Light control switch circuit

Towns around the world are considering and installing LED street lights to help save energy, reduce costs, protect the environment, and improve lighting conditions for citizens. Despite this trend, little attention has been paid to the issue of controlling the on and off times of these lights.

Proper control can achieve significant energy savings because lights may be switched on too late, too early, wasting energy, or providing insufficient light. Using a twilight switch can significantly reduce energy consumption in all types of light fixtures (Figure 1). It provides a cost-effective, compact and reliable way to control lighting schedules.

This circuit does not use relays. Therefore, it has no moving parts and there is no contact wear. It uses a TRIAC (three-terminal alternating current) and can switch hundreds of watts of power.

The circuit consumes almost no power. It uses a charge pump to power the circuit from the AC mains, drawing less than 37mW on a 220V rms AC line. It uses only a few low-cost components.

Using potentiometer R1 on the circuit board, you can adjust the brightness level of the circuit and switch the light on and off. The circuit will automatically turn on the light at sunset and turn off the light at sunrise. The circuit can be used for incandescent lamps, fluorescent lamps, or LED lamps.

The circuit uses an LDR (light-dependent resistor) to measure the ambient light level (Figure 2). Note that the LDR spectral response should be chosen to be close to that of the human eye for good performance. It uses a hysteresis comparator because the basic comparator structure can produce oscillations, or output instability when the light level is close to the boundary between natural light and darkness. Hysteresis creates two switching thresholds in the circuit: an upper threshold voltage of 8.47V for a rising input voltage change from natural light to darkness, and a lower threshold voltage of 7.75V for a falling input voltage change from darkness to natural light. The relationship between the 82kΩ and 4.7kΩ resistors controls the hysteresis voltage of 0.72V. This value is sufficient to avoid false triggering due to light noise.

When the ambient light is lower than the level set by R1, the input voltage VI rises to a value greater than the upper threshold voltage, and the comparator output decreases, turning the TRIAC on. When the ambient light rises above the level set by R1, the input voltage drops below the lower threshold voltage, and the comparator output increases, turning the TRIAC off.

A mechanical insulation must be provided between the LDR and the light to prevent a feedback path to the LDR. Otherwise, the light will cause an oscillation at the output of the comparator, causing the state of the lamp to oscillate. The BTA16-600SW is available from many sources and can be used to switch lamps over 2000W.

The comparator drives a Vishay IL4216 or BRT12-F optocoupler with TRIAC output (Figure 3). The optocoupler then drives the BTA16-600SW TRIAC to control the lamp.