Why does power slump when 8 I2C Temp Sensors are activating a 24V Electro Mechanical Relay

Parts :
8each Adafruit ADT7410 I2C temp sensor
1 each TCA9548A Adafruit 8-1 Mux
1 each RPI 3 B+
1 each '244 octal buffer
1 each 953-1C-24DGF-2 24V EM relay (expand to 8 when functional)
1 each Homelylife (Amazon) DC12V to 24VDC transformer, 3A
1 Power Supply (3.3V, 5V, 12V)

8 digital I2C temp sensor modules from Adafruit are input into a multiplexer and each temperature is displayed on the screen. When GPIO5 is high, it switches the 24V EM relay to gnd thereby turning it on. When relay is on, it turns on 120VAC to the load.

Good:The 8 temp sensors work fine by themselves.
Good:The relay switches on and off fine when controlled by GPIO5, by itself.

Not Good: However, if I turn on the 24V EM relay thru GPIO5 then the moment I start reading the 8 temps, the relay turns off. I don’t know why , my guess is that perhaps the circuit is unable to provide the surge current needed ? I’m not using bypass caps. not sure the value and where they are needed.

ADT7410 High Accuracy I2C Temperature Sensor Breakout Board : ID 4089 : $5.95 : Adafruit

I2C Mux

12V to 24V 3A transformer

24V EM relay

Detailed Circuit layout shows 8 temp sensors and just 1 EM relay to drive the AC load thru a transistor (BC337)
debug.pdf (1.2 MB)

Please advise how I can improve the circuit so it works properly.

1 Like

Hi @Rob199027, I’ve had a look at your schematic and nothing jumps out as an obvious problem. Might you upload your code? Perhaps there is an insidious bug hidden in there.
The power requirements of the I2C hardware shouldn’t be problematic, each sensor would draw such little current.


Hi Rob

Why does power slump

Broadly this will happen when a power source will not supply the required current.
This can happen in 2 ways.

  1. Something is calling for more current than is expected due to some external cause or a fault.
  2. The power supply has not got enough current capability in the first place.
    You don’t provide any details of that 3 output power supply.

You might have to do a bit of voltage probing here.
The first question here is exactly what is that octal buffer and why do you need it. Need the IC type to look at data sheet.

Assuming you need the buffer what is that 256Ω “pull up” resistor.

When it fails the questions asked should be.

Is the relay driver transistor turning off

If so why. Is there any output from the buffer. Is there any input to the buffer. Is there any output from the GPIO. Is the 5V sagging to the point where the add on components no longer work.

A bit of intelligent sleuthing with a multimeter will probably tell the story.

If you are sourcing the 5V for all the add ons from the RPi this source may run out of grunt when all 8 of the sensors are powered up. It would appear the power supply has a 5V output. You might do well to source the 5V for all your add on bits from there.

A bit of advice. DON’T start throwing capacitors all over the place without knowing where to put them and why. Quite often you can do more harm then good by doing this in and indiscriminate manner.
Cheers Bob


Hi Rob.
Just another thing I noticed.
You have not shown any ground connection from sensors to Mux to RPi. They are there aren’t they? They need to be.
Cheers Bob


I just found out the SW, written by someone else, was turning off the GPIOs as part of the initialization.