Often we require a variable timer to set On/Off the equipment after a prescribed time period. Sometimes, the ‘start’ signal may be issued from other circuit also. The circuit presented here is a simple and cost effective timer which can switch 'on' a relay for a preset time. The N/O or N/C pins of relay can be used to switch On or Off the electrical/electronic equipment/gadget.

ATtiny13A is a powerful Microcontroller from Atmel having all features of standard MCU, yet it occupies less space (8 pins only) available at unbelievable lowest price. Due to its compactness, it makes the circuit simpler.

The circuit consists of very simple controls, i.e., one variable resistance (VR1) to set the time delay, one push button switch (SW_START) to start the timer and two miniature switches, one for selecting time for delay in minutes or seconds (SW_MIN_SEC) and the other one is for selecting the starting of timer by rising edge or trailing edge (SW_RISE_TRAIL).

Once, the timer is started by pressing or releasing (depends on SW_RISE_TRAILS switch status) the SW_SWITCH, the 5V Relay will be ON and holds the relay coil for the time set by VR1.

The process indicator LED indicates the process in four ways.

1)In seconds mode set by SW_MIN_SEC switch (connected to Ground) and timer is off mode, then it blinks quickly with about 20% duty cycle.

2)In minutes mode set by SW_MIN_SEC switch (open) and timer is off mode, then it blinks quickly with about 80% duty cycle.

3)In seconds mode set by SW_MIN_SEC switch (connected to Ground) and timer is ON mode, then it blinks seconds pulse with about 20% duty cycle.

4)In minutes mode set by SW_MIN_SEC switch (open) and timer is ON mode, then it blinks seconds pulse with about 80% duty cycle.

To set the timing, VR1 is turned to set from 0 to 100 appoximately. If VR1 is turned to minimum side, then the timer value is 0 and if VR1 is turned to maximum side, the timer value is 100. The value may be Minutes or Seconds depending on the SW_MIN_SEC switch.

In case, the timer is to be controlled by another circuit, the positive (or high) signal can be injected at SIGNAL INPUT across the 20K resistance.

The program (AnalogTimer.HEX) can be burned using any AVR burner, using/matching the six pins of ISP PORT as shown in the circuit.

A DC power supply as shown below can be used to power the circuit.