Automatic Light Controller with 7806 – Build Your Own!

Description

Voltage regulator ICs (78xx series) ensure a stable output voltage irrespective of fluctuations in the input supply, when a common terminal is connected to ground. Any voltage around zero volts connected to the common terminal is added to the output. Consequently, an increase in the common terminal voltage is reflected at the output. Conversely, if the common terminal is disconnected from ground, the full input voltage is available at the output. This characteristic is utilized in the present circuit.

When the common terminal is connected to ground, the regulator’s output is equivalent to the rated voltage, and immediately upon disconnection from ground, the output increases up to the input voltage. The common terminal is controlled by a transistor, which functions as a switch at that terminal. For automated control of light, a light-dependent resistor (LDR1) is connected to the base of the transistor. In this manner, the voltage regulator is able to operate a light bulb automatically based on ambient light conditions.

To obtain the power supply for the circuit, 50Hz, 230V AC mains is stepped down by transformer X1 to produce a secondary output of 12V, 250 mA. The secondary output of the transformer is applied to a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1, and fed to the input terminal of the regulator (IC1). The common terminal (pin 2) of IC1 is connected to the ground line of the circuit through transistor BC557 (T1). The transistor is biased by R2, R3, and VR1, along with LDR1.

Circuit diagram:

The grounding of IC1 is controlled by transistor T1, while light is sensed by LDR1. Using potentiometer VR1, it is possible to adjust the light-sensing level of transistor T1. The output of IC1 is fed to the base of transistor T2 (via resistor R4 and zener diode ZD1) and relay RL1. LED1, connected across the positive and ground supply lines, functions as a power-‘on’ indicator. During daylight, the resistance of LDR1 is low and during nighttime it is high. During daylight, when light falls on LDR1, pnp transistor T1 conducts.

The common terminal of IC1 connects to ground, and IC1 outputs 6V. Consequently, transistor T2 does not conduct, and the relay remains de-energized. The light bulb remains ‘off’ because the mains connection is not completed through the relay contacts. During nighttime, when no light falls on LDR1, it offers a high resistance at the base junction of transistor T1. Therefore, the bias is significantly reduced, and T1 doesn’t conduct. Effectively, this removes the common terminal of IC1 from ground and it directs the full input DC to the output. Transistor T2 conducts, and the relay energises to light up the bulb as the mains connection completes through the relay contacts.

As LDR1 is in parallel to VR1+R3 combination, it effectively applies only half of the total resistance of the network formed by R3, VR1, and LDR1 to the junction at T1 in total darkness. In bright light, it significantly reduces the total effective resistance at the junction. The circuit is simple and can be assembled on a small general-purpose PCB. Use a heat-sink for IC1. Ensure that LDR1 and the light bulb are well separated. This circuit can be used for streetlights, tubelights, or any other home electrical lighting system that needs to be automated.