Power advise

I'm designing a circuit that will have:

  • ATmega328 running at 3.3V
  • 2x 74C923
  • 2x PCF8574
  • 30x MX Cherry switches, each with a currant rating of 10mA

This PCB will then connect via a ribbon cable to a couple of PCBs each holding 6x MAX7219, with 7-segment LEDs, 1x 2-digit, 1x 4-digit, and 10x 3-digit.

My question is can someone advise on the power requirements for this circuit and hopefully give me a link to a web page that shows me how I would design the power into the circuit. My initial plan was a DC power jack connected to a 7805 regulator with a capacitor on both the input side and the output side. But, as this would supply 1 AMP I doubt that will be sufficient. I'm hoping someone can advise please.


What is your electronics, programming, arduino, hardware experience?
Is this a school/college/university project?

You will need to get the specs on the LED arrays that you are using as they will be the majn part of your power consumption.

Tom.... :slight_smile:

Seven segment displays use very little power these days, as most LEDs are very efficient. A 5V adaptor to the USB port of the Arduino will work in most cases. Those are available in 2A versions.

The 7-segment LEDs I'm using each have a forward current of 20mA. So, that's 24 x 20mA which is 480mA. The MX Cherry switches each have a forward current of 10mA, using a total of 300mA. So, assuming the 7805 giving 5V and 1000mA should be sufficient?

I'm quite new to Arduino, but I have 20 years programming experience with C and C++.

The MX switch spec is the maximum current they are rated for. In typical operation, they'd be handling under 1mA each, since you're now powering anything through them or anything.

The forward current rating on the 7-segment LEDs is probably the maximum current you can put through each segment (20mA per LED is very common spec) - however, the actual current going through them is set by the series resistors you use (ohm's law) - I often run 20mA rated LEDs at 2~5 mA, not to save energy, just because they're too bloody bright with the full 20mA going through them,.

I'm running (or will be) the 7-segment LEDs via MAX7219 driver ICs. I have a 16K ohm resistor connected to P18 [lSet]. So, I recon the power output from a 7805 will be ok for my whole setup.

“A 5V adaptor to the USB port of the Arduino will work in most cases. Those are available in 2A versions.”
Just have that problem of the 500mA PTC fuse after the USB connector.

MAX7219 also needs 3.5V minimum for a High with its Vcc at 5V. Will need something to boost the Arduino’s <=3.3V outputs up to a higher level.

With MAX7219, each chip only turns on 8 of up to 64 LEDs at once, so your current need is a on the high side.
With the switches, inputs to the Atmega? Use the internal pullup resistor, switch will only pass
3.3V/30K = 0.1mA when closed. wire switch to connect pin to Gnd when closed.

74c293 is an obsolete part?