Road Ice Alarm

The circuit uses a thermistor and three sections of a LM3900 quad op amp IC. When the temperature drops to 36 degree F the LED indicator flashes about once each second. The flashing rate increases as temperature drops to 32 degree F when the LED remains on. Amplifier I compares the thermistor's resistance to the resistance of the standard network connected to its noninverting input. Its output-fed to the noninverting input of op amp III-varies with temperature. Op amp II is a free-running multivibrator feeding a pulse signal of about 1 Hz to the inverting input of op amp III. This amplifier compares the outputs of op amps I and II and turns on the LED when the multivibrator's output level drops below op amp I. The monitor is calibrated by placing the thermistor in a mixture of crushed ice and water and adjusting the 20k ohms pot so the LED stays on. (Radio-Electronics 1979). Figure FB7-11 in Encyclopedia of Electronic Circuits on CD-ROM Vol 1.