Breadboard

CHAPTER 8: Hacking with Sensors 195

Figure 8-3 Attaching leads to the sensor connection, then its output turns on. In this case, that will light the LED and sound a buzzer.

The trimpot supplies a threshold voltage to the negative input of the comparator. To use the gas detector, the trimpot is turned until the LED just goes out. It will come back on if the output from the sensor increases enough to exceed the value at the comparator’s negative input. The sensor has rather unusual connections. It has six connections, but some of them are doubled up and connected behind the scenes (see Figure 8-1). The H connectors supply a heating element that warms the catalyst layer between A and B. When methane is detected, the resistance between A and B falls. R2 forms a voltage divider with the sensing element. One benefit of the sensor basically being two resistors—one acting as a heater and the other as a sensor—is that the pin connections are reversible. The sensor leads are thick and at a strange spacing, so they will not fit in breadboard. For this reason, we solder some leads to them (Figure 8-3).

Rather than solder wires to all the leads, we can just solder the following connections:

● A red positive supply lead to all the pins on one side of the sensor (the two A pins and one H connection) ● The resistor R2 between B and the GND side of the heater ● A GND lead to the GND side of the heater (black) ● An output lead to B (yellow)

Breadboard

Figure 8-4 shows the breadboard layout for the gas detector, while Figure 8-5 displays the project itself.

The breadboard layout is very straightforward, but do make sure that the IC is the correct way around. When it is all assembled, I will leave you to find your own way of testing it. Just a note that breathing on the sensor will also set it off.