Using Adafruit Trinket Pro 5v to power 3.3v RFID reader

How should I power the RFID reader with the 5v Pro Trinket? I was going to use a (3.3V 950mA LD33V LD1117V33 Voltage Regulator) would that work or do i need resistors?

Also I will be powering the trinket with a 9v powerpack (6 AA batteries) What would I need to correctly power this project?

How I planned on doing it was

9V (supply) -> adafruit pro trinket 5v -> 5v servo (Hitec 31055S HS-55) and 3.3 RFID reader (desc below)

This is my first project with the arduino and I want to make sure i get it right. The project is pretty simple. Read RFID unlock drawer. I already wrote the sketch and tested it on my UNO board. Just need to understand how todo the voltage drop safely and correctly.

RFID reader (

Thank you!!!

You need to have a re-think on the equipment you plan to use. The first issue will be current draw - the Pro Trinket can only supply 150mA from its 5v regulator, your servo has an no-load draw of 150+ mA and you want to power an RFID as well.

The second issue is of level conversion for 5v to 3.3v parts. Apart from powering the RFID at the correct voltage the signal lines will need conversion too, and in two directions (3v>5v and 5v>3v).

The best source of information is the datasheet, it will tell you how much current each item can supply or consume.

Have a look at alternative boards, but bear in mind that your overall power requirements may necessitate a separate power supply for the peripherals. Level conversion can be done with simple components and a bit of soldering, or with an off-the-shelf board - there are plenty to choose from.


So what I ended up doing was getting a step down converter to take the 9v down to 5v and then made a power bus for all the devices to connect to that, so the adafruit would not have to supply the power to anything but itself. Everything seems to be working great except getting the reader to 3.3v

What i did initially was I took resistors and split the 5v input. ( 5v -> 2k -> (3.3v out) -> 1k -> ground) This worked great when it was solo.. However after I got everything soldered up and turned it on the first time the RFID reader was only getting about 2.1v when it was connected. If I disconnected the power from the board that pin would go back up to 3.3v? What would make that happen?

My other option i was looking at was getting the Bi-Direction Level shifter from adafruit But they only come in 4 path or 8 path. I would like to use the 4 path but I am not sure i can find a way to make that work with the RFID reader since it uses 5 connectors (plus power).

Using a voltage divider for power isn't a good idea, too little current provided on the output. You really need a 3.3v regulator that can handle your power requirements. If you want to the job properly then the 8-channel converter will be needed, though some would argue that you only need to convert the 5v downwards as the 3v won't hurt the 5v side.

You might be able to use a series of voltage dividers for the signal lines, most of the forum post I've read say it can be done (example here: Level shifting with a voltage divider - General Electronics - Arduino Forum) but you'll still need to get a 3.3v supply for the RFID power whichever route you choose.

If you are sending a signal "downhill", i.e., from 5 v. to 3.3v. You just need a voltage divider. If you are sending a signal from the 3.3 v. to the 5 v. side, you need a level shifter. (Some 5 v. chips seem to read 3.3 high, but I wouldn't rely on it.) You can make your own from just a 2n7000 MOSFET and 2 10 K. resistors.

Just google up "level shifter", and you'll happen across the circuit.

Considering the fact that we'll probably be mixing 5 v. and 3.3 v. hardware for quite a while into the future, I bought a lot of the simple parts needed, including 3.3 v. regulators. when you buy 20 or so at a time, and plenty of resistors/MOSFETS, the prices are pennies per level shifter.