Car Ice ararm circuit

Posted on Apr 19, 2012

The circuit uses a thermistor and three sections of a LM3900 quad op amp IC. When the temperature drops to 36Â°F the LED indicator flashes about once each second. The flashing rate increases as temperature drops to 32Â°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 outputâ€”fed to the noninverting input of op amp IIIâ€”varies with temperature.

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Op amp 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 20 kfi pot so the LED stays on.

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