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30 Minute Bedside Lamp Timer Circuit Diagram
30 minutes operation
Blinking LED signals the last six minutes before the device shuts down
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 BC238 25V 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 objective of this circuit is to power a lamp or similar appliance for a specified duration (30 minutes in this instance), followed by automatic deactivation. This is suitable for applications such as nighttime reading, wherein the bedside lamp will automatically switch off when the user falls asleep. Upon activation via pushbutton P1, the LED illuminates for approximately 25 minutes. However, during the final six minutes prior to the shutdown, the LED performs a blinking sequence lasting for two minutes, followed by a two-minute period of inactivity, and then again a two-minute blinking cycle, signaling the imminent termination of the power supply. If an extension of the light duration is desired, the user can press the P1 button again to add an additional half-hour of illumination. The lamp can be deactivated simply by pressing the P2 button.
Circuit operation:
Q1 and Q2 establish an ALL-ON ALL-OFF configuration, consuming minimal current when switched off. Activation of P1 initiates the circuit, engaging the relay and powering both ICs. The lamp receives power through the relay's SPDT switch. IC2 is reset by applying a positive voltage at pin 12. IC2 begins to oscillate at a frequency determined by R4 and C4. After approximately 30 minutes, the signal at pin 3 transitions to a high state, triggering the circuit’s shutdown via C3. During the final six minutes before deactivation, the LED executes a blinking pattern by connecting to pins 1, 2, and 15 of IC2. The blinking frequency is generated by IC2’s internal oscillator at pin 9. To increase the current flowing through the two gates of IC1, a piezo sounder can be connected to pins 1 and 14. The oscillation frequency can be altered by substituting IC2 with a different brand, with Motorola’s ICs typically exhibiting faster speeds. The operating time can be adjusted by modifying the values of C4 and R4.
This circuit was recognized with a publication in ELECTRONICS WORLD "Circuit Ideas", October 1999 issue, page 819.
circuit from http://www.redcircuits.com/