Patent ID: 8026470

Claim:
A photosensor circuit, comprising: a normally open 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 power circuit including a half-wave rectifier coupled to the power source that is configured to provide a power signal to the pulse width modulator circuit and a regulated power signal to the photo control circuit during one of the halves of the line cycle of the AC power source, wherein the power signal comprises a square wave and the power circuit further includes a rectifier circuit and a voltage divider circuit configured to generate the regulated power signal from the power signal; wherein the photo control circuit comprises: a phototransistor, the phototransistor having a first terminal coupled to the regulated power signal through a first resistor 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 first terminal of the phototransistor that filters the output current of the phototransistor to provide a light level signal voltage; and 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.