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.

Description
BACKGROUND OF THE INVENTION
The present invention relates to photosensor circuits and, more particularly, photosensor circuits for light level switching control.
Photo controllers are devices that automatically turn electrical devices on and off in response to the ambient light level. They are used, for example, on street lights to automatically turn them off during the day and on at night. They are also used on billboard lighting systems to turn the billboard lights on early at night, off late at night during periods of low vehicular traffic, on again during early morning rush hour periods when high traffic levels resume, and then off during the daylight hours. Photo controllers may also be used in reverse, for example, to turn a golf course water fountain on during the day and off at night.
A variety of devices, including photo controllers, may make use of power converters to convert relatively high voltage alternating current to relatively low voltage direct current as is used in many conventional electronic devices. Some conventional power converters make use of large, high-voltage resistors to drop the voltage. However, these resistors are typically inefficient and generate high heat. The heat generated from the resistors may require that the resistors be housed in a large package and include heat dissipating elements, such as heat sinks. Also, the high heat generated by the resistors can lead to problems with reliability and longevity in the resistors and in other electronic components situated near the resistors.
Another conventional approach to power conversion is the use of a switch mode power converter. The switch mode power converters typically require six transistors or a micro-controller to implement. The requirement for multiple transistors or a micro-controller may cause the implementation of switch mode power converters to be cost prohibitive in some applications, such as in photo controllers.
A small, low cost, efficient switch mode power converter and a photosensor circuit including the same are described in U.S. Pat. No. 6,903,942 (“the '942 patent”), which is hereby incorporated herein by reference as if set forth in its entirety.
It is also known to provide a photosensor circuit including digital circuitry as incorporated in the 3100 Series photocontrol, available from TE Connectivity. The photosensor circuit for the 3100 Series photocontrol is shown in FIG. 1. As seen in FIG. 1, a microcontroller U1 outputs a signal GP4 that drives a drive transistor Q2 to turn on and off the drive transistor Q2 as part of a pulse width modulated control circuit including capacitors C5 and C6 and diode D2 (operating at a frequency of approximately 50 Hertz (Hz) to control the voltage across the relay coil of the relay K1. Relay K1 is shown as a normally open relay and, when light is detected by photransistor Q1, the coil current of relay K1 is turned off to turn off a load LOAD, such as a street light. A direct drive signal is provided by including capacitor C3 to adjust voltage levels of the signal to the base of the drive transistor Q2 from the signal GP4. In addition, a half wave rectifier (diode D1) is included to provide power for the photocontrol circuit.
A regulated power supply is also shown to provide the power supply signals VSS and VDD to allow operation of the microcontroller U1. In the illustrated embodiment, VSS is coupled to the AC line NEUTRAL to provide the DC ground reference. The VDD signal is provided by the resistor R7 coupled to the half wave rectifier D1. In addition, The VDD signal is coupled to the emitter of the drive transistor Q2. As such, current will also be provided through the drive transistor Q2 but only when the transistor is on.
A small, low cost, efficient switch mode power converter including a regulated power source for a microcontroller is also described in co-pending U.S. patent application Ser. No. 13/190,727 (“the '727 application”), which is hereby incorporated herein by reference as if set forth in its entirety.
Regulated power sources may also be used in photocontroller circuits not including a microcontroller, such as described in U.S. Pat. No. 8,026,470 (“the '470 patent”), which is hereby incorporated herein by reference as if set forth in its entirety.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide photosensor circuits including 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 of the photosensor 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. The pulse width modulator circuit includes 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 of the photosensor 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 the comparator of the pulse width modulator circuit. 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. 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.