Design Ideas: May 9, 1996

An ordinary transistor can serve as an uncalibrated temperature sensor (Figure 1) for an accurate, PC-compatible thermometer. Unadjusted accuracy is more than 0.5°:C over the -50 to +50°C range, and the only limitation on resolution is the time devoted to measurement acquisition. For example, 0.1°:C resolution requires less than 1 sec. The thermometer derives all its power from the RS-232C COM port. Therefore, a laptop computer running a simple data-logging program yields an inexpensive, portable temperature-logging system.

The key to the thermometer's accuracy is the way the circuit uses the temperature-sensing transistor, Q₁. Op amp IC₁ drives Q₁'s emitter voltage to force the transistor to sink the current from the R₁ to R₂ current source. As IC_2A toggles, it generates a square wave at Q₁'s emitter, with a peak-to-peak amplitude given by:

where T_Q is the absolute temperature (in Kelvin) of Q₁.

This amplitude varies from 46.11 mV at -50°:C to 66.77 mV at +50°:C for virtually any npn silicon transistor. Accuracy depends only on the tolerance of the nominal 10-to-1 R₁/R₂ ratio. Temperature-to-digital conversion involves a voltage-to-frequency-ratio scheme: Difference amplifier IC₃ compares the square wave from Q₁'s emitter to the signal developed across R₃ by switches IC_2A and IC_2B, acting concurrently with R₄, R₅, and the 2.500V reference IC₄ develops. IC₃ amplifies the difference with a gain of 250 and applies the result to synchronous rectifier IC_2C.

The rectification action pumps charge into integrator IC₅. IC₅'s output connects to comparator IC₆, which closes the voltage-to-frequency feedback loop to IC_2B. Conversion timing comes from the PC's transmission of strings of CHR$(07) characters over the RS-232C interface, which is formatted for 9600 baud, 1 stop bit, 5 data bits, and no parity). The arrival of each character initiates a sequence of actions, among which are the connection of R₂ to 2.500V by IC_2A and the connection of the right end of C₁ to ground by IC_2C. IC₆ simultaneously compares the output of IC₅ to a threshold voltage near 1V.

If IC₅'s output is more positive than the threshold, IC₆'s output goes positive, connecting the right end of R₅ to ground. If IC₅'s output is less positive than the threshold, IC₆'s output stays negative and causes R₅ to remain connected to 2.500V. In the former case, C₁ charges negative, and the CHR$(07) character echoes back to the receive side of the COM port. In the latter case, C₁ charges positive, and no character echoes. Thus, the feedback loop constantly drives IC₅ toward the threshold and echoes a character to the PC each time a negative C1 charge condition requires it.

As a result, the fraction of transmitted characters that echo back to the COM port varies linearly with temperature from 0% at - 50°C to 100% at +50°C. So, for example, to perform a measurement with 0.1°C resolution, the PC would transmit 1000 characters over 0.73 sec, tally the number of echoed characters, subtract 500, and divide by 10. (DI #1860)