A couple standard solutions for your project would be:
1) isolate your readout unit's internal ground and with a high current voltage follower, float it midway between vehicle ground and +10V
2) use a low-cost ($5-10) DC to DC converter module to generate -5V from your regulated +5V supply (or -9V you can then regulate to -5V).
Personally, I would do neither and instead follow BMW's example: keep everything digital, attach a microcontroller to the instrument cluster's vehicle speed output, and have it count the output's +12V digital pulses, and show the speed. I might also build a Mach meter rather than a speed-of-light meter as I think you'll find passing the sound barrier is challenge enough.
Good luck! |