A very simple cont down timer electronic project circuit diagram can be designed using a 555 timer circuit and some other common electronic parts .
This count down timer electronic project circuit will activate a relay on-to-off or off-to-on (mode selected by a jumper) as soon as the counting period is over .
The circuit consists of 3 parts: an oscillator, a ripple counter and two switching transistors.
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.
The output pulse from pin 3 of the 555 is fed to a the clock input pin 10 of the 14-stage binary ripple counter, the 4020 .
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.)
The 4020 ripple counter advances its count on each negative transition of the clock pulse from the 555.
When power is applied to the circuit C3 is in a discharged state so pin 11 will be pulled high. C3 will quickly charge via R4 and the level at pin 11 falls thus enabling the counter. The 4020 then counts clock pulses until the selected counter output goes high. D1 provides a discharge path for C3 when the power is disconnected.
As soon as power is applied to the circuit counting begins and the output from the 4020 goes to a transistor switch arrangement.
This electronic project circuit must be powered from a simple 12 volt DC Power supply circuit.