Question 9.11: You want to connect an analog panel volt meter to a Raspberr......

Assuming you have a 5V volt meter, you can use a PWM output to drive the meter directly, connecting the negative side of the meter to ground and the positive side to a GPIO pin (Figure 9-14). However, if the meter is the common 5V kind, you’ll only be able to display voltages up to 3.3V.

If you want to use almost the full range of a 5V volt meter, you will need a transistor to act as a switch for the PWM signal and a 1kΩ resistor to limit the current to the base of the transistor.

To make this recipe, you will need:

• 5V panel meter (“Miscellaneous” on page 382)
• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• Two 1kΩ resistors (see “Resistors and Capacitors” on page 380)
• Transistor 2N3904 (see “Transistors and Diodes” on page 381)

The breadboard layout for this is shown in Figure 9-15.

Discussion

To test the volt meter, use the same program as you did for controlling the brightness of the LED in Recipe 9.2.

You will probably notice that the needle gives a steady reading at either end of the scale, but everywhere else jitters a bit. This is a side effect of the way the PWM signals are generated. For a steadier result, you can use external PWM hardware like the 16-channel module used in Recipe 10.2.

See Also

For more information about how old-fashioned volt meters work, see Wikipedia.

For more information on using a breadboard and jumper wires with the Raspberry Pi, see Recipe 8.10.