The circuit in Figure 1 is a triple, relative-humidity sensor and radio transmitter. Sensors 1 and 2 form two gated oscillators with natural frequencies of 10 and 5 kHz, respectively, at relative humidity of 50%. The gated oscillators use variable resistances R2 and R3, respectively. Together, these two oscillators generate FSK-modulated outputs at output of IC1B, Pin 6. The oscillator for Sensor 3 causes switching of the FSK signal at IC2B.
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| Figure 1. | Using FSK modulation, you can generate three independent relative-humidity measurements with one circuit. |
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IC2B’s natural frequency is 1 kHz at relative humidity of 50%. As the HS1101’s capacitance changes from 160 to 200 pF (180 pF at relative humidity of 50%), the oscillator frequencies change by approximately ±20% for relative humidity of 0 to 100%). You can tune the RF generator, IC1A, to the desired frequency of 27 to100 MHz for FM transmission. The following represents various ways to monitor the signals at the receiver end (not included in the design):
- Sensor 3 signal is the FSK-modulated signal at the receiver: 1 kHz±20% for relative humidity of 0 to 100%.
- Sensor 1 signal is the top FSK frequency, 10 kHz, on the carrier wave. It measures 10 kHz±20% for relative humidity of 0 to 100%.
- Sensor 2 signal is the bottom FSK frequency, 5 kHz, on the carrier wave. It measures 5 kHz±20% for relative humidity of 0 to 100%.
The difference between the top and the bottom FSK modulating frequencies provides the difference in the relative-humidity signals.
You can replace the Sensor 3 circuit with any TTL oscillator circuit with a range of 100 Hz to 1 kHz. You can then generate the frequency from any other type of sensor. This frequency then becomes available at the receiver without affecting the relative-humidity signals from sensors 1 and 2. You can even use a TTL-based ASCII output to replace the Sensor 3 circuit to pass the ASCII data along with relative-humidity signals.
