This timer circuit can be used to switch OFF a particular device after around 35 minutes. The circuit can be used to switch OFF devices like radio, TV, fan, pump, etc after a preset time of 35 minutes. Such a circuit can surely save a lot of power.
The timer, based on EXAR’s Type XR2240, is intended for use in photographic and printed-circuit departments, as a standby switch for TV and radio equipment, as a battery charging monitor, and similar applications. The timing can be set between 15 minutes and 24 hours 45 minutes with two BCD (binary coded decimal) switches.
The timing is programmed in steps, T, which are determined by the time constant R1( C5 + C6)= 15 minutes. The BCD switches allow up to 255 such periods to be selected. The timer functions are controlled internal switch bistable (flip-flop) and switch S1 and S2. To keep the period accurate, C5 and C6 must be carefully selected.
Diode D1 lights when the timer is switched on. The voltage at junction D1-R2-R3-R8 is then so low that T1 remains off. When the set time has elapsed, pin 10 of IC2 becomes logic high, whereupon the supply voltage exists at the junction. The LED then goes out. T1 conducts and relay Re] is energized, so that its contact changes over.
Hardware Components
The following components are required to make the Duration Timer Circuit
| S.no | Components | Value | Qty |
|---|---|---|---|
| 1 | DC Supply | 5-12V | 1 |
| 2 | 555 Timer IC | 1 | |
| 3 | Variable Resistor | 1M | 3 |
| 4 | Diodes | 1N4148 | 3 |
| 5 | Switch | 1 | |
| 6 | LED | – | 3 |
| 7 | Resistor | 10K, 470Ω | 3, 3 |
| 8 | Ceramic Capacitor | 0.1µF, 1000pF | 3, 2 |
| 10 | Electrolytic Capacitor | no value show | 3 |
The supply voltage to the timer is stabilized by regulator IC1. Capacitors C1 -C4 smooth the supply voltage to prevent the timer being triggered by pulses on this voltage.
Notes.
- Assemble the circuit on a good quality PCB or common board.
- The circuit can be powered from a 9V PP3 battery or 12V DC power supply.
- The time delay can be varied by varying the values of C1&R1.
- The push button switch S2 is for starting the timer and S1 for stopping the time.
- The appliance can be connected via contacts N1 & N2 of the relay RL1.
- The IC U1 is 2 input quad NAND gate 4011.
