5 to 30 Minute Timer
This 5 to 30 minute timer circuit achieves times ranging from 5-30 minutes using the 7555 integrated circuit (CMOS version of the popular 555 timer).
How the 5 to 30 Minute Timer works?
The 7555 IC works as monostable multivibrator, delivering at its output, a positive pulse for a time set by the values of R and C.
The pulse time is given by the formula: 1.1 x RC. Where R is the series combination of the 8.2M resistors (see diagram) and C is a capacitor of 33uF. Obtainable times are: 5, 10, 15, 20, 25 and 30 minutes, changing the switch position.
The 7555 timer output (pin 3) is connected to the 2N2222 NPN transistor through the 4.7K resistor. When the output of 7555 goes high, the transistor passes to the saturation state and activates the relay. The relay can control anything, for example, a radio, a lamp, a small alarm, etc.. The type of device that the relay can control, depends on the current the relay can withstand.
It is important to use the 7555 timer and not the typical 555 IC, so you can work smoothly with the 8.2M resistors. The 33uF capacitor should have little leakage. The 1N4001 diode is used to protect the transistor 2N2222 when the relay is deactivated.
Bear in mind that the times of this timer are not accurate due to possible variations in the values of the elements used (resistors, capacitors)
- The power supply (B) to the circuit can have a value between 5 and 15 volts.
- The relay voltage must be the same voltage as the source.
Original circuit source by Andy Collinson