Question 12.6: You have a resistive sensor that you wish to use with an MCP......

Use a potential divider with one fixed resistor and the resistive sensor to convert the resistance of the sensor into a voltage that can be measured with the ADC.

As an example, you can remake the light sensing Recipe 12.2 to use the MCP3008 instead of the step response technique.

To try this recipe, you will need:

• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• MCP3008 eight-channel ADC IC (see “Integrated Circuits” on page 381)
• 10kΩ resistor (see “Resistors and Capacitors” on page 380)
• Photoresistor (see “Resistors and Capacitors” on page 380)

Figure 12-12 shows the arrangement for this, using a breadboard.

You can use the exact same program as Recipe 12.4 (adc_test.py). Covering up the light sensor with your hand changes the readings. You also need to set up SPI on your Raspberry Pi, so if you haven’t already done so, follow Recipe 8.6.

These readings may be quite a bit different, depending on your photoresistor, but the important thing is that the figure changes as the light level changes.

Discussion

Choice of fixed value is not very critical. If the value is too high or low, then you will just find that the range of readings is rather narrow. Select a resistor value somewhere between the minimum and maximum resistance of the sensor. You may need to experiment with a few resistors before deciding on one that suits your sensor over the range of readings that you’re interested in. If in doubt, start with 10kΩ and see how that works.

You can swap out the photoresistor for pretty much any resistive sensor. So, for instance, you could use the gas sensor of Recipe 12.3.

See Also

To measure light intensity without the complication of an ADC, see Recipe 12.2. For an example of using more than one channel of the ADC at a time, see Recipe 12.8.