Powering multiple devices + arduino from one power supply

Hey all,

I’m working on a RFID door lock as a side project. Here is a parts list I’ve purchased for the project thus far:

  1. http://www.adafruit.com/product/923 - RFID reader (5v)
  2. http://www.adafruit.com/products/1482 - tags to read only ID from
  3. http://www.radioshack.com/product/index.jsp?productId=22472146 - servo to rotate deadbolt (5v)
  4. http://store.arduino.cc/product/A000066 - arduino uno as the brain

The problem I’m having is I’d like to be able to power all of these devices from a single power source. My theory for the solution to this problem was to connect all of these devices in parallel and as long as I had a power source that could safely supply the amperage it would work. I’ve attached a (very) crude sketch of my theory. The numbers in the sketch are the pin numbers going to the arduino.

Needless to say this does not work (should it?). By wiring each component individually and testing them one at a time, I can read the ID’s of the tags and print them to the serial monitor, and the servo can do the sweep test provided in the examples, not at the same time.

So my question is, what am I missing ? How do I power the arduino, a servo, and the rfid reader all from a single power source ? I hope to use this information for future projects as well and I know it depends on the project but more generically
How do I power an arduino and N number of components from one supply?

what am I missing ? How do I power the arduino, a servo, and the rfid reader all from a single power source ?

I guess that means you're meeting with some disappointment, there's some trouble. With components allotted to a couple of different supplies then everything is OK? Do you know for a fact that the power supply is dipping unacceptably? Maybe it's about supply capacity (current, regulation). Perhaps there are other issues. Believe it or don't - sometimes a 5v, 100A supply won't be the cure. At what point does the project crater?

Sorry I forgot to specify. What should happen is the reader will read the ID and check if it's one I've specified, if so it will rotate the servo from 0 to 90, delay 1 second, and reset back to 0 (for testing)

What happens when I hook everything up according to my drawing is the reader does not read the ID's of the tags and the servo does not sweep (0 to 90).

However, when I power just the servo from a 5v phone charger, the arduino from usb and the reader from the 5V pin on the arduino they work, meaning the reader will read the ID then the servo will rotate.

Sorry I forgot to specify.

V-USB is good for < 500 mA, and of ten < 200 mA. A servo alone can push that to the limit.
It’s just not a “robust” power source.

I don’t know what the phone charger is rated for, but from what you’ve noted it takes some burden off the V-USB (“5V”).
Have you tried running everything from that?
(Disconnect Arduino from USB.)

Motors, even servo motors, with their surge demands and so on make life interesting.
It’s a guesstimate, but a 5V, 4A (“20W”) supply ought to do it.
You don’t want to be near the edge in your worst-case assumption.

I figured it out, thank you for all your help Runaway Pancake. I wired everything up like my original drawing but didn’t connect the ground of the arduino to the ground of the charger (which was 5v 1A). But everything is working now thanks again !

erosenzweig, you said you wired it as per your diagram but without the ground of the arduino connected to the ground of the charger and it worked. I'm trying this with a PIR motion sensor and a servo, rather than the scanner plus a servo and it doesn't work. I tried it by connecting a 9V wall plug in the arduino's power jack and no servo movement. I also tried connecting the wall plug to a breadboard to the servo and PIR with only the positive going to the arduino and got no power to the arduino (not surprising). I have regulated the servo power to 5V because it can only handle 7.2V. Could you explain how you got your's working another way please?

Hi, this is a very old thread, you would be better to start a new thread with a full description of your project and what it is supposed to do, and what it isn't doing.

Tom... :)