Clock & Timer Circuit Diagrams


A Turn-Off Type Time Relay with Schmitt Trigger Connection is a specific type of time-delay relay that operates based on a Schmitt trigger circuit. A Schmitt trigger is an electronic circuit that converts a varying input signal into a digital output signal. It is commonly used in applications where a stable digital output is required from an input signal with varying voltage levels.

Here’s a description of the components and their functions in the schematic circuit diagram of a Turn-Off Type Time Relay with Schmitt Trigger Connection:

Input Signal:

  • The input signal to the Schmitt trigger circuit. This signal can come from various sources, such as a sensor or a switch.

Schmitt Trigger Circuit:

  • The Schmitt trigger circuit processes the input signal and provides a stable digital output. It consists of components like resistors, capacitors, and operational amplifiers configured in a way that the output switches from high to low (or vice versa) at specific voltage thresholds, providing hysteresis and noise immunity.

Time Delay Circuit:

  • The time delay circuit determines the delay period before the relay is turned off. It typically includes resistors and capacitors configured as an RC network or a timer IC like the 555 timer. The values of these components determine the delay time.


  • The relay is an electromagnetic switch that opens or closes the circuit contacts based on the presence or absence of the magnetic field generated by the relay coil. When the coil is energized, the relay switches off, breaking the circuit.

Power Supply:

  • Provides the necessary power to the entire circuit, including the Schmitt trigger circuit, time delay circuit, and the relay coil.


The Schmitt-triggered arrangement of the transistors can be summarized as follows: If the voltage supplied to the relays doesn’t reach zero or its maximum value, the relay contacts might vibrate due to insufficient magnetic force from the relay coil. This continuous vibration is highly undesirable as it accelerates contact wear and tear, leading to rapid deterioration. To prevent this, transistors are set up in a triggered manner to minimize relay contact oscillation.

The Schmitt-triggered relay circuit functions as follows: Initially, T1 is off when capacitor C is uncharged. Consequently, the voltage at the collector of T1 (point A) remains at the maximum level corresponding to the chassis. Consequently, T2 immediately conducts, activating the relay and associated components. Capacitor C starts charging through resistors R1 and the potentiometer, eventually triggering T1. As the voltage at the collector of T1 (point A) decreases, the voltage across resistor R3 (point B), connected to the emitter, rises. This swift voltage change prompts transistor T2 to rapidly switch off due to two distinct electrical effects.


  1. The drop in voltage at the collector of T1 causes T2 to switch off.
  2. The voltage generated across resistor R5, connected to the emitters of T1 and T2, works to decrease the base current of T2.

Pressing button B in the circuit discharges capacitor C, instantly turning off T1. This increase in voltage at the collector (point A) of T1 leads to the conduction of T2. Consequently, the Schmitt triggering mechanism enables swift transmission or interception by the relay.


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