DIY Micro 1S Low Voltage Warning

by kobuki01 | June 16, 2016 | (6) Posted in Projects

Hullo everyone!

Before everything else, these are the skills you need to make this circuit:

  1. Basic knowledge of parallel and series circuits
  2. Decent soldering skills
  3. Patience


So here's my simple-ish solution to running out of batteries if you're using a 1S FPV mulitrotor.

I'll admit, you probably can't mount this on a Cheerson CX10, but if you were making something using a Quanum WISP 110 or a Diatone 110, I think this little circuit I made will be super handy.

I based it of on FPVHobby's 1S 3.5 Low Voltage alarm:


But I'm from the Philippines. And to be quite honest, our customs isn't exactly very friendly to anyone's wallet. So I decided to go searching for a means of making my own version of the thing.


All I wanted to do was find a way to make an LED bulb turn on as your voltage dropped to 3.5v. I'm pretty sure that's what FPVHobby's thing did. And the internet has plenty of solutions but none were simple enough, nor easy to make.


Then I found this circuit on RCgroups:


It was simple enough and the components weren't difficult to find. In fact, all I had to do was rip open some broken laptop chargers and I found the most important component, the "TL431" bit. This is the "switch" that turns your LED on when your voltage drops to 3.5v

I'm no electrical engineer, but I could read. This circuit ONLY NEEDS 5 components:


1. TL431 shunt regulator. It looks EXACTLY like your typical transistor, but it's kinda not. As long as you see the numbers "431" on it, you're good to go. The one I got in the charger was labelled "KIA431A" and it works. If your soldering skills are up for the challenge, you can easily get one from an old power supply or charger.

(Top row: cheap 5 volt power supply, cheap phone chargerApple iPhone chargerBottom row: MagSafe power adapter, KMS USB charger, Dell ATX power supply)

And these images will help you place the TL431 properly in your circuit:


2. A 2v LED bulb. The voltage rating is kind of important otherwise the light will never turn off. Although you can use an LED that's rated for a lower voltage, all you need to do is add another diode in series BEFORE the LED bulb.


RESISTORS: Pay attentions to the colored strips of the resistors

3. 3.9k ohm resistor

4. 470 ohm resistor

5. And lastly a 10k ohm resistor



All these you can either pull out of old electronic junk or purchase from a local electronics shop.



This is my first attempt at assembling the circuit

As you can see here, I didn't have a 3.9kohm resistor. So I put a 3.3kohm and a 680ohm in series to get approximately 3.9k ohms. And I have a 4148 diode right beside the TL431.



Connect all negative (or ground) connections to the Anode pole of the TL431. It'll help.


Now, i know it's no OSD. And i'm pretty sure it's slightly heavier than the one from FPVHobby. But for a DIY solution, I think it's reasonable. And a whole lot cheaper. FPVHobby's was $7.40, mine cost less than $1

How can I then use this for FPV? I could place the LED at the very corner of the camera, where it's peeking out at the very corner of your monitor.

Happy flying and God bless you! :D


danield on June 18, 2016
Good Idea, thanks for sharing.
Log In to reply
Ernesto Bacani on June 23, 2016
Yo! Brother? Where you from the Philippines? I wish we can have time flying together. Nice idea! keep it up!
Log In to reply
kobuki01 on June 24, 2016
Hey! I'm from Quezon City. :D I fly in UP Diliman when i have time on the weekends. :D thanks!
Log In to reply
BManTYA on June 26, 2016
Can you post a pic of the backside, please?
Log In to reply
BManTYA on June 26, 2016
Also, why did you need to add the Diode?
Log In to reply
kobuki01 on June 27, 2016
Hey BMan, I'll try to make another one cause i put hot glue on the back and then covered it with heat shrink. :) the diode is there to regulate the voltage going into the LED. If your LED is rated for 1.8V, it might not turn off at all :)
Log In to reply
kobuki01 on June 27, 2016
* it might not turn off at all without the diode
Log In to reply
BManTYA on June 27, 2016
So if I used a 3v LED or 3v Piezo Buzzer, I shouldn't need the diode, correct? I'm going to attempt this without the board to save even more on weight. Every 0.Xg makes a big difference when you fly micros. Lol. I'm great at soldering and backward engineering (I do IT), but I'm no engineer to calculate things on circuits.
Log In to reply
kobuki01 on June 30, 2016
I'm not entirely sure if a 3v LED will turn on at the right voltage though. I have yet to experiment myself. I'm just waiting for my adjustable power supply to arrive :) but best of luck to you!
Log In to reply
BManTYA on June 30, 2016
Kobuki, check this out.
Log In to reply
kobuki01 on July 3, 2016
I don't completely understand the whole graphic yet. :( but I'll play around with this simulator. Thanks a bunch!
Log In to reply
BManTYA on July 3, 2016
This is the same as yours, but doesn't need the Diode. It uses a 2mA, 2.1V LED. It also uses less Current so it won't drop your run time hardly at all. The 3 pieces in the middle (The triangle (OpAmp), 30k Resistor and 2.5 volt source) are what makes up a TL431. Just consider those 3 pieces as the TL431. The Outer resistors and LED are what makes the most difference. It can even be used as a discharger for bad batteries. It will take a while (only draws 1.7mA), but if you leave it plugged in, the LED will keep steeling voltage until the battery is completely drained so that you can safely puncture and throw away the battery. Takes longer, but way safer than the water or a regular discharger.
Log In to reply
kobuki01 on July 5, 2016
Aaahh. Alrighty! Thanks a bunch! In my case, I was using an 1.8 LED bulb, that's why i needed the diode. The bulb wouldn't turn off without it. Haha. But thanks a bunch BMan!
Log In to reply
torq5252 on April 9, 2017
Man, I wish I had found this sooner! I build custom vaporizers (can't have just one hobby, right?) and have been looking for a small easy voltage warning light for individual li-Ion cells. Most hobby stuff is too big and expensive for it to be reasonable. This is a perfect solution, and I have all kinds of junk around that has these components already. (dead laptops and such) Thanks for sharing this!
Log In to reply

You need to log-in to comment on articles.

DIY Micro 1S Low Voltage Warning