Description
Parts:
Circuit operation:
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 diagram: - R1 = 1K
- R2 = 4K7
- R3 = 10M
- R4 = 1M
- R5 = 10K
- C1 = 470µF-25V
- C2-C4100nF-63V
- C1 = 470µF-25V
- C2-C4 = 100nF-63V
- D1-D4 = 1N4002
- D5 = 5mm. Red LED
- IC1 = CD4012
- IC2 = CD4060
- Q1 = BC328
- Q2 = BC547
- P1,P2 = SPST Pushbuttons
- T1 = 9+9 Volt Secondary 1VA Mains transformer
- RL1 = 10.5V 470 Ohm Relay with SPDT 2A 220V switch
- PL1 = Male Mains plug
- SK1 = Female Mains socket
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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