Long Delay Timer Circuit: 35 Minute Battery-Powered Design

Suitable for battery-powered devices, Fixed 35 minutes delay

This timer was primarily designed to deactivate a portable radio after a specified duration: this allows individuals to sleep comfortably on the beach or in a hammock, confident that the receiver will automatically switch off after a set time, thereby conserving battery resources.

Circuit operation:

R1 and C1 establish a remarkably lengthy time constant. When P2 is momentarily closed, the near-zero voltage at the positive lead of C1 is applied to the high-impedance inputs of the four gates within IC1, which is configured in parallel. The four parallel-connected gate outputs from IC1 then transition to a high state, making the battery voltage accessible at the Emitter of Q1.

When P2 is released, C1 commences to charge gradually through R1, and when the voltage at its positive lead reaches approximately half of the battery voltage, the IC1 gate outputs drop to zero, halting Q1.

This transistor can directly power a portable radio receiver or various other devices requiring a current of up to roughly 250mA. Connecting a Relay across the Emitter of Q1 and negative ground enables the operation of devices needing significantly higher voltage and current levels via its contacts.

Depressing P2 for 1 to 5 seconds initiates the circuit, after which it will deactivate automatically after about 35 minutes. This delay period can be adjusted by modifying the values of R1 and/or C1. P1 serves to interrupt the timer’s operation if needed.

LED D1 is optional and can be useful for signaling relay activity when the load is positioned remotely from the timer.

Circuit diagram:

Parts:

Notes: