The low-cost percentage-relative-humidity radio transmitter in Figure 1 operates in a cold-storage warehouse for vegetable storage at temperatures of 1 to 5°C. It is generally difficult to collect such data from a low-temperature area with high humidity and low illumination. The transmitter design is simple: It uses a readily available, capacitor-type percentage-relative-humidity sensor for which the capacitor value increases with humidity. Generally, these sensors offer accuracies well within 5%. Humirel (www.humirel.com) relative-humidity sensors work well with this circuit; you can also use other types with low leakage resistance. The R₁C₁ product gives the time constant for the audible-modulating, 1- to 2-kHz signal oscillator, which you can gate to stop the communication. This oscillator starts the RF oscillator, which has a time constant, R₂C₂, equating to a 10- to 50-MHz RF band. The last inverter is a power driver for the tuned filter and antenna. The circuit requires a 3 to 5V battery. Two AAA cells can power it for approximately 15 days. If you need a high modulating frequency, then you can reduce R₁ to 1 MΩ, changing the modulating signal to the range of 10 to 20 kHz.

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