Digitally control room light intensity

Many people favor different light and temperature settings for different rooms depending upon their mood or whether they are working or relaxing. The circuit in Figure 1 controls the intensity of the artificial light in a room and monitors the temperature of two zones. The two main circuit blocks are the PIC16C67 master controller and the ADT7516 temperature-sensor interface, which includes a four-channel ADC and a quad voltage-output DAC. Other components include a photodiode and an op amp that monitor the ambient light; a rotary potentiometer that sets the light intensity; an LED bar array and display driver, which indicate the light-intensity setting; a light-dimmer-control circuit; and a 16×two-character LCD, which indicates the temperature of the two zones.

On power-up, the PIC16C67 configures its ports to control the LCD and the ADT7516. The ADT7516 has a dual interface, comprising I²C and SPI, so the master communicates in SPI mode. The ADT7516 operates in SPI mode, once the controller initializes the LCD.

The ADT7516 senses both its internal temperature and the temperature of a remote thermal diode (Q₁ configured as a diode), and the PIC16C67 displays these temperatures on the LCD. One of the ADT7516's analog inputs monitors a potentiometer that you adjust to set the required light intensity. The PIC controller reads the potentiometer value from the ADT7516 and outputs a corresponding DAC value. The DAC controls an LM3914 LED-bar-array controller that shows the potentiometer setting on the array. If you set the potentiometer halfway, for example, then half of the LEDs turn on, indicating that you want an intensity that is half of what the light source can deliver.

A second DAC output controls a DIAC-based (X1) light-dimmer circuit. This dimmer circuit operates like any other light dimmer, except that the DAC controls it instead of a potentiometer. A photodiode monitors the intensity from the light bulb. An OP07 amplifies its output and feeds it into one of the ADT7516's analog inputs. The PIC controller uses the potentiometer and photodiode values, which the ADT7516 digitizes, to maintain equilibrium between the light intensity and the required light setting. If the photodiode reading is less than the potentiometer setting, the controller increases the dimmer DAC value; it decreases the dimmer DAC value if the reading is greater.

One of the features of the ADT7516 is its round-robin mode, in which it constantly monitors all of its measurement channels. The master need not initialize any conversions during its operation; all it has to do is read back from four value registers and act according to its program. This circuit ensures a constant light intensity within a room, saving power when daylight takes over as the main light source. It also extends the lifetime of a light bulb, thus saving on maintenance bills in a large office environment. You can also extend the application to include control of air conditioning and to memorize heat and light settings that suit individuals tastes.

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