Circuit diagram:
Parts:
R1________1K 1/4W Resistor
R2________4K7 1/4W Resistor
R3_______10M 1/4W Resistor
R4________1M 1/4W Resistor
R5_______10K 1/4W Resistor
C1_______470µF 25V Electrolytic Capacitor
C2-C4____100nF 63V Polyester Capacitors
D1-D4____1N4002 100V 1A Diodes
D5_______5mm. Red LED
IC1______4012 Dual 4 input NAND gate IC
IC2______4060 14 stage ripple counter and oscillator IC
Q1_______BC328 25V 800mA PNP Transistor
Q2_______BC547 45V 100mA NPN Transistor
P1,P2____SPST Pushbuttons
T1_______220V Primary, 9 + 9V Secondary 1VA Mains transformer
RL1______10.5V 470 Ohm Relay with SPDT 2A 220V switch
PL1______Male Mains plug
SK1______Female Mains socket
Device purpose:
The purpose of this circuit is to power a lamp or other appliance for a given time (30 minutes in this case), and then to turn it off. It is useful when reading at bed by night, turning off the bedside lamp automatically in case the reader falls asleep...
After turn-on by P1 pushbutton, the LED illuminates for around 25 minutes, but then it starts to blink for two minutes, stops blinking for two minutes and blinks for another two just before switching the lamp off, thus signaling that the on-time is ending. If the user want to prolong the reading, he/she can earn another half-hour of light by pushing on P1. Turning-off the lamp at user's ease is obtained by pushing on P2.
Circuit operation:
Q1 and Q2 form an ALL-ON ALL-OFF circuit that in the off state draws no significant current. P1 starts the circuit, the relay is turned on and the two ICs are powered. The lamp is powered by the relay switch, and IC2 is reset with a positive voltage at pin 12. IC2 starts oscillating at a frequency set by R4 and C4. With the values shown, pin 3 goes high after around 30 minutes, turning off the circuit via C3. During the c6 minutes preceding turn-off, the LED does a blinking action by connections of IC1 to pins 1, 2 & 15 of IC2. Blinking frequency is provided by IC2 oscillator at pin 9. The two gates of IC1 are wired in parallel to source more current. If required, a piezo sounder can be connected to pins 1 & 14 of IC1. Obviously, timings can be varied changing C4 and/or R4 values.
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