Patent ID: 7804200

Claim:
A photosensor circuit, comprising: a relay coil configured to control application of an alternating current (AC) power source having a negative half and a positive half of a line cycle to a load; a pulse width modulator circuit 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 coupled between the relay coil and a neutral bus that controls the average voltage across the relay coil responsive to the pulse width modulated signal; a photo control circuit configured to control application of the pulse width modulated signal to the drive transistor responsive to a detected light level; and a half-wave rectifier coupled to the power source that is configured to 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; wherein the photo control circuit comprises: a phototransistor, the phototransistor having a first terminal coupled to the power signal and a second terminal that outputs a current responsive to a level of light detected by the phototransistor; a low pass filter circuit coupled to the second terminal of the phototransistor that filters the output current of the phototransistor to provide a light level signal voltage; a select transistor that couples the pulse width modulated signal to the drive transistor during the one of the halves of the line cycle of the AC power source responsive to the light level signal voltage having a selected level; and a capacitor coupled between the second terminal of the phototransistor and the neutral bus that 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, wherein the low pass filter circuit includes a discharge transistor coupled between the second terminal of the phototransistor and neutral bus and/or a diode coupled between the second terminal of the phototransistor and the neutral bus that rectifies the positive feedback through the first terminal of the phototransistor.