RX/TX leds on custom board.

Hi all.

I have a custom board that I’m putting together to allow me to use and attiny85 chips and selectable speed ( 16 & 20 Mhz ).

From the schematics ( UNO ) I notice the atmegaXX controlling the USB also controls the TX/RX leds. Is there a valid way to add these without messing up serial transmission ( have read about people needing to unplug devices before upload works ).

Would a transistor ‘switch’ be suitable? Or would it create noise on the line/be too low-impedance?


Just connect 1 LED and a resistor to the Rx line and 1 LED and resistor to the Tx line.

Thanks for the reply, I have attached a diagram, is it what you are intending.

yes, but higher resistance, like 1K, so there is a lower load on the line, especially for whatever offboard is driving the Rx line.

I've read this is one way to do level shifting. by using a blue LED (forward voltage is 3.3v) connected to rx/tx pin to gnd, and without using a series resistor and just using the 10k internal pullup. will this really work? I'm thinking of using it to level shift rx/tx for xbee and scl/sda for i2c (bidirectional level shifting)

Yes pYro_65, that will work but as i said and CrossRoads said use a 1K resistor. Will not work doughboy and it may damage something. Always use a series resistor with a LED and use transistors or a logic IC for level shifting.

how about adding a series resistor then the led and tap between the resistor and led to go to the 3.3 logic level circuit. I can't think of any reason this will not work on one way level shifting. I have to test this on two way level shifing. A 3.3v zener diode can be used in place of the blue LED. I'm sure there are other creative ways to solve a problem by thinking out of the box.

Thanks for the help guys, I'll start with a 1K, do up a sketch to vigorously test the serial then go larger if I receive more than an acceptable count of errors ( hopefully none, not sure of serial standards ).

You could always add a simple transistor driver circuit or logic buffer IC for the LED`s, if you want them to be brighter.

Personally I would use a buffer chip so there are no question about will it work or not. That said the schematic you showed with a resistor and LED should do the job.


I was originally planning to use a transistor circuit, is this not the same as what is in a buffer IC. I have not wanted either as not enough room, however I can squeeze it in now I think. Going to see what is available at jaycar ( might get bigger PCB too ).

I think I might have a use for a buffer now, I can use it for the serial lights, pin 13, and one RGB led I want attached to 3 pwm lines.