This little circuit converts temperature measured by NTC (negative temperature coefficient) resistor into a digital signal. The resistance of NTC, which is an inverse function of the ambient temperature, determines the frequency of an oscillator built around the familiar TLC555 timer. The astable circuit is designed to give a pulse repetition frequency (p.r.f.) of about 250 Hz at 25oC that rises with temperature. The non-linear relationship between temperature and oscillator frequency is not a problem here, because it is relatively simple to `straighten’ by arithmetic operations performed by a computer or a microcontroller system.
Basically, three temperatures are measured, and the corresponding oscillator frequencies are stored as reference points, which serve to interpolate other values in between.
The components used are all surface mount devices (SMDs), but the printed circuit board allows the use of a traditional NTC in position R1.
To ensure a fast response to temperature changes, the completed PCB is fitted into a small metal tube with a diameter of 13 mm or larger. Great care should be taken to isolate the board and the components from the metal tube; if necessary, use beat-shrink sleeving! The tube is sealed hermetically with potting compound or two-component resin glue, through which three wires are passed: + supply; output signal & ground.
The current consumption of the converter is smaller than 1 mA.
The multifunction measurement card for PCs is perfect for processing the converter output signal. Use is made of the frequency meter input and the program modules found in the Turbo Pascal library PMEASURE.PASc on diskette ESS1751, and in Borland’s Numerical Toolbox which provides some handy interpolation routines.
The TLC555 is a monolithic timing circuit fabricated using the TI LinCMOS™ process. The timer is fully compatible with CMOS, TTL, and MOS logic and operates at frequencies up to 2 MHz. Because of its high input impedance, this device supports smaller timing capacitors than those supported by the NE555 or LM555. As a result, more accurate time delays and oscillations are possible. Power consumption is low across the full range of power-supply voltage.