I've built a standalone Arduino and upload sketches via this method From Arduino to a Microcontroller on a Breadboard. I would like to hook up an LED across TX like on the Arduino board, so I simply wired it to the TX pin with a resistor going to ground. And it works on a sketch where I am simply sending serial data at a 9200 baudrate. Both the Arduino TX and my ATmega TX LEDs blink correctly.

However, my application involves a baudrate of 19200, but when I upload this sketch, the TX pin on my ATmega stays on constantly, even if no serial data is being sent. What's even more interesting is that the Arduino TX led does blink correctly for the same sketch. Can anyone help?

  • \$\begingroup\$ Can you show us how you wired your LED? Or at least tell us what exact pin it's wired at? Technically, the pins you are using are not the ones from the serial interface (pins 2 & 3), so you're most likely using some other pin that are used during ISP programming (MISO, MOSI etc). \$\endgroup\$
    – Ricardo
    Aug 9, 2014 at 14:45
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    \$\begingroup\$ If you were using the TX serial pin, you'd have to wire the LED to Vcc (with the series current limiting resistor) and not ground, as UART (serial interface) is idle HIGH (i.e. they stay at Vcc levels when nothing is transmitted). So, knowing what pin you wired your LED with will be useful. \$\endgroup\$
    – Ricardo
    Aug 9, 2014 at 14:46
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    \$\begingroup\$ @ZackB - if you look closely at an Arduino, you will see that the TX and Rx LEDs are not connected to the ATmega's Tx and Rx pins. On an Arduino, they are driven by pins on the USB to serial interface chip. I have not looked at the specs, but you might find at high baudrates, or long cables (or other things with reduced drive signals) that the communications start to become unreliable because the LEDs put an extra load on the connection. You might want to consider driving the LEDs indirectly with a MOSFET or darlington transistor. \$\endgroup\$
    – gbulmer
    Aug 9, 2014 at 14:54
  • \$\begingroup\$ @Ricardo I wired the led to vcc and I get no activity whatsoever. I'm not really sure how to post a visual of how the led is wired, but it's connected to the tx pin on one end, and a resistor to ground on the other. \$\endgroup\$
    – ZackB
    Aug 9, 2014 at 15:00
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    \$\begingroup\$ @Ricard In contradiction to my previous comment - After correctly wiring to Vcc, the led does work! Thanks. (I forgot that leds are really just diodes) \$\endgroup\$
    – ZackB
    Aug 9, 2014 at 15:06

5 Answers 5


Caveat: Even though the OP accepted my answer as the best one, another, better answer was posted after that, that you may want to read before reading mine. As noted by Chetan Bhargava, my solution may draw too much current to drive the LEDs from the serial lines.

Below is part of the schematic of a RS232-to-UART converter that I've made. In it, I connected LEDs (and their respective series limiting resistors) from the RX and TX lines to the Vcc line, just the way you could connect yours. Wire the anodes to Vcc and the cathodes to the TX/RX lines, with the current limiting resistor in series (either before or after the LED).

RX and TX LED connections

The LEDs must be connected to Vcc and not to ground because UART lines (i.e, the ATmega serial interface) are idle HIGH, i.e., they stay at Vcc levels when nothing is transmitted.

Note what gbulmer said in his comments, though:

... you might find at high baudrates, or long cables (or other things with reduced drive signals) that the communications start to become unreliable because the LEDs put an extra load on the connection. You might want to consider driving the LEDs indirectly with a MOSFET or darlington transistor.

I have had no problems with those LEDs attached to the serial lines up to 78600 bauds, but you might if you go faster.

If you were interested in connecting indicator LEDs as feedback in your ISP programmer, you could do the following. The ArduinoISP sketch (firmware) already drives three indicator LEDs:

  1. Heartbeat on D9: it blinks (fadding) to show that the sketch is working properly;
  2. Programming on D7: it's on when the actual programming is taking place;
  3. Error on D8: on when something goes wrong.

These indicators work perfectly with the ArduinoISP sketch.

To wire these LEDs, use the schematic below:

ArduinoISP shield

The schematic is for an Arduino Shield that I've made for programming ATmegas and ATtinies, for use with the ArduinoISP sketch. I hope this helps.

If you really want to attach LEDs to the transmitting lines, please answer the questions I posted as comments, then I'll update my answer.

