I am needing to know how I could make an led light turn on when voltage on a 18650 battery goes below 3.2v

The battery will be at 4.0 volts fully charged.

Thanks in advance guys!!

 

There are many LEDs that have a Vf of ~1.9V

 

I'm pretty new to this whole electrical thing, Could you put it in layman's terms for me?

I'm building an electronic cigarette "mod" and need the led to indicate when my internal battery drops below 3.2v, so I know when to charge it without having to pull it out and put a multi-meter on it. Will be a sealed unit with usb charging,

 

Hello,

All led colors have a different Vled.
Here is a list with the led colors and their average led voltages:
http://www.oksolar.com/led/led_color_chart.htm

This list comes from the led page of the EDUCYPEDIA:
http://educypedia.karadimov.info/electronics/composemiconductorsled.htm

Bertus

 

The led that I'm tryng to use has a forward voltage rating of 1.7-2.6V

 

Then it should be possible to choose a current limiting resistor such that you have less than a two-to-one change in LED current as the battery discharges.

What is the current rating of your LED? How bright do you want it to be? How long do you want the battery to last?

 

Then it should be possible to choose a current limiting resistor such that you have less than a two-to-one change in LED current as the battery discharges.

What is the current rating of your LED? How bright do you want it to be? How long do you want the battery to last?

The current is rated 28mA, Near full brightness would be good since its only a 0.8MCD LED.

I really appreciate your help guys

 

I am needing to know how I could make an led light turn on when voltage on a 18650 battery goes below 3.2v

The battery will be at 4.0 volts fully charged.

Thanks in advance guys!!

If you want an LED to turn ON below a certain voltage, a comparator circuit should be used.

 

Another possibility is a joule thief, which will light any LED from 1.5VDC. They are very common, just Google around for the plans. They use one transistor, a couple of caps and resistors, and a coil.

LEDs, 555s, Flashers, and Light Chasers

 

Pardon my ignorance, But I have no idea how to design said circuit.

 

Thanks Bill, I will check it out

 

If you need more help just ask. No one is born knowing this stuff, and I like what I call do nothing circuits.

 

...and I like what I call do nothing circuits.

Mechanical equivalent of a "do nothing circuit".

 

It may be easier to string together a couple LEDs to get close to 3.2 Vf, and then connect that, through a resistor, to your battery. Then, when the batter falls under 3.2V and the LEDs go out, or when the LEDs rapidly start to go dim, you'll know you need to charge. Maybe connect a "test switch" so that the LEDs aren't always on when the battery is charged and needlessly draining it.

 

I believe that this circuit will work if you choose the resistors properly, but it's crude and inefficient, which may be a problem if you're using battery power. Also, the LED would light up gradually, getting brighter as the battery voltage drops.

https://dl.dropboxusercontent.com/u/28291527/circ131102.jpg

 

There are much better out there. For example, a quick search on Google found this...

Make a Joule Thief

 

It may be easier to string together a couple LEDs to get close to 3.2 Vf, and then connect that, through a resistor, to your battery. Then, when the batter falls under 3.2V and the LEDs go out, or when the LEDs rapidly start to go dim, you'll know you need to charge. Maybe connect a "test switch" so that the LEDs aren't always on when the battery is charged and needlessly draining it.

Really? Given that newer generation LEDs start at 2.5V Vf, and go as high as 3.6V Vf or more it is really impractical. Older LEDs drop a lot less due to different materials, but trying to match a voltage to LEDs suggests you think you do not need a current limiting resistor, which is far from the truth. The old generation of LEDs are rare too.

I would suggest reading the link in post #9.

 

Note that the assignment here is to turn on an LED when the voltage drops, not turn it off! The Joule Thief is fine, but what's its function here?

 

I second that.

 

I'm pretty new to this whole electrical thing, Could you put it in layman's terms for me?

I'm building an electronic cigarette "mod" and need the led to indicate when my internal battery drops below 3.2v, so I know when to charge it without having to pull it out and put a multi-meter on it. Will be a sealed unit with usb charging,

The shops around my way have Vapourlires or E-lites, the Vapourlites already have a function for the blue LED in the end to flash when the battery needs charging - the E-lites effectively cost double because you have to buy the USB charging bit separately, so I haven't tried those.

There is possibly a question mark over the reliability of the suction sensor in the Vapourlite - especially if you refill from bottles of the fluid instead of buying expensive refill tips (the fluid can leak into the battery compartment and contaminate the suction sensor) I have stripped a couple and found that the battery charge control chip can be used with bigger/other lithium cells.

A quick google came up with various third party suppliers offering tips with various different resistance - allegedly affects the flavour. I'd be interested to see any discussion that develops on which tips/batteries etc are compatible with what.