Photosensor circuits including a regulated power supply comprising a power circuit configured to provide a regulated power signal to a comparator of a pulse-width modulator
Photosensor circuits include a relay coil configured to control application of an alternating current (AC) power source to a load. The circuit includes 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 to the relay coil controls the average voltage across the relay coil responsive to the pulse width modulated signal. A photo control circuit is configured to control application of the pulse width modulated signal to the drive transistor responsive to a detected light level. A power circuit coupled to the power source is configured to provide a regulated power signal to a comparator of the pulse width modulator circuit.
1. 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, the pulse width modulator circuit including a comparator;
a drive transistor coupled to the relay coil 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, coupled to the power source, that is configured to provide a regulated power signal to the comparator of the pulse width modulator circuit, wherein the power circuit includes the drive transistor coupled between the relay coil and the regulated power signal and a second transistor coupled between a base of the drive transistor and the regulated power signal and wherein the drive transistor conducts current passing through the relay coil to the regulated power signal when the drive transistor is on and the second transistor conducts current not passing through the relay coil to the regulated power signal when the drive transistor is turned off.
2. The photosensor circuit of claim 1 , wherein the power circuit includes a rectifier coupled to the AC power source.
3. The photosensor circuit of claim 2 , wherein the rectifier comprises a full-wave rectifier.
4. The photosensor circuit of claim 2 , wherein a collector of the second transistor is coupled to the base of the drive transistor, an emitter of the second transistor is coupled to the regulated power signal and an output of the comparator is coupled to the base of the second transistor and wherein the power circuit further comprises a resistor coupled between the base of the second transistor and the rectifier.
5. The photosensor circuit of claim 4 , wherein the resistor comprises a first resistor and a second resistor and wherein the first resistor is coupled between the rectifier and the base of the drive transistor and the second resistor is coupled between the base of the drive transistor and the base of the second transistor.
6. The photosensor circuit of claim 5 , wherein the power circuit further comprises a third resistor coupled between the output of the comparator and the base of the second transistor.
7. The photosensor circuit of claim 2 , wherein the pulse width modulator circuit is configured to control a frequency of the pulse width modulated signal and wherein the frequency of the pulse width modulated signal is at least about 500 Hertz (Hz).
8. The photosensor circuit of claim 7 , wherein the photosensor circuit includes a relay including the relay coil and wherein the relay comprises a normally open relay.
9. The photosensor circuit of claim 7 , wherein the photosensor circuit comprises a street lighting photo control device and the load comprises street lighting and the relay coil is configured to activate to switch on the street lighting responsive to detection of less than a minimum light level by the phototransistor.
10. The photosensor circuit of claim 2 , wherein the photo control circuit includes:
a phototransistor having a first terminal coupled to the regulated power signal and a second terminal that outputs a current responsive to a level of light detected by the phototransistor; and
a microcontroller configured to measure the detected light level and to turn off the drive transistor responsive to the detected light level meeting a selected condition, wherein the power circuit is configured to provide the regulated power signal to the microcontroller.
11. The photosensor circuit of claim 10 , wherein the microcontroller is configured to disconnect the output of the comparator from the base of the second transistor to turn off the drive transistor.
12. The photosensor circuit of claim 10 , wherein the photo control circuit further includes:
a resistor coupled between the microcontroller and the second terminal of the phototransistor and a charge capacitor coupled between the second terminal of the phototransistor and a low voltage reference of the regulated power signal, wherein current flowing through the phototransistor charges the charge capacitor; and
wherein the microcontroller is configured to periodically discharge the charge capacitor and to measure a voltage across the charge capacitor a selected time after the charge capacitor is discharged to measure the detected light level.
13. The photosensor circuit of claim 10 , wherein the microcontroller is configured to turn off the drive transistor at a selected delay time after the detected light level is determined to meet the selected condition.
14. The photosensor circuit of claim 10 , wherein the comparator and the microcontroller are included in a single microcontroller chip.