Electronic Design

Build A Digital Temp Monitor

This Idea for Design was originally published Dec. 2, 1993.

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 .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 it's shut off when not operating, it will obviously last much longer.

Update by Scott Wayne
Analog Devices Inc., One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106

Temperature is the most widely measured real-world phenomenon. Whether we are monitoring an infant's temperature in a hospital, the temperature of a vat of beer in a brewery, the temperature of a comet by a NASA deep- space probe, or the temperature outdoors by the meteorologist on the TV news, it seems that just about everyone wants to know how hot or how cold something is. Conseqently, there's always a keen interest in temperature-measuring circuits.

The 1993 Idea for Design, "Build a Digital Temp Monitor," requires a fairly complex circuit. In this design, an AD590 is used to produce a current that is proportional to absolute temperature (PTAT). This 1-mA/K current is converted into a 2-mV/K voltage by a 2-kΩ precision resistor. It's then scaled to 10 mV/K by one half of an OP220 op amp and two more precision resistors. After that, the output of an AD580 precision 2.5-V reference is scaled to 2.7315 V by the OP220's second half. This voltage is tweaked with a potentiometer, and it is used to convert from the Kelvin to the centigrade scale. A digital multimeter (DMM) provides the analog-to-digital (A-D) conversion and displays the result. A dual comparator supplies a low-battery detection function.

Today, the entire function could be accomplished with a single AD7814, a low-cost microprocessor, and a low-cost LED or LCD display (see the updated figure). The new design provides savings in terms of cost, size, and power consumption. It doesn't require trimming, and it will operate with a single 2.7- to 5.5-V power supply. The AD7814 is a complete temperature-monitoring system. It's available in a 6-lead SOT-23 package or an 8-lead µSOIC package. Containing a bandgap temperature sensor and a 10-bit A/D converter, it can digitize and monitor temperature readings to a resolution of 0.25°C.

The AD7814 has a flexible serial interface that allows it to be connected directly to most microcontrollers. This interface also can be used to put the device into standby mode. The AD7814 consumes only 250 µA in normal mode, and 1 µA in standby mode. A PIC16C6x microcontroller can have its synchronous serial port (SSP) configured as an SPI master with the clock-polarity bit set to logic one (see the updated figure, again). In this mode, the serial clock line idles high between data transfers. Data is transferred to and from the AD7814 in two 8-bit serial- data operations.

The low-battery detector can be devised in many different ways. The 1993 design uses a dual comparator, several resistors, and a voltage reference, comparing a divided-down supply voltage with the reference voltage. This approach could still be used, but it's not very efficient. Many microcontrollers contain on-board peripherals, such as voltage references, comparators, analog-to-digital converters (ADCs), and brown-out detectors. These could act as a low-battery detector, but at the expense of increased cost and pin count.

Perhaps the best option would be to use a microprocessor supervisory circuit. It would reset the microcontroller if the battery voltage goes too low, and it also would light the LED. An ADM811 offers a reset output during power-up, power-down, and brownout conditions. It has a reset threshold that is accurate to 100 mV, while consuming only 5 µA. And, it's available in a 4-lead SOT-143 package.

The new design uses less power, and it's smaller and less expensive than the 1993 design. Also, it doesn't require a DMM, and it provides more accurate readings. It is a true standalone device. Or, it could be integrated with other functions. In addition to monitoring temperature, the microcontroller can be used with setpoints to control temperature. Alarm functions could be added for over-temperature and under-temperature conditions. Basically, the possibilities are endless.

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