Daftmike's low battery indicator guide

[daftmike]

Re: Low battery indicators

Here is a small circuit for a low battery indicator.

The green led(referred to later as led1) is the power indicator, it is on as long as the circuit is powered. The red led(led2) will come on when the battery voltage drops to a level set by the resistors.
For my purpose I'll have it set at ~6.9V, I will use the following values:
R1= 2K
R2= 1K2
R3= 560R
Transistor= BC547

The green led will have around 2V across it when it's lit, the BC547 has a base-emitter voltage of 0.6V. This means that the transistor will need around 2.6V at the base to turn on.
Resistors R1 and R2 form a voltage divider, the point between them will be at a voltage of Vin*[R2/(R1+R2)] when this voltage drops below 2.6V the transistor will turn off and the red led will come on. R3 limits current to the leds.

Using the above values, the circuit will draw under 10mA.

I would recommend using a 2K variable resistor for R1 and R2= 1K, this will allow the adjustment of the low battery point to values below ~7.8V.

To modify this circuit to use different colour leds or to use a different transistor you will need to know the base-emitter voltage of your transistor (VBE), this will be found in the datasheet and is normally around 0.6-0.7V for most low power npns. You also need to know the voltage across your leds, blue and white leds are normally ~3V.

1.)Add VBE to Vled1, call it Vtrigger.

2.)Choose the voltage you want the low-power led to come on, call it Vlow.

3.)Choose R3 to limit current to the leds, any value ~500-1K is fine.

4.)Choose R1,R2 values such that; Vlow * [R2/(R1+R2)] = Vtrigger, start by making R2= 1K and work out a value for R1, adjusting both if necessary to find common values. They need to be 1K or higher to limit current at the transistor base.

5.)Alternatively make R2= 1K and use a suitably sized pot for R1, making R1 a 2K pot in series with a 1K resistor will allow you to adjust Vlow to any value ~11-3V.

EDIT:
Since there's been some demand for this circuit and calculating the correct resistors is difficult for some, I've written a little spreadsheet that will work out all the values for you, it's in excel format and is linked below.

http://www.mediafire.com/?sharekey=3c28 ... f6e8ebb871


EDIT 2:
daftmike's low battery indicator mk2

(the green led is "battery ok" the red led is "battery low")

The transistors can be any npn type but should all be the same, the leds can be any colour, and the potentiometer can be changed for a 2k or 5k etc.
You should adjust the pot until the low battery light comes on when the desired low battery voltage is applied to the circuit.

This circuit has a few improvements over my original:
It only lights one led at a time, a green led for battery ok will turn off when the red led for low battery comes on (or whatever colours you choose),
It can use any colour led without changing the circuit.
It is much more precise and will respond to changes of only a few millivolts.
You can set your own low battery voltage without calculating the resistor values.
It will work down to a 3v supply(suitable for a gameboy )

However it requires quite a precise potentiometer, ideally you would use a multi-turn cermet pot. It's also a little more complex than before.

enjoy

EDIT 3:
I made a further guide on how to build the mk2 circuit here: http://forums.modretro.com/viewtopic.php?f=28&t=1897

 

[daftmike]

Re: Low battery indicators

In making such a simple circuit I used specific components, it's their voltage characteristics that enable it to work with so few parts. Having said that, if you need to use a different transistor then using a pot for R1 will let you adjust the trigger point to compensate.

The pins on a TO-92 package, go 1,2,3 with the flat face towards you, but some transistors have different pin connections so you need to be careful, a quick google usually brings up a datasheet that should tell you.
http://www.onsemi.com/pub_link/Collateral/2N4401-D.PDF

If you do use a 2N4401, be aware the pins are in a different order to how I labelled the diagram, so you should go by the picture in the datasheet and match it to my diagram.

The leds must have a Vf of about 2V, this is essential to how the circuit works, most green and red leds are around 2V anyway, but I recommend the older style indicator leds with the coloured package to be most suitable, after all you want to draw as little power as possible with a circuit like this, if you wanted to use blue or white leds then the circuit would have to be changed and it would end up drawing more power.

The top line is V+ and the bottom ground, as is the convention when circuit drawing.

If you have more questions I'll do my best to try to help.

