As photovoltaic or solar arrays---which harvesting sunlight for electricity using photovoltaic shingles or solar panels---increase in popularity, the demand for higher efficiency products will also increase. A simple electronic addition to a solar array can make it track the sun, increasing its power output by up to 60 percent. The tracking function keeps the solar array pointed into the sun all day long.
Tracking can be achieved by timing a motor to move the array in an arc. You can track more accurately and effectively using additional solar cells in the diagonal corners to sense the sun's position and trigger the motors to adjust the array. This technique has the advantage of maintaining its accuracy regardless of the season and does not require complex lookup tables.
Create a simple timer circuit and set it to count down to zero. The same basic circuit can count down to zero from 99 minutes and 99 seconds or from 99 hours and 99 minutes. Modify the timer to count up as well as down and you can create a stop watch.
A classic senior project, the line-following robot, presents a simple exercise in motor control. Set a light-sensitive transistor at each forward corner of your circuit board. Mount two DC motors under the circuit board. Use analog or digital electronics to control the motors.
When both light sensors find equal light intensity, both wheels spin equally fast because the robot is centered. If one light sensor "sees" less light than the other, the electronics make a motor correction to realign and center the robot with the line.
Use a balanced bridge for your sensor input and an instrumentation amplifier to output 2 volts maximum into a digital multi-meter. Scale your digital thermometer to measure 0 to 200 degrees Celsius or Fahrenheit. Use a thermistor (temperature sensitive resistor) as your sensor.
Barr suggests the design should minimize or account for temperature variations other than those being measured. One way to do this is to mount the sensor on a long wooden pole to isolate your body heat from the point you are testing.