Abstract
This chapter fopcuses on the ice-warning indicator. This unit automatically operates a small warning light on the car's instrument panel whenever there is a danger of ice on the road. The device uses a thermistor to sense the air temperature just above the road surface, and closes an electronic switch whenever this temperature falls below 0°C. The design is such that the operating point of the electronic switch is independent of the temperature of the actual transistor circuitry, and of variations in the car's battery voltage. The thermal backlash of the circuit is a mere 0.5°C, tha is, the electronic switch closes when the thermistor temperature falls to 0°C, and opens again when it rises to +0.5°C. The chapter presents in a diagrammatic form the negative ground version of the unit, in which one side of the indicator lamp is taken to the car's chassis. The circuit is made up of two sections, the first being a temperature-sensing differential amplifier (Q1 and Q2), and the second being a regenerative switch (Q2 and Q3).
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