The Smartec Type SMT160-30-18 is a temperature sensor with a digital output, and housed in a TO-18 case: Pins 1 and 3 are the supply input pins. With a nominal 5 V supply, the current drain is not more than 200 μA. At pin 2, a short-circuit-proof and TTL compatible rectangular output voltage with a pulse repetition frequency (p.r.f.) of 3 kHz is available. The value of the frequency is not all that important. because the actual temperature data are stored in the duty factor (that is, the ratio of the pulse width to the pulse spacing). There is a linear relationship between the temperature. T and the duty factor:
duty factor = 0.32 +0.0047x T.
Thus, at a temperature of -45 °C, the lowest at which the sensor can be used, the duty factor is 0.109. The maximum temperature at which the sensor can be used is 130 °C. The sensors are calibrated to an accuracy of ±0.25 °C during production.
In principle, it would be feasible to apply the rectangular signal to a moving coil meter. This would indicate a value that is directly proportional to the average voltage level of the rectangular signal and thus with the duty factor and the temperature. In practice, it is, however, much more sensible to connect the sensor to a digital Input port of a peripheral interface or a microcontroller. Sampling the rectangular signal enables the computer system to carry out temperature measurements with a minimum of external components as shown in the diagram. A suitable PC measurement card is published Ref. 1. Connector K1 is a 26-way box header that is linked to connector K6 of the measurement card via a short length of a flat cable. The 5 V supply is taken directly from the computer. On the prototype, L1-C1-C2 proved essential to prevent jitter of the signal, which caused the first digit after the decimal point to move to and fro. The program for controlling shown here is available, with other 13 basic routines for the measurement card, from, Elektor Electronics (Publishing) under reference ESS 1753.
Ref. 1. ‘Multifunction measurement card for PCs’, Elektor Electronics, February 1992, p. 14.