Can I Power 4x LEDs and 4x Phototransitors off Sparkfun's Arduino Pro Mini 528?

Hi all,

Beginner here. I’d really appreciate some advice. Can you tell me whether the setup I propose will exceed the amperage limits of the Arduino Pro Mini 528? I have the 5 volt version. https://www.sparkfun.com/products/11113

I’m doing a project involving an optical slot sensor that contains an IR LED and an IR phototransistor. When an object is put in between the slot, the phototransistor outputs a voltage that corresponds with the amount of light transmitted across the slot distance.

The sensor in question is the Omron EE-SX1140. Relevant links directly below

Manufacturer datasheet: http://www.omron.com/ecb/products/pdf/en-ee_sx1140.pdf
Digikey page: http://www.digikey.com/product-detail/en/omron-electronics-inc-emc-div/EE-SX1140/OR1034-ND/1789461

I want to hook up 4 of these sensors to my Arduino Pro Mini 528. This essentially means that I am hooking up the following to the Pro Mini board:

-4 LED’s running off the VCC pin (I am using a 191 Ohm resistor in front of these. They draw 20 mA with this configuration) I am sinking these into ground

-4 phototransistors running off their own digital output pins (2, 3, 10 and 11). I am branching the emitter prongs on the transistors with a 68K Ohm resistor to ground, and sending the analog voltage to individual analog input pins (A0 through A3)

Please let me know if I am in danger of frying my board and how I can improve

I made a quick mockup below. I tried to make it as clear as possible

http://i.cubeupload.com/y2DVwd.png

Your math for current draw of the leds with 191 ohm resistor in series is wrong as far as I can tell - I'd expect 20mA per LED (0.02 A), give or take a few mA from that spec sheet.

So you're looking at 80mA.

If you're powering it off regulated 5v (ex, from FTDI adapter - you can get 500mA out of USB typically, or regulated 5v supply connected to Vcc) , you're golden.

If you're using an external power supply with a higher voltage feeding the RAW pin, the limit is lower; you're limited by the heat dissipation of the regulator. You typically need at least 7 volts going into the regulator to get 5v out, and the more you have in excess of that, the less current it can supply continuously due to heat. The exact limits (on current, and minimum voltage) depend on the exact regulator used on the board - the vast majority of pro mini's are clones (of course they are, the clones are $2 each!), which probably don't use the same regulator that the official ones do.

I think the LED will need more like 30 mA instead of 0.2 mA. You can drive it up to 50 mA as long as the ambient temperature is less than 25°C.

^ Whoops, I originally wrote 0.02 mA for the LEDs, but really mean to type 20 mA.

Here is the math I used to get to 191 Ohm (closest resistor available for purchase on Digikey)

190 Ohm = (5.0 volts - 1.2 Forward volts) / 0.02 Amps.

I can’t figure out how to get this image to display on the forum. It provides some context for the component data, and where I got the 20 mA.

Also tested it with a power supply. It indeed drew 20 mA


(Edit - you were close! Moderator)

Can you tell me what the Arduino limits are exactly for the Sparkfun Pro Mini 5V?

It would put me at ease if I had a better understanding of the constraints

-How many amps are you allowed to pull off each pin?
-How many amps can you pull off the entire board?
-How many amps can you sink into the ground pin?

I've been Googling around but can't find a clear answer.

Thanks again

DrAzzy:
So you're looking at 80mA.

If you're powering it off regulated 5v (ex, from FTDI adapter - you can get 500mA out of USB typically, or regulated 5v supply connected to Vcc) , you're golden.

If you're using an external power supply with a higher voltage feeding the RAW pin, the limit is lower; you're limited by the heat dissipation of the regulator. You typically need at least 7 volts going into the regulator to get 5v out, and the more you have in excess of that, the less current it can supply continuously due to heat. The exact limits (on current, and minimum voltage) depend on the exact regulator used on the board - the vast majority of pro mini's are clones (of course they are, the clones are $2 each!), which probably don't use the same regulator that the official ones do.

-How many amps are you allowed to pull off each pin? 20mA.

  1. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
    ATmega48A/PA/88A/PA/168A/PA/328/P:
    1] The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150mA.
    2] The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150mA.
    If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current greater than the listed test condition.
  2. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
    ATmega48A/PA/88A/PA/168A/PA/328/P:
    1] The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100mA.
    2] The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100mA.
    3] The sum of all IOL, for ports D0 - D4, RESET should not exceed 100mA.
    If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater than the listed test condition.

-How many amps can you pull off the entire board?
-How many amps can you sink into the ground pin?

Onboard regulator is only good for 150mA. If you have offboard VCC, than the chip can control 300mA within the limits above.
With offboard VCC source, I'd keep the total current thru the board to 300-400mA (with VCC coming on one and going thru the chip and other Vcc/Gnd pins).