Simple Circuit Breathing LED

[Zack91]

Hi everyone,

So I found this (https://makezine.com/projects/add-a-simple-circuit-breathing-led-effect-to-your-pumpkin/) tutorial on Make for a breathing LED circuit using a TL071 OpAmp.

I've built the circuit as per the schematic but my LED won't turn off all the way.

I've asked the author on the tutorial but he hasn't replied.

So my questions are:
1. What do I need to change to turn off the LED completely?
2. If I want to change the power supply to 12V, what changes need to be made to the circuit?

Thanks!

 

[Harald Kapp]

The TL071 is not suited for this application. Its output can only go to negative supply + 1.5 V (typically, worst case +3 V!). Therefore with a unipolar power supply the lowest output voltage reachable is between 1.5 V and 3 V which will turn on the LED.
You could use an operational amplifier with so called rail-ti-rail output to get 0 V output voltage.
A much simpler solution which allows you to use the TL071 is to simply put 2 LEDs in series. This will increase the turn-on voltage to a value higher than the lowest output voltage of the operational amplifier.

For 12 V operation no changes are necessary. You may want to tweak the value of the 470 Ω current limiting resistor in series with the LED to adjust the brightness. If, on the other hand, you put 2 LEDs in series, even this tweak may not be necessary.

 

[Zack91]

Wow, so easy! Should have paid more attention in class. Thank you very much Harald! Much appreciated.