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Simple Circuit Breathing LED

Zack91

Oct 9, 2017
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Oct 9, 2017
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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.
image04-620x564.png


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

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Nov 17, 2011
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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

Oct 9, 2017
6
Joined
Oct 9, 2017
Messages
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Wow, so easy! Should have paid more attention in class. Thank you very much Harald! Much appreciated.
 
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