Read negative voltages with an MCP3008

Question

How can I read a negative voltages with the MCP3008? I tried to do that with this code but I could not read them; the largest readings are 0.00.

This is my code:

import RPi.GPIO as GPIO
import pigpio
import time
import spidev

GPIO.setwarnings(False)

spi = spidev.SpiDev()
spi.open(0,0)
pi = pigpio.pi()
if not pi.connected:
 exit()

def analogInput(channel):
 spi.max_speed_hz = 1442      #f_cpu/frequency 1442
 adc=spi.xfer2([1,(8+channel)<<4,0])
 data = ((adc[1]&3)<<8) + adc[2]
 return data

def Volts(data):
 volts=(data*3.3)/float(1023)      #1024
 volts=round(volts,2)
 return volts  

dc=718000
pi.hardware_PWM(18, 70000, dc) 
while True:
 out = analogInput(0)
 out1 = Volts(out)
 print(out1)
 time.sleep(3)

Answer 1

It cannot.

However, you can make a voltage divider that will make it into a positive voltage, by using a positive voltage as one side of your divider.

You say the voltage you want to measure is -12V to -9V. If you have +3.3V available, you could use a voltage divider with about a 4:1 ratio, like so (4.7:1 in this case):

^{simulate this circuit – Schematic created using CircuitLab}

Your output voltage will be $$V_{out} = 3.3 + \frac{10}{57}V_{in}$$

So when the input is -12V, the output will be 1.195V. When the input is -9V, the output will be 1.721V.

If you have a different voltage than +3.3V or your ADC can only measure up to 1V or something, you'll need different resistor values, but this is the general idea.