As shown in Fig. 7, the multiplexing card is made of one CD 4051 and two
CD 4052 multiplexers (denoted by MUX-1, MUX-2 and MUX-3, respectively), two
relaxation oscillators and a voltage-to-frequency converter (VFC). The eight input
signals arising from the different meteorological sensors are selected sequentially
by the MUX-1 multiplexer. The series of data obtained at the MUX-1 output are
demultiplexed by the MUX-2 chip. They are then transmitted to the MUX-3 multiplexer
either directly or through one of the three devices shown in Fig. 7 (i.e., both
relaxation oscillators and the VFC), following the type of sensor used. The relaxation
oscillator, built using the TL084 operational amplifier, is intended to convert the
variations of capacitive sensors (like aneroid barometers) into frequency variations.
Similarly, the NE555 relaxation oscillator makes possible the transformation of the
variations of resistive sensors (e.g., thermistors) into frequency variations. The VFC
is based on an AD537 chip. The frequency of the signal produced by this device is
chosen so as to vary from 0 to 100 kHz for an input signal varying from 0 to
10 V. Anemometers are amenable to voltage/frequency conversion. However, the
utilisation of the VFC is especially justified in the case where the meteorologicalsensors are far from the data-acquisition system and produce weak signals varying
slowly in time. In the case of the measurements of relative humidity, the pulses
provided by the related conditioning circuit are directly transmitted to MUX-3
through MUX-1 and MUX-2.