This design idea illustrates what may be the simplest and smallest temperature transmitter in the VHF 50- to 100-MHz range. As seen in the figure, the design employs an ON SEMI single-gate IC (NL27WZ14). The sensor is a negative-temperature-coefficient 1-kΩ SMD thermistor. It causes an increase in the oscillator frequency as temperature increases. Since the design of the circuit is not critical, the value of C1 can be selected high enough to lower the transmission-frequency band down to 1 MHz if necessary. The thermistor-resistance value, however, is critical and should not exceed 3 kΩ. If the resistance value exceeds this limit, the circuit may not oscillate properly.
The data is transmitted as a slowly modulated FM signal (i.e., the frequency shifts as temperature changes). Therefore, rapid signal modulations, typical in audio FM receiver systems, are not involved in this design. At the receive end, the receiver locks on to the transmission frequency and monitors the frequency variation directly as the temperature signal. C1 must not have a positive temperature coefficient or frequency compensation will be required to cancel the temperature dependency in the R1C1 time constant.
To limit the size of the transmitter, a small wire hanging from the tiny pc board is used as the antenna. Other types of antennas may also be used.
Since this circuit draws less than 10 mA from a NiCd cell, its operating time with a small cell is reasonably high (i.e., a few days). Power can also be supplied from other types of cells in the 3- to 5-V dc range.