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FEZ Domino Output Power


#1

We ran into a rather strange problem with a FEZ Domino. We were using a benchtop power supply to provide 9V to the FEZ. The FEZ was only powering a small 4x20 LCD and reading values from two pressure sensors.

Total consumption of the system was 0.2A as shown on the power supply.

We couldn’t get a relay rated for 3V to trip and measured the digital pins, and we were getting anywhere between 1.5V and 2.5V on the digital out pins. We also tried using the one analog out pin to send 3.3V but again, we were only seeing 1.5V out.

What is the allowable draw through the FEZ? Why would we see such low voltages coming out of the digital pins and the analog out? I couldn’t see anything in the spec sheet that would indicate why this is a problem?


#2

This is expected and normal. You can’t use a digital pin on any micro to control a relay directly, not FEZ not any other :slight_smile:

Google “relay control microcontroller” and the first hit is http://www.dnatechindia.com/Tutorial/8051-Tutorial/Interfacing-Relay-to-Microcontroller.html


#3

Ok. So aside from the relay problem why would we see such low voltages on the output?


#4

I guess the other thing is I am looking at my Arduino kit, and it was shipped with a relay. I quickly just breadboarded it up, and it trips the relay just fine?


#5

Maybe it worked but that is terrible to the micro. The inductive load will damage the micro sooner or later.

You used arduino? this is the very first google hit for “arduino relay” http://www.arduino.cc/playground/uploads/Main/relays.pdf

See even arduino ask you to no connect it directly :slight_smile:


#6

Ok no problem. So that’s cool, but the larger point I am asking is:

Why the low voltages from the FEZ?


#7

Because you are doing something very wrong!


#8

The current that you are pulling out of the micro drops the voltage.

Ohms law.

Cu Wim.