Countdown Timer Circuit with Relay Switching

Parts List:

R1 = 390K R2 = 470K R3 = 1K R4 = 100K R5, R6 = 27K R7 = 4K7 POT = 100K trimpot IC1 = NE/LM555 nmos IC2 = 4020

C1, C3 = 1uF/16V C5 = 470uF/16V C2 = 10nF C4 = 100nF Q1, Q2 = BC547 LED D1, D2, D3 = diode 1N4004 Relay RWH-SH-112D SPDT switch

This is the circuit diagram of countdown timer circuit. The “execution” of the timer output will be conducted by a relay. There will be 2 options:

• the relay can turn ON when power and counting start then turn OFF after the count period, or
• it can do the opposite. The relay will turn ON after the end of the count period and stay on so long as power is supplied to the circuit.

The 555 is configured in the standard astable oscillator circuit designed to give a square wave cycle at a period of around 1 cycle/sec. A potentiometer is included in the design so the period can be set to exactly 1 second by timing the LED flashes

The counter output wanted is set by a jumper. Eleven counter outputs are available: 8 counts, 16 32 64 128 256 512 1024 4096 and 8192 counts. If the 555 is set to oscillate at exactly 1.0Hz by the on-board trimpot then the maximum timer interval which can be set is 8192 seconds (just over 2 hours.) At the end of the counting period a pulse is output on the pin with the jumper on it.