An Autonomous Wireless Temperature Sensor for Measuring Human Body Temperature

by Gustavo C. Martins

Sensor networks are used in many application areas, such as biomedical sensors, predictive maintenance and RFID tags, which generally needs a long lifetime and small sizes [1]. For these systems emerges the necessity to develop energy harvesting integrated circuits. With no batteries we can develop sensors that has a longer lifetime, are smaller and, possibly, cheaper.

This project consists in a temperature sensor tag powered by an RF signal emitted by a reader. The readerreceives the reflected signal from the tag, which contains information on the temperature measured. The Figure 1 shows a block diagram of the system containing the blocks to be implemented.

Figure 1: Block diagram of the system

The blocks are listed below:

  1. Impedance Matching: matches the input impedance of the rectifier to the antenna impedance. It assures the maximum power is transmitted to the rectifier [2];
  2. Rectifier (RF-DC): converts a high frequency AC signal to DC [3];
  3. Voltage Limiter: limits the output voltage of the rectifier, so that it remains in a safe level [4][5];
  4. Voltage Regulator: regulates the voltage that will be used to power the sensor. This voltage isn’t dependent of temperature [5];
  5. Oscillator: is the temperature sensing element. The output signal frequency varies with temperature [2];
  6. Backscattering modulation: modulates the reflected signal by modulating the input impedance [6].



[1] R. Vullers, R. Schaijk, H. Visser, J. Penders, and C. Hoof, “Energy Harvesting for Autonomous Wireless Sensor Networks,” IEEE Solid-State Circuits Magazine, vol. 2, no. 2, pp. 29-38, 2010.
[2] J. W. M. Rogers and C. Plett, Radio Frequency Integrated Circuit Design. Artech House Publishers, 2010, p. 513.
[3] A. J. Cardoso, L. G. de Carli, C. Galup-Montoro, and M. C. Schneider, “Analysis of the Rectifier Circuit Valid Down to Its Low-Voltage Limit,” IEEE Transactions on Circuits and Systems I: Regular Papers, vol. 59, no. 1, pp. 106-112, 2011.
[4] Fernandez, E.; Beriain, A.; Solar, H.; Garcia-Alonso, A.; Berenguer, R.; Sosa, J.; Monzon, J.M.; Garcia-Alonso, S.; Montiel-Nelson, J.A.; , “Low power voltage limiter design for a full passive UHF RFID sensor,” Circuits and Systems (MWSCAS), 2011 IEEE 54th International Midwest Symposium on , vol., no., pp.1-4, 7-10 Aug. 2011.
[5] Balachandran, G.K.; Barnett, R.E.; , “A 110 nA Voltage Regulator System With Dynamic Bandwidth Boosting for RFID Systems,” Solid-State Circuits, IEEE Journal of , vol.41, no.9, pp.2019-2028, Sept. 2006
[6] K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards, Radio Frequency Identification and Near-Field Communication. Wiley, 2010, p. 478.