| 摘要 |
<p>Photosensor circuits include a relay coil configured to control application of an alternating current (AC) power source to a load. A pulse width modulator circuit is configured to generate a pulse width modulated signal having a pulse width that varies responsive to an average voltage across the relay coil. A drive transistor controls the average voltage across the relay coil responsive to the pulse width modulated signal. A photo control circuit controls application of the pulse width modulated signal to the drive transistor responsive to a detected light level. A half-wave rectifier provide a power signal to the pulse width modulator circuit and the photo control circuit during one of the halves of the line cycle of the AC power source. The photo control circuit includes a phototransistor. A low pass filter circuit filters the output current of the phototransistor to provide a light level signal voltage. A select transistor couples the pulse width modulated signal to the drive transistor during one of the halves of the line cycle of the AC power source responsive to the light level signal voltage having a selected level. A capacitor coupled between the second terminal of the phototransistor and the neutral bus modulates an amount of positive feedback through the first terminal of the phototransistor proportional to a current flowing through the relay coil to provide hysteresis to control of the relay coil by the photosensor circuit.</p> |
