Twilight switch automatically switches on and off the lighting of your home, garden or other place. When it gets dark, the device switches on the output relay. The twilight switch is equipped with circuitry that eliminates the possibility of oscillation if the illumination level is at the operating threshold.
• Photo resistor as a light sensor.
• Adjustable sensitivity.
• Hysteresis eliminates oscillations when the lighting is at the operating threshold.
• Active status indication with LED.
• Output relay with contacts 230V AC / 8A.
• Power supply: 12-15V DC.
• PCB dimension: 37×57mm.
• Electronic Kit – soldering required.
Schematic of the twilight switch is shown in Figure 1. It is based on the TL071 operating amplifier working as a comparator. The TL071 compares the voltage from the potentiometer PR1 to the voltage from a resistive divider made up of a resistor R3 and a photoresistor. The decrease in the intensity of light incident on the photoresistor causes an increase in the voltage on the pin 2 of the operational amplifier U1. If the voltage on the pin 2 will be higher than the voltage on the pin 3, the amplifier output voltage appears, causing conduction of the transistor T1 and switching on the relay RL1. The PR1 potentiometer allows for adjusting the sensitivity threshold over a wide range. The C3 capacitor eliminates the possibility of oscillation if the illumination level is at the operating threshold. Relay contacts are switched on at dusk and off at dawn. Using the changeover contacts can reverse the operation of the device. The high load capacity of the relay contacts enables the switch to be used in a variety of applications. The switch can be powered by 12V DC from any AC adapter or a battery.
Assembly and test
The assembly is typical and should not cause the problems. It runs in standard way starting from the smallest components and ending with the largest ones.
The device assembled from the tested components does not require any adjustment and operates immediately after the power is turned on. The twilight switch will not only be used for lighting control but also in photography, modeling and other applications. In the active mode twilight switch consumes a current of approximately 100mA.
PR1: Trimmer potentiometer 100kΩ
D1: 1N4007 or similar
D2, D3: 1N4148 or similar
D4: LED diode
X1, X3: 2-pin terminal block connector
X2: 3-pin terminal block connector