Arduino Blinker Tester

Thread Starter


Joined Feb 26, 2011
Hello AAC Forum,

Have a couple of old Arduinos which could be useful. So decided
to put together a test circuit using a 555 timer as a signal to the

The sketch is attached but basically it says: When pin A0 is getting a signal turn pin 9 HIGH then LOW.
Then repeat at pin 10 and then pin 11.
It works great except after a few minutes the LED connected to 555 pin 3 burns out which makes the signal
to the Arduino stop and the LEDs attached to the Uno pins 9, 10, and 11 stop working. So I tried a 1500 ohm
resistor but had the same result. So a 10k resistor was substituted but although the LED seemed to last a little longer
it finally gave up the ghost.
It looks like too many amps are being pulled thru the LED.

Is there a way of preventing the LED at 555 pin 3 from burning out?


Allen Pitts
Dallas TX



Joined Mar 30, 2015
It looks like too many amps are being pulled thru the LED.

Is there a way of preventing the LED at 555 pin 3 from burning out?
What you described doesn't make sense. 10mA isn't going to burn out a typical LED. Something else is going on.

BTW, the wiring diagram for the 555 timer is a pain to read. Most would be able to determine circuit function from the following with a glance:


Joined Jan 8, 2017
You are trying to pull input A0 to about 10 volts positive. (Assume + 12 output from pin 3 on the 555 less the voltage across the LED (about 1.8 volts for a red LED.) This current must must be passing through the input protection diode on the ATMEGA328 input. As there is no option to provide a pull down resistor (Only a pull up option is available.) on the ATmega input it must allways be seeing A0 as a high signal. (So there would be no changing input on the Arduino. As your circuit works at all I think you have the LED connected in the REVERSE polarity to that shown on your diagram. I think this way the output of the 555 when low will pull the A0 input down from + 5 volts to about + 1.8 volts. Now when the 555 output is high the LED is reverse biased with about 7 volts. (12 volts - 5 volts.) This will probably exceed the reverse bias rating of the LED causing it to fail after a time. For your circuit to work as shown you would need to add a pull down resistor on the A0 input.


John P

Joined Oct 14, 2008
Mixing devices running off different power supplies is a thing that you need to be careful about. That first diagram certainly shows a circuit that shouldn't be used, though I wouldn't want to predict how it's likely to fail.

And when you have an Arduino, it's all totally unnecessary. Connect your 12V to the Arduino alone, and then take 5V from the Arduino's internal regulator and use it to operate the 555. There isn't much power available there, but it should be enough.

panic mode

Joined Oct 10, 2011

btw, to protect LED from burning, make sure it is protected. 1K resistor you have is good enough to limit current but.... during times when LED is reverse biased, voltage across the diode is exceeding save limits. so the ED fails when it is not lit.

typical LED will have forward voltage drop of 1.8...3.6 V
but it will only tolerate about 5V in reverse.

to see this connect second LED antiparallel to it. that way LEDs are protecting each other - one that is forward biased limits reverse voltage for the other. you can of course use signal diode too (1N914 for example).


Joined Aug 21, 2008
To that I would add that some LED manufacturers say it is ok to have "no more than 0 volts reverse bias", so unless you have a specification saying otherwise, it is safest to not allow any reverse bias.