= alternating green & red lightThe design is based around a type 324 integrated circuit that contains four op.amp blocks. Each op.amp block is used in a different way to process the sensor signal. The first, IC1a, creates a constant current through the sensor diode, D1. IC1b amplifies the diode voltage variations, applies voltage of a sensible magnitude and moves the voltage in the right direction. The voltage output is directly proportional to the temperature readings from the sensor. IC1c is a comparator that compares the voltage output of IC1b to the desired thresholds of 0C and 4C and controls the red/green LED switch over at 4C. IC1d is a secondary comparator, configured as an oscillator, which controls the red/green LED when the readings fall below 0C.The simple design of the frost box lends itself to construction by a “do it yourself” person. A materials list is available along with instructions and the recommended layout for component installation. Additional advice on technique is provided in the instructions to ensure success.Several tests should be conducted prior to installation and use. First, using a voltage meter, the resistance across the power leads, and between the +5V and 0V lines on the circuit board should be measured to see if there are any shorts. Then with the meter check that IC1a is receiving a 5V supply across its power pins (11 and 4). If things check O.K., remove the power and install IC1a in its socket. Once all the components are installed together, it’s time to calibrate VR1 and perform an integrated system test. To simulate weather conditions below 0C, place some ice from the freezer with some water in a pitcher. A thermometer is used to validate the temperature of the chilled water used in testing. Once testing conditions are set up, apply power to the circuit. Then dip the diode in the water, being careful to keep the sensor away from the ice. Calibrate VR1 by adjus...
