It's longer than usual, so here are some chapters:
Introduction 0:00
What is a moving coil meter 01:04:10
How to change what the meter reads 03:04:14
Proving these eBay meters are not inaccurate! 04:10:21
Taking the meter to bits 05:11:22
Adapting the meter to display illuminated pressure 07:02:04
Illuminating the meter 14:23:11
Assembling the customised meter 17:15:09
Getting the meter to display what you want with a microcontroller 22:34:24
What is Pulse Width Modulation (PWM) 23:42:06
Controlling the meter with PWM 30:38:16
Adding the BMP180 pressure sensor using I2C communication 34:04:24
Writing an Arduino sketch to control the meter 40:59:22
Replacing the Arduino with an Adafruit Trinket 44:53:03
Fixing the sensor and Trinket into the meter 48:22:17
Testing the illuminated barometric pressure meter 49:22:23
Final assembly 51:48:08
There's a mistake in the sketch I rewrote in the kitchen! The FloatMap command should read:
int pwmOutput = FloatMap(pressureValue, 970, 1040, 0, 254); 970 NOT 0
Useful links:
Adafruit Trinket tutorial - https://learn.adafruit.com/introducing-trinket
BMP180 library - https://github.com/LowPowerLab/SFE_BMP180
eBay meter listing - https://www.ebay.co.uk/itm/194360326840?_skw=20+volt+analog+meter&epid=1749308535&itmmeta=01JHZS11PJZ2NHQJM3SMAG7EQZ&hash=item2d40c73ab8:g:LCwAAOSwoINk8NDR&itmprp=enc%3AAQAJAAAA8HoV3kP08IDx%2BKZ9MfhVJKljht4b2UyBQ%2FblET%2BxfMkaMmm6oYhqtQ5h%2B40IZW29r%2Bhh%2FiZ64Qe9JfFvjSYdtQ%2BuNxVr8PHO7%2Fg5WUzjRbznZ3n4dAApgwb1BkAyc6omeCS%2BebihoQbVQgQIQfeXeAyxkH%2BfdQ2EurszAoo9ttimMOsroeRgu6O3S%2BJf3tUheNp6Gy8lz2rRsFebHl%2F8LSoMP%2FXqQ1VL54DtwhCWwfwLfAu5upxHwnm9XhhbxYfXJ%2FXvnBEWaFS3u6b4y9yekgRLkLT%2FQcbCehjBxCT8%2F6jVweDDQ7Xh1jB%2FJRBDVpUMAQ%3D%3D%7Ctkp%3ABFBMuJuE-Y9l
BMP180 barometric pressure sensors - very reasonably priced on Amazon (link too long!)