trouble with power

Hey there folks. I'm making an ultrasonic range finder with a 4 digit 7-segment display powered by an arduino duemilanove with a ATMega328. I eventually want to get it working with the microcontroller off the arduino.

The components I'm using with the arduino are:

  • Common Anode 4 Digit 7-Segment Display
  • driven by 4x 595 shift registers
  • Parralax Ping Ultrasonic Rangefinder (I may eventually trade it out for an EZ0)

The rangefinder only works when I have a 5V supply voltage. I can get the whole unit to work if I power the display off of the 3.3V output and the Ping off of the 5V output on the arduino.
Is it possible to power it all off of just the 5V output? When I try, the display works but the ping doesn't. My meter reads 4V across the Ping. This is because of all the power my display is drawing. I know this is all caused by my newbieness. I've tried a putting in a 7805, but it dropped down to 2V. I also tried putting in a cap, but that didn't help either. Any ideas?

I would use an external 3V regulator to power your display.

There is a 3V regulator on board that could separately power the display but you could run into current limitations, how much current does your display take?

Another option is to find a spare Power Supply out of a PC and turn it into a benchtop unit,

PC PSU's have most commonly


and some have others as well.

I recommend using a PC PSU for most of my projects involving TTL CMOS Logic and Programmable units because the regulation on these units is very stiff, and changes according to the load placed on the unit.

They also offer superior output short circuit protection should you make a mistake.

after the projects are finished, if they need to go mobile, then i switch to the 78xx series and Zener Diodes depending on power needs.

Thanks for the replies. The problem though is that even though I have a regulated 5V supply, I can't get the display and the rangefinder working on the same 5V power supply.
The datasheet says the display draws 20mA per segment, so eight segments per number (including the decimal) that's 32 segments. So max: 640mA, a little more at peak when they turn on. Sounds like a lot to me. I think I messed up my meter though, so I need a new one, so I can't check it right now.
The display works fine at 3.3V off the arduino's output while the 5V runs the Ping. So obviously the current's there.
I'm trying to figure out how to regulate the ping without drawing all the power from the display. :\

a little more at peak when they turn on.

No they are not like light bulbs what they draw is what they draw. However if you can't get the two working at the same time off the same power supply then you might have to increase the amount of decoupling you have.

The datasheet says the display draws 20mA per segment, so eight segments per number (including the decimal) that's 32 segments

But aren't you multiplexing the display?
If so, then you shouldn't be drawing more than 8*20mA = 160mA at any one time.

Thanks Mike. Your web site is really helpful. I'm going to give it a try.

Well, I tried de-coupling the ICs and that didn't work. Finally, I tried just putting some two 10 Ohm resistors on the anodes of the display and suddenly it worked. I didn't think of it until now since the display was rated at 5V. Thanks for your help guys.