Cities and towns worldwide are considering and installing LED streetlights to help save electric energy, reduce costs, protect the environment, and improve lighting for their citizens. Despite this trend, the lamps’ turn-on/ turn-off time control is receiving little attention.
A suitable control can achieve an important energy saving because lights can operate too late, too early, or both, wasting energy or providing insufficient light. Using a twilight switch can significantly reduce energy consumption in all types of lamps (Figure 1). It offers a cost-effective, compact, and reliable way of providing lighting time control.
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| Figure 1. | This charge pump feeds the twilight switch from the ac line with high efficiency. |
The circuit does not use a relay. Therefore, it has no moving parts, and it is not prone to contact oxidation. It uses a TRIAC (triode alternating current) that can switch hundreds of watts.
The circuit requires little power. It uses a charge pump to feed the circuit from the ac line, drawing less than 37 mW for a 220 V-rms ac line. It uses just a few low-cost components.
You can adjust the circuit’s darkness and illuminance level that switches the light on and off using only onboard potentiometer R1. The circuit automatically turns on the lamps at nightfall and turns them off at daybreak. You can use it with incandescent lights, fluorescent lights, or LEDs.
The circuit uses an LDR (light-dependent resistor) to measure the ambientlight level (Figure 2). Be sure that the LDR you use has a spectral response similar to that of the human eye to achieve good performance. It uses a hysteresis comparator because a basic comparator configuration oscillates or produces a noisy output when the illumination level is close to the edge between natural light and darkness. Hysteresis creates two switching thresholds in the circuit: The upper threshold voltage is 8.47 V for the rising input-voltage change from natural light to darkness, and the lower threshold voltage is 7.75 V for the falling input-voltage change from darkness to natural light. The relationship between the 82-kΩ and the 4.7-kΩ resistors controls the 0.72 V hysteresis. This value is adequate to avoid the false triggering that light noise can cause.
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| Figure 2. | The circuit uses a light-dependent resistor to measure the ambient-light level. |
When the ambient light falls below the level that R1 sets, the input voltage, VI, rises above the upper threshold voltage and the output of the comparator decreases, switching on the TRIAC. When the ambient light rises above the level that R1 sets, the input voltage decreases below the lower threshold voltage and the output of the comparator increases, switching off the TRIAC.
You must provide a mechanical isolation between the LDR and the lamp light to prevent the formation of a feedback path to the LDR. Otherwise, the lamp light will cause an oscillation at the comparator’s output and then in the lamp’s state. The BTA16- 600SW, which is available from many sources, is suitable for switch lamps operating at more than 2000 W.
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| Figure 3. | The comparator drives a Vishay IL4216 or BRT12-F optocoupler with a TRIAC output. |
The comparator drives a Vishay IL4216 or BRT12-F optocoupler with a TRIAC output (Figure 3). The optocoupler, in turn, drives the BTA16-600SW TRIAC that controls the lamp.
