Delay circuit with NE555 timer

This circuit design was used to switch on device via a LED photocell arrangement (optocoupler) using components R1, C1, D1 and Q1. It produces a delay on powering up to ensure correct sequencing of certain equipment. A very simple delay timer using a single transistor and an R-C timing circuit.

C1 charges toward the power supply voltage (12 volts) through R1. When the voltage reaches 9.1 volts, zener diode D1 conducts, forward biasing the transistor Q1, turning it on. At about 9.8 volts, the zener diode and base of Q1 conduct heavily, taking the charging current from C1. When power is turned off, the circuit is reset by C1 discharging through R1 into the now zero voltage supply. The time can be varied by changing R1, C1.

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