Aref lites up Uno board

I have 3 pairs of LEDs (Red, Green, Blue) coupled to 3 photodiodes.

I'm using a 5 volt reg to supply the 3 LED's independantly from an Aruduino board. I wanted to supply them separately but use the Aref when I use the analog inputs from the Photodiodes mounted opposite the LEDs

I just noticed that when I connect the Aref jumper from the 78L05 Voltage regulator on the LED board the Arduino board power LED lights up too. I did not expect that to happen.

I have not yet plugged the USB cable from the Arduino into the computer. I know I should preload the Aref instruction in the sketch. I just wish to double check before plugging into the UBS port... If I do then it will connect a 5 volt supply from the USB port. Do you know if that is OK?

Procyan: I have 3 pairs of LEDs .... coupled to 3 photodiodes.

You lost me right there. I think this needs a diagram, but that might just be me.

I think this post is in the wrong part of the forum....

sorry, I have 3 LED to PD pairs. Fritz attached

I was advised to not use the 5 volt supply from the Arduino board…so I’m using a 5 volt vreg for the LED’s.

Fritz attached

I meant a schematic…

Here’s his pic inline:


Never mind. I think it is too complicated. I plugged the output from a 78L05 into the Aref and the board lit up. I was surprised.

Now I've sent an old fritz to show the idea of using LED/PD pairs. but they are no longer connected to the PWM pins...their rapid on/off translated right through the op amps and showed up in the monitor as random "off"

I should have redrawn it all before asking...just wondered about the Aref thing

thanks...don't hate me

Procyan: don't hate me

Ok .... wan't going to, though 8)

No I meant I asked too quickly...I'll draw it all up then come back. Just thought it was a simple question about Aref jumper to external 5v supply.

Powering up AREF when the board is powered down is risking burning out your Arduino. Derive AREF only from Vcc if at all possible. Remember to call analogReference (EXTERNAL) in setup() before any call to analogRead() too.

The same applies to any I/O pin. Don't back-power.

You must not power any pin when the chip is not powered. Per datasheet spec, voltage on a pin should not exceed Vcc or go below Gnd (I think they spec half a volt of margin in either direction). The pins are protected with internal diodes to Vcc and Ground, but these are not meant to carry much current. When you apply power to a pin when it's not powered, the protection diode to Vcc conducts, so the board is powered... through the protection diode which is not meant to carry that much current. This is a good way to burn out pins, so don't do it.

So - don't ever apply a voltage to a pin when the chip is not powered, or a voltage higher than the chip is powered with or lower than ground - this applies to almost all IC's. (Obviously, there are exceptions, and this is in all cases described in the datasheet)

OK Thank you. Now i first power on the Uno, then plug in the Aref, then the LED/PD powersupply.

and it works! Glad I didn’t kill that board…got lucky.

many thanks team


Why are you connecting anything to Aref? It seems to me like you are not understanding what the Aref pin is for. Why was Aref not shown in that mess of a diagram you posted?

in that mess of a diagram you posted?

Fritzing diagrams are a complete disaster: slick graphics that obfuscate and confuse everyone.