Circuit Theory

The Sunrise-Sunset Light circuit operates like the Smart Switch, except you don’t have to use a mini pushbutton to start the timing function. The mini pushbutton has instead been replaced with a light sensor called a photocell. A photocell is a variable resistor that changes its resistance based on the amount of light touching its surface. Light falling on a photocell will decrease its resistance value. No light will increase its resistance value. Figure 2-3 shows the resistor-capacitor (RC) timing circuit with a photocell variable resistor symbol.

Figure 2-3. RC timing circuit with a photocell (variable resistor)

TECH NOTE

Another type of variable resistor is a 3-pin electronic part known as a potentiometer. By rotating its shaft, the internal resistance value changes. Potentiometers are used in electronic products like radios and TVs to control the volume or sound level.

A photocell is a small electronic component with two leads protruding from a light-sensitive pellet. On top of the pellet is an etched series of squiggly lines. The lines etched on its surface are the resistance portion of the photocell. An actual photocell part can be seen in Figure 2-4.

Figure 2-4. Photocell (a light-dependent resistor)

TECH NOTE

LDR (light-dependent resistor) is another word used for photocell.

If you know someone with a DMM (digital multimeter), have him attach your photocell to it. By waving your hand over the photocell, you will see the light-sensitive part change its resistance based on the amount of light touching it. This variable resistance feature will be used to turn on an LED. In the Sunrise-Sunset project (a light-activated switch), the green LED will be on first. Placing your hand over the photocell briefly will turn on the red LED. After the RC timing circuit has completed its charging-discharging cycle, the red LED turns off followed by the green LED turning on.

TROUBLESHOOTING TIP

Due to the sensitivity of the Sunrise-Sunset Light Switch, for best operating results, use ambient (i.e., natural) lighting when testing the device.