay. Short input pulses have no impact when the relay is already actuated.
Customizing Pulse Length for Relay Activation
When relatively long input pulses are applied while the relay is inactive, the circuit operates as a standard relay. It promptly activates upon receiving the pulse and deactivates as soon as the pulse ends. It is essential for the relay’s hold voltage not to exceed half the supply voltage. A lower hold voltage permits a shorter input pulse to activate the relay efficiently.