Clock & Timer Circuit Diagrams

Variable Time Switch Schematic Circuit Diagram

A Switch with Selectable Time Ranges

This switch is equipped with two selectable time ranges, altered via a push-button. In the prototype, these time intervals were set to 5 minutes and 20 minutes, but customization is straightforward. A unique feature is the ability to turn off the load, such as a lamp, by pressing the switch three times consecutively. When S1 is pressed, IC3 is activated via D1 and NAND gate IC2b. Simultaneously, capacitors C3 and C4 begin to charge. Upon releasing the switch, IC3 receives a clock pulse from IC2a through R2. Consequently, its QA output goes high, enabling relay T2. The relay contact closes, and the lamp is powered.

Automatic Switch-Off Mechanism

Upon actuation, the QA output activates transistor T1, effectively short-circuiting R4. This action initiates the discharge of capacitors C4 through R3. After approximately 5 minutes, the potential across C4 drops to the lower trigger level of IC2b, leading to the reset of IC3. Consequently, the relay disengages, turning off the lamp.

Selective Time Control Activation

Upon two consecutive presses of S1, the counter receives two clock pulses, causing its QB output to go high. Transistor T1 remains inactive, allowing C4 to discharge through R3 and R4, a process taking approximately 20 minutes. To indicate the selection of the longer duration, the lamp briefly goes out immediately after the second press of S1. This effect is achieved by 1C2d, which generates a pulse through the differentiating network R9-C5 when the QB of IC3 becomes high.

Instantaneous Load Deactivation

With three successive presses of S1, both QA and QB of IC3 become high, causing the output of IC2b to go low, leading to the rapid discharge of C4. This results in an instantaneous switch-off of the lamp.

Permanent Control with Rotary Switch

The rotary switch S2 offers the option to either permanently activate or deactivate the load. In its central position, the relay is controlled by the timing circuit, ensuring precise and selective control of the load.


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