PC controls light dimmer

摘要

Using the simple circuit in Figure 1, you can control the light intensity in your room or work area from your PC. The heart of the circuit is a low-power D/A converter that converts digital words from a computer's parallel port to analog-voltage signals. To isolate the dc low-voltage part of the circuit from the high-voltage part, the circuit uses an op-toisolator, which prevents any direct electrical connection between the two sections. The optoisolator triggers triac T_1, which behaves like a switch. In each power cycle, T_1 switches on, the ac supply voltage connects to the load (lamps), and current starts flowing in the triac. At the end of a half-period, when the current drops to zero, T_1 turns off and awaits another trigger in the opposite direction. This additional trigger occurs in the second half-period of the power cycle. A lower triggering voltage makes T_1 conduct at an earlier point in and stay on for a larger fraction of the cycle. The larger fraction corresponds with transferring more power to the lamp, resulting in a higher intensity.