Python script to drive a tachometer on Raspberry Pi GPIO using PWM

The components needed to drive a standard automotive RPM gauge are pretty simple. You need:

• 12V power source (or battery) for the gauge
• Optocoupler 4N25 to separate the gauge signal and Raspberry Pi GPIO.
• 75 (or so) Ohm resistor for optocoupler input
• 20kOhm resistor for pull-down for gauge signal line (might depend on gauge)

The script assumes BCM GPIO 4 (Pin 7 in Rasperry Pi) to be used for driving the gauge. Pin 9 is used for the ground. Opto-isolator acts like normal led, so putting a small ~75 Ohm resistor makes it ok with 3.3V.

The Hz is calibrated to specific hardware using the calibration array in the beginning of the script. You probably need to adjust the numbers here to make it match your hardware. Mine had huge variations especially between 4 to 6.

Run the script by giving the operating more as the first parameter:

• test - Goes trhough test cycle setting values from 1 to 8
• [value] - displays the given float value between 1-8
• cpu - displays CPU load percentage from 0 up to 80%
• network - displays network inbound traffic in MB/s.
• mqtt [server] [topic] - subscribes to MQTT broker / topic for values.

Optional 'quiet' parameter disables the console output.

For example the following puts the script to refresh CPU on the background

sudo ./tacho.py cpu quiet &

Note, that because GPIO on RPi needs root access 'sudo' must be used.