When designing a precision outdoor temperature sensor it is a good idea to electrically isolate the sensor from the signal conditioning circuitry to protect it from voltage spikes such as might be induced by lightning. Digital signal transmission is preferred over analogue as the circuitry is more straightforward, communication is more reliable and subsequent processing of the temperature readings is easier. In the design shown here the signal and the power for the converter circuit are both carried on just two wires. A type PT1000 temperature sensor is used. This is capable of withstanding temperatures of well over 130 °C (such as might be found in solar heating systems). The voltage dropped across the sensor is taken as input to an Analog Devices AD654 votage-to-frequency converter. The power rail is then modulated with a square wave signal whose frequency is dependent on the measured temperature.
The signal can be carried on a cable over a great distance. At the receiver end an optocoupler provides for electrical isolation. T1 forms a current source that delivers a constant current of 1 mA into the temperature sensor R2. The current can be adjusted for calibration using trimmer potentiometer P1. The voltage across the sensor is taken to the VIN input (pin 4) of the voltage- to-frequency converter IC1. R4 and C1 are chosen so that the conversion factor is 10 kHz per volt.
The temperature is given by the formula
T = (f – 10000) / 38
where T is the temperature in °C and f the frequency in Hz. The frequency therefore ranges from 8.8 kHz (at –30 °C) to 15.7 kHz (at +150 °C). The output transistor of IC1 has its collector at pin 1 and its emitter at pin 2. Pin 1 is connected to the positive signal line via resistor R5, and pin 2 is taken directly to the negative signal line. The demodulation function is carried out by the circuit around T2. The value of the current sense resistor R6 is chosen so that when converter IC1 is in its quiescent (off) state T2 is not switched on. When the output transistor of the converter turns on, extra current is drawn from the supply via R5, making the total current drawn considerably higher. In turn, the voltage drop across R6 increases significantly and T2 is turned on. A large collector current now flows through R7, R8 and the LED inside optocoupler IC3. The phototransistor inside the optocoupler is now also turned on.
Finally, at connector K4 the signal is available with a low impedance, suitable for further processing. So that we can arrange for the circuit to operate from a single supply we use an isolating DC-DC converter. This not only provides the 12 V needed by the sensor circuit, but also offers up to 1000 V of electrical isolation.