Been using 3 x MCP3008 on a Pi1 for a couple of years for ADC to read 20 zone inputs on an alarm panel.
Worked perfectly. Values were consistently about 600 - 700 /1023 when doors / windows closed and dropped to ~150/1023 when opened. Power came from 12vdc Alarm panel Battery stepped down to 5Vdc
Changed to Pi3, used existing SD card and the readings are now 300 - 600 /1023 and fluctuate too much for solid readings.
All wiring is exactly the same
A: Changed PSU to 5Vdc 3 Amp Get 4.90v, and 3.30v exactly
B: Powered via 5v pin in case of volt drop through polyfuse
C: Powered via 2.5A USB wall Wart
I've checked about 6 times Putting SD card back in Pi1, all back to normal, put SD in Pi3 same problems.
Has something changed in Pi3 that creates this problem