The figure below shows a very useful project of a 555 adjustable repeat timer circuit. Previously we have presented some 555 timer circuits on this website which can switch on a relay for a preset time period and switch it off when that preset timer period is completed, and for again starting these
timers you have to press a switch.
The circuit given here will automatically starts itself when the preset time period is completed. It has been done by using two 1N4148 diodes D1 and D2. You can also adjust the ON and OFF time periods with the help of 5M variable resistors.
The LED1 and LED2 are used to indicate the ON and OFF conditions of the relay or the output of 555 IC. The relay is derived through a 2N4401 transistor. A 1N4007 diode across the relay is used to block the back EMF coming from the relay. The circuit can be operated from 4.5V to 12V DC. The relay should be used according to the operating voltage. For example if you are operating the circuit with 6 volt then use a 6 volt relay and if your are operating it with 12V then use a 12V relay.