Page 116 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

A simple sensor setup is shown in Fig. 5.46. The resistive element
must be heated to become sensitive. The sensor incorporates its
own heating unit, which is separately powered. The heater requires
a regulated 5 V for proper operation and draws about 130 mA.
The resistive element can be read like any other resistive sensor
used thus far.
The potential for these gas sensors is greater than what is implied in
the simple schematic. The gas sensors are not precise instruments.
In other words, their response varies slightly from device to device.
This “analog” property can be used to create a more sensitive smell
detector.

Let’s arrange eight sensors. The resistive element from each sensor
is connected to an A/D convertor. A comparator circuit wouldn’t do
in this situation because precise and subtle variations in response
are what we are looking for. To calibrate the device, a small
amount of a known gas (smell) is released by the eight sensors.
The response of each detector is measured by the A/D convertor
and recorded by the main computer. Since the responses of the
detectors will vary, an eight-number pattern is created for each
smell.

Pattern matching is well established in neural networks. A neural 95
network can be built using the information gathered that can not
only measure but recognize different smells.

5.45 Toxic gas sensor

Team LRN Sensors