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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 optoisolator, which prevents any direct electrical connection between the two sections. The optoisolator triggers triac T1, which behaves like a switch. In each power cycle, T1 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, T1 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 T1 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.
The output voltage of the D/A converter sets the triggering point. The DAC, after one stage of buffering, provides enough current to drive the optoisolator. IC3 generates a 2.5V reference; the crystal oscillator and capacitors C1 through C4 set the DAC's timing characteristics. The DAC1220 (Burr-Brown Corp, www.burr-brown.com) connects to the parallel port with three wires for serial transfer of the digital codes. The Pascal program of Listing 1 reads the PC's keyboard; when you press Q or W, the routine increments or decrements a digital code and sends it to the DAC. The DAC then controls the lamp's intensity. Upon power-up, the DAC receives a digital code of zero, which corresponds to a 2.5V output (the reference voltage). You then adjust potentiometer R3 such that the lamp is half on. Using the keyboard, you can change the light intensity to the desired level. The dc part of the circuit consumes only approximately 5 mA. (DI #2401).