# Incorrect values while attempting to read TMP36 + MCP3008

I'm learning how to read an analog thermistor using an MCP3008 and a Raspberry Pi Zero W, along with a Smraza T Type GPIO Breakout board.

Specifically, I'm following the Analogue Sensors On The Raspberry Pi Using An MCP3008 tutorial on Raspberry Pi-Spy.

I am consistently getting values like this:

``````--------------------------------------------
Light: 972 (3.14V)
Temp:  183 (0.59V) 9.03 deg C
--------------------------------------------
``````

The light value seems fine but the temperature is dead wrong (it's not 9C in my living room, even in a Kansas City winter). This is the second time I've tried a tutorial with the MCP3008 and the TMP36 and the result seems to be the same.

Here's my code:

``````#!/usr/bin/python

import spidev
import time
import os

# Open SPI bus
spi = spidev.SpiDev()
spi.open(0,0)
spi.max_speed_hz=1000000

# Function to read SPI data from MCP3008 chip
# Channel must be an integer 0-7
return data

# Function to convert a voltage level,
# rounded to specified number of decimal places.
def ConvertVolts(data, places):
volts = (data * 3.3) / float(1023)
volts = round(volts, places)
return volts

# Function to calculate temperature from
# TMP36 data, rounded to specified number
# of decimal places.
def ConvertTemp(data, places):

# (approx)  Temp    Volts
# 0         -50     0.00
# 78        -25     0.25
# 155       0       0.50
# 233       25      0.75
# 310       50      1.00
# 465       100     1.50
# 775       200     2.50
# 1023      280     3.30

temp = ((data * 330) / float(1023)) - 50
temp = round(temp, places)
return temp

# Define sensor channels
light_channel = 0
temp_channel = 1

delay = 5

while True:
# Read the light sensor data
light_volts = ConvertVolts(light_level,2)

# Read the temperature sensor data
temp_volts = ConvertVolts(temp_level, 2)
temp = ConvertTemp(temp_level, 2)

# Print out results
print "--------------------------------------------"
print("Light: {} ({}V)".format(light_level, light_volts))
print("Temp:  {} ({}V) {} deg C".format(temp_level, temp_volts, temp))

# Wait before repeating loop
time.sleep(delay)
``````

Here are some (admittedly difficult-to-look-at) pictures of my connections:

Is there anything immediately apparent that seems wrong? The light sensor seems to work (admittedly, it's harder to tell just how bright the room is in a quantifiable way like it's possible to do with temperature) but the temperature sensor is off.

EDIT: Changed pin to 3.3v and adjusted values accordingly

I'm afraid it all looks fine to me.

I would try two test readings on the TMP36 channel.

1. If you connect ground as an input it should return -50 degrees centigrade.
2. If you connect 3V3 as an input it should return 280 degrees centigrade.

If it does that shows the software is correct which suggests either a problem with the TMP36 or the wiring.

• Well, the values are as you said they'd be with those wiring configurations. I tried switching out the TMP-36 for the others I have and all gave the same temperature range. I also swapped out the MCP3008 with the same result. Could this be a problem with the breakout board I'm using? – nerdenator Jan 22 '18 at 0:07
• Does the temperature vary if you heat up the TMP36? Perhaps a hair dryer for a few seconds. – joan Jan 22 '18 at 8:16
• it definitely changes if i squeeze my thumb and forefinger around it. – nerdenator Jan 22 '18 at 14:59
• The only thing I have to suggest is to double check that the voltage being fed to the MCP3008 is the same as the voltage you expect (3V3). – joan Jan 22 '18 at 16:47

As it turns out, putting a .01uF capacitor between the ground and vout of the TMP36 resolved this.