# Bedside Light Timer Schematic Circuit Diagram

#### Tackling Bedtime Lighting Challenges: Introducing the Timer Solution

For parents of young children, managing bedtime lighting can be quite a challenge. Many kids prefer having the bedside light on even after the storybook is closed, and later they might fall asleep with the light still on, wasting energy. This situation often leaves parents dealing with the task of switching off the light without disturbing the sleeping child. The timer design presented here offers a practical and elegant solution to this common domestic issue. Utilizing a handful of affordable components, this timer allows parents to determine the duration the bedside light remains on after bidding goodnight and activating the timer.

#### Timer Activation and Light Control: Functional Overview

Upon pressing switch S1, bistable ICib toggles, generating a debounced clock pulse at the input of the second bistable, ICia. This action leads to the high output of Q, triggering a low-power thyristor, Th. The complementary bistable output, Q, goes low, enabling timer IC2. The connected load, a small bulb with a maximum rating of 60 W, is switched on and continues to stay lit until counter IC2 resets ICia. The counter, specifically a Type CD4541, features an onboard oscillator operating at a frequency, f, calculated in Hz using the formula f = 1/(2.3 × Rtc × Ctc), where RTC and CTC denote the resistor and capacitor linked to pins 1 and 2, respectively.

#### Determining Delay Time: Calculating Delay in Seconds

To set the delay time in this circuit, the resistor connected to the Rs input at pin 3 should have a value of approximately 2R-rc. The 4541’s scale factor is fixed at 65,536 (2^16), achieved by connecting its A and B control pins to the positive supply rail. Consequently, the OUT pin changes state after 32,768 clock pulses. By defining logic levels at pins 5, 10, and 9, a logic low level at the OUT pin is selected when the RESET pin is logic high. The delay, denoted as ‘t’ in seconds, can be calculated using the formula t = 2.3 × 32,768 × R5 × C5.

#### Power Supply and Circuit Components: Safety Considerations

The circuit is directly powered by the mains, with transistor T1 functioning as a 10-V zener diode. An LED, D2, serves as an orientation aid for the child and is integrated into the digital push-button, S1. It’s essential to note that the actual supply voltage of the circuit can vary between 6 V and 12 V, depending on T1’s characteristics. While the specific voltage value is relatively unimportant, it is crucial that ICia can supply a trigger current of approximately 200 μA to the thyristor.

#### Construction and Safety Precautions: Assembling the Circuit Securely

Construct the circuit on the provided printed-circuit board (refer to Fig. 2) and enclose it in a suitable ABS enclosure. Prioritize safety by ensuring proper insulation and securing the input and output cables with strain reliefs. To minimize risks, design the enclosure’s top panel with a minimal clearance for the key top, preventing accidental contact with the circuit.

#### Warning: Safety Precautions for Handling Dangerous Voltages

Due to the presence of dangerous voltages at multiple points within the circuit, rigorous electrical insulation is paramount. Avoid working on the circuit while it is connected to the mains. Exercise extreme caution during setup, adjustment, and usage, ensuring that no part of the circuit can be touched to prevent potential hazards.

#### PARTS LIST

##### Resistors:
• R1 = 180 kΩ, 0.5 W
• R2 = 220 kΩ
• R3, R4 = 470 kΩ
• R5 = 180 kΩ
• R6 = 390 kΩ
##### Capacitors:
• C 1 = 22 μF, 16 V
• C2 = 100 nF
• C3, C4 = 1 nF
• C5 = 47 nF
##### Semiconductors:
• D1 = 1N4007
• D2 = LED, high-efficiency (see S1)
• D3 = 1N4148
• B1 = B380C1500
• T1 = BC547A
• Th1 = BRX49
• IC1 = 4027
• IC2 = 4541
##### Miscellaneous:
• S1 = digitast push-button switch with integral LED
• K1, K2 = PCB-mount terminal block, pin spacing 10 mm, ABS enclosure about 100 γ150 γ125 mm

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