Posted on Feb 6, 2014

IC1, an Intersil ICL7106, contains an a/d converter, a 3 1/2-digit LCD driver, a clock, a voltage refer-ence, seven segment decoders, and display drivers. A similar part, the ICL7107, can be used to drive seven segment LEDs. The probe body is a five-inch length of light-weight aluminum tubing. The leads from the circuit are connected to the body a

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nd tip of the probe. The sensor functions as a variable resistor that varies Q1`s base current, hence its collector current. The varying collector current produces a varying voltage across 100 © resistor R7, and that voltage is what IC1 converts for display. The LCD consumes about 25 A, and IC1 consumes under 2 mA, so the circuit will run for a long time when it is powered by a standard 9-V battery. Current drain of the two 1. 5-V AA cells is also very low: under 300 A. To calibrate, rotate R3 to the center of its range. Then place the end of the probe into a glass of water and adjust R8 for a reading of 100. When you remove the probe from the water, the LCD should indicate 000. You might have to adjust R3 slightly for the display to indicate 000. If so, readjust R8 with the probe immersed. Check for a reading of 000 again with probe out of water.

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