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    \$\begingroup\$ @Ricardo - is the first diagram really part of the schematic, or have you just sketched something to answer this question? I think it is broken, and can not work as shown. The schematic shows Rx connected directly to +5V, so it is very unlikely that would work. Also, would you show the value of R1 and R2. Further, that is an R@232 to UART and the question does not mention RS232. \$\endgroup\$
    – gbulmer
    Aug 9, 2014 at 18:41
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    \$\begingroup\$ @Ricardo - okay, that's better. However, I now read the circuit as having a nearby piece of electronics which has done the RS232 signal level to 5V UART signal level conversion. That is exactly not what I am warning ZackB about. A local piece of electronics to act as a buffer and hence drive the electronics is what I suggested. I am relaxed about how the problem is approached. However, I think your circuit is not evidence that ZackB's circuit is robust and reliable. I'm just trying to be clear to minimise confusion. \$\endgroup\$
    – gbulmer
    Aug 9, 2014 at 19:09
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    \$\begingroup\$ @Ricardo in figure "RX and TX LED connections" (Figure 1), wouldn't there be a lot of current be drawn from RX and TX lines to drive the LEDs? \$\endgroup\$ Jun 16, 2015 at 21:48
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    \$\begingroup\$ @Ricardo it all depends on amount of current drawn and TTL output driver on the TX and RX lines. By looking at some FTDI cable datasheets, it says that they can sink 6-8ma. Not sure if drawing 5ma from the pins is a good idea. \$\endgroup\$ Jun 16, 2015 at 22:28
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    \$\begingroup\$ @Ricardo yes, it would be possible with BJT with high hFE and also possible with MOSFETs. \$\endgroup\$ Jun 17, 2015 at 18:46

In general it is not a good idea to draw a lot of current from your Rx and Tx lines to drive LEDs. The extra current draw can reduce your Fan-out to your target.

To avoid extra current draw from your lines, you can use a dual OpAmp like LM358D to drive those LEDs.

enter image description here

LM358 TI

The high impedance of the OpAmp inputs will not draw much current from your Tx and Rx lines and will preserve the fan-out on the lines.

I have simulated this schematic in LTSPICE and physically verified with LM324 (TI) on a breadboard. Sorry did not have a LM358D handy but they work similarly.

Edit: Thanks to pointers by arudino.tyro. I was able to try out the new schematic with LM358D. Old schematic is here: http://i.stack.imgur.com/GHauy.png

  • \$\begingroup\$ If I am not mistaken this is op-amp used as a buffer? Is that how it is called? \$\endgroup\$
    – 71GA
    Nov 21, 2015 at 11:10
  • \$\begingroup\$ What is a typical value for R1 and R2? What is the typical output current at 1 & 7 assuming 5V Vcc and 5v TTL? I think the maximum current rating of 1 and 7 are 40mA. But what is the typical value with the given values? Thanks \$\endgroup\$ Feb 6, 2016 at 8:42
  • \$\begingroup\$ @arudino.tyro the values of R1 and R2 depend on the current rating of the LED, the amount of current you want to pass through the LED and (or) how bright you want LED to be in on state. You can google for LED current calculator. \$\endgroup\$ Feb 7, 2016 at 2:54
  • \$\begingroup\$ @ChetanBhargava, I used your circuit and it remains on all the time. I believe I've found the problem: TTL levels are true low if the voltage is 0.7V or lower, and they are high when they are 2V or higher. So with your circuit you "compare" two voltages. One is the TTL level at IN- and the other is at IN+. If the comparator output just should be the (inverted) state of the signal lines, then you should compare them with a fixed voltage between 0.7V and 2.0V. Here is the problem: You compare it with the output. (OUT is connected to IN+) \$\endgroup\$ Feb 11, 2016 at 7:35
  • \$\begingroup\$ @ChetanBhargava, If the output is high (5V) then the IN- input should be higher as 5V to make the output switch. The same is when the output is low, then you need an input voltage lower than 0V for the output to switch. Problem solved by 1. disconnecting OUT from IN+, 2. supplying 1.5v from 5v via a voltage divider (or probably a Zener diode) . I'd be thankful if you could fix your schematics. Thank you \$\endgroup\$ Feb 11, 2016 at 7:37

I can not imagine, that the circuit posted by Chetan Bhargava works correctly since when I simulate it the LED is as pointed out by arudino.tyro in a comment constantly on. Moreover the behavior is different in comparison with the simple resistor plus LED circuit proposed by Ricardo.

UART is HIGH when IDLE, so you have to turn the LED on when the bus is LOW. You might expect to see some blinking with the "wrong circuit", when a transmission is ongoing, because the LED is still toggled, right? However, I think, this is too fast, that you can see it with your eyes (e.g. 9600 kHz).

My proposal to solve this problem is the circuit below. It is basically a voltage follower (non-inverting amp. with a gain of 1), but the output is connected to the LED circuit, which is hooked up to 5 V. This results in current flowing through the lamp once the input is negative.

UART LED schematics.

According to LTSpice current is only flowing through the LED when a transmission is ongoing (you see this as a "longer ON state" when transmissions are ongoing). If there is a small pause between a couple of transmissions you will see some blinking (LED turns OFF).

UART LED simulation.

  • \$\begingroup\$ I build up my circuit on a board for I2C and UART. I2C, which I simulated as well, one does only see longer transmissions or if the bus is corrupt (which is nice as well). You are welcome to check it out: github.com/m3x1m0m/UppSense2017/blob/master/pcbs/backplane/… With your circuit the LED might blink as well, but it is constantly on when no transmission is ongoing, because UART is HIGH in idle mode. \$\endgroup\$
    – m3x1m0m
    Nov 14, 2017 at 7:48

@Chetan Bhargava your circuit will always be on all the time. Because UART is HIGH in idle mode. To avoid this problem Please change the direction of LEDs. As shown in the picture given below.

enter image description here


I think this will work


simulate this circuit – Schematic created using CircuitLab


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