Low-cost relative-humidity transmitter uses single logic IC

Texas Instruments SN74HC132

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.

China PCB Prototype and Fabrication Manufacturer

This percentage-relative-humidity transmitter uses 10- to 50-MHz, tunable RF, and 1- to 2-kHz on/off amplitude modulation.
Figure 1. This percentage-relative-humidity transmitter uses 10- to 50-MHz, tunable RF, and 1- to 2-kHz on/off
amplitude modulation.

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%. HS1101 relative-humidity sensors work well with this circuit; you can also use other types with low leakage resistance. The R1·C1 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, R2·C2, 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 5 V 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.

Materials on the topic

  1. Datasheet TE Connectivity HS1101LF
  2. Datasheet Texas Instruments SN74HC132

EDN

You may have to register before you can post comments and get full access to forum.
User Name