The joy of incorrect specifications

A project log for Portable Trollmaster 3000

Spreading the voice of Eduard Khil

Dr  SalicaDr Salica 12/02/2014 at 22:152 Comments

It's nice to have a fun idea that seems doable (see my previous log entry), but now I need to be a little more concrete. All I have for the moment is an Arduino Uno, so I need to order a Trinket pro. But wait !? There are two flavors : 5v and 3.3v ??

And so starts the endless stream of questions : which one do I buy ? What is the voltage of the FM transmitter ? Does it matter ? Can I mix voltages ? What about i2c ?

A look at the FMBerry project page shows the following picture :

It seems that the MMR-70 works on 3.3 volts and so is the Raspberry Pi. I should probably order the 3.3v Trinket. But can I use my 5 volts Arduino in the meantime ? Maybe, maybe not.

I found that 5v and 3.3v hardware cannot always be connected together unless the later is "5 volts tolerant". A mere 1.7 volts out of spec might fry the hardware. I also found that some magic device called a level shifter can convert the voltage between two i2c systems. But beware, some shifters are not compatible with i2c ...

So at this point the plan is to buy a 3.3v trinket, wait for the package to arrive, and stop worrying about this voltage stuff.

Not so fast.

I happen to own an old Sony-Ericsson mobile phone compatible with the MMR-70. So I decided to hook it up and check that it runs on 3.3v. I was not pleased with the result :

... ... ... 2.8 volts between ground and pin 11 ... Not 3.3v as the photo with the labelled test points stated ! And so starts the endless stream of questions again.

But in the end I guess it's not so problematic : after all, the FMBerry project works and uses a 3.3v RasPi without problem.

I think I will order a 3.3v Trinket pro and a level shifter just to be sure, but still try to connect the Trinket and the MMR-70 directly.


Ryan wrote 12/03/2014 at 00:35 point
If you look on the other side of the board, there's an ATMEGA32L. This device can work between 2.7 and 5.5V. Since 2.81 > 2.7 and 2.81 < 5.5, it works! You can see the datasheet here:
There are a couple other devices on there that may be more sensitive to higher voltages. I'll let you find those datasheets.

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Dr Salica wrote 12/04/2014 at 15:35 point
Thanks for the tips !
The datasheet for the FM transmitter ( ) states that the operating voltage must be between +2.7 V to +3.6 V . So I guess I can use a 3.3 V signal without frying it :-)

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