Electronic Design

Build A Digital Temp Monitor

The AD590 proportional-to-absolute-temperature (PTAT) device combined with an autoranging digital multimeter (DMM), such as Fluke's Model 77, can make a stable digital temperature-monitoring device. This holds true particularly for long distance temperature measurements, up to 100 ft.

Looking closely at the circuit (see the figure), U1A forms a current-to-voltage and voltage-amplifier circuit for the total range of the AD590, -50° C to +150°C. The output of U1A then becomes the positive lead of the DMM. U1B forms a 0°C reference and calibration circuit, based upon the AD580 precision voltage reference device. This, in turn, becomes the negative lead of the DMM. The AD590 has a certain amount of calibration error, depending upon the device's grade. The 500-Ω trimmer pot removes the error at 25°C.

U2 is a basic comparator that forms a low battery voltage circuit. If the battery falls below 5.1 V, the LED will turn on, signaling that the battery should be replaced.

To calibrate the device, U1B's output should be set to 2.730 V (0.0°C) by adjusting the 500-Ω pot. Next, the AD590 must be attached as shown in the figure. Then, with a calibrated temperature measuring device, the 500-Ω pot should be adjusted for the correct temperature reading at 25°C. The reading on the DMM will have to be moved two decimal places to the right for final reading. For example, if the meter reads 0.285, the actual temperature is 28.5°C. An autoranging DMM, such as the Model 77 or an equivalent, is recommended.

The battery will last about two weeks in continuous operation. If its shut off when not operating, it will obviously last much longer.

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