Oscillator:-
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. A jumper connection is provided so the LED can be turned off. As soon as power is applied to the circuit counting begins. 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 (or 14020.).
parts required:-
Circuitdiagram:-
Ripple Counter:-
The
counter output wanted is set by a jumper. Ten counter outputs are
available: 8/16/32/64/128/256/512/1024/4096 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 of the countdown timer period a pulse
is output on the pin with the jumper on it. The 14020 ripple counter
advances its count on each negative transistion of the clock pulse from
the 555. So for each output cycle of low-high-low-high the count is
advanced by two. It can be set to an zero state (all outputs low) by a
logic high applied to pin 11.
In
this circuit C3, R4 and D1 are arranged as a power-on reset. 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 14020 then counts clock
pulses until the selected counter output goes high. D1 provides a
discharge path for C3 when the power is disconnected.
You
can change the components values of R1 and C1 to set the 555 count
frequency to more than 1.0 Hz. If you change the count to 10 seconds
then a maximum timer delay of 81920 seconds, or 22.7 hours, can be
obtained.
Transistors:-The output from the 4020 goes to a transistor switch arrangement. Two BC547 are connected so that either switching option for the relay is available. A jumper sets the option. 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. Note that the reset pin of the 555 is connected to the collector of Q1. This enables the 555 during the counting as the collector of Q1 is pulled low