 

[Basement_Modder]

Re: Low battery indicators

I love how you can just whip up a diagram thats 10 times better then Mario's (no offense) in a matter of seconds. I will be totally using this!

 

[Basement_Modder]

Re: Low battery indicators

Did it a little by trial and error - which resistor would you change to make the red light come on at 7.1v?

Alternatively make R2= 1K and use a suitably sized pot for R1, making R1 a 2K pot in series with a 1K resistor will allow you to adjust Vlow to any value ~11-3V.

 

[XCVG]

I've done the research and calculations for 2N4401 and 2N3904 transistors. I've set Vlow for 6.9V like Daftmike did. Ugggh algebra. Unfortunately I think I've screwed up the math, so I need someone to check it and fix it.

2N4401:
VBE=0.75V
Vtrigger=VBE+VLED, Vtrigger=2.75V
Vlow*[R2/(R1+R2)]=Vtrigger
6.9*[1000/(R1+1000)]=2.75
R1/1000+1=.398
R1/1000=-0.602
R1=602 ohms

2N3906
VBE=0.65V
Vtrigger=2.65
I can't figure this one out at all.

I also made a remake of the schematic in ExpressSCH, I'll post it once I find a place to put the file.

 

[Alie]

Re: Low battery indicators

I tried changing the red LED to a yellow one, didn't work. Green and red are fine.

I think it's beacuse yellow leds have a different voltage than the green ones.
BTW thanks daftmike! I was just searching for a circuit like this and it's really useful!

I found out another circuit that does almost the same and that lazy people that don't want to do the calculations can use :



In this circuit you have only to change the zener value to set the point at when the red led turns on (for example if you want the led to turn on at 6,8 volts you have to use a 6,8v zener diode).
If i'm not wrong you can add a green led right after the 470 ohm resistor if you want a "power on indicator"
However it sucks a little more current than daftmike's one, i think about 20mA but that's not too bad.
The circuit under the schematic it's daftmike's one, it's really functional and well made :clap: (i've used a bc107 instead of the bc547).

PS: excuse me for my bad english and my bad handwriting

 

[N00BModdr]

Hey, I'm new, but I've been reading these for a while. I'm also a noob at this, (references to name) can someone explain the diagram and explanation to me in really simple terms? rays:

 

[eurddrue]

Hey, I'm new, but I've been reading these for a while. I'm also a noob at this, (references to name) can someone explain the diagram and explanation to me in really simple terms? rays:

Hey, there will be none of that :lol2: (inside joke) Now, from what I read on the benheck forums, you want to replace the LED in your gameboy color with a bicolor LED so that when the batteries are dying, it changes from green to red? I don't know if it will work on the color due to LEDs taking different voltages, and the SP having a higher voltage. 3.7v = SP 3v = color. I can read this schematic, but it still won't make much sense to you, no offence. I recommend getting the book, Electronics for dummies, if you're serious about this hobby. It tells you everything about resistors and whatnots. I also recommend Hacking video game consoles, by Benjamin Heckendorn.

 

[N00BModdr]

Ok, thanks! I'll read the book, and skip that part of my mod for now.

 

[eurddrue]

Ok, thanks! I'll read the book, and skip that part of my mod for now.

I generally scowl upon this, but why don't you make a thread telling all of the mods you want to do? There are a lot of people on here that are educated on the gameboy series. Bibin, PalmerTech, Jleemero, and me (lulz) know quite a bit.

 

[N00BModdr]

I'll do that.

 

[Basement_Modder]

If you adjusted the circuit a bit, N00Bmoddr, you could run it at the voltage of your GBC and just desolder the existing LED and put the 2 new ones over it. You may not want to do this on a clear-cased GBC, though.

 

[N00BModdr]

My GBC is green (solid) how would I have to adjust the circuit?

 

[eurddrue]

If you adjusted the circuit a bit, N00Bmoddr, you could run it at the voltage of your GBC and just desolder the existing LED and put the 2 new ones over it. You may not want to do this on a clear-cased GBC, though.

Yes, show us pl0x.

 

[Basement_Modder]

If you adjusted the circuit a bit, N00Bmoddr, you could run it at the voltage of your GBC and just desolder the existing LED and put the 2 new ones over it. You may not want to do this on a clear-cased GBC, though.

Yes, show us pl0x.

You would need to adjust some resistor values, and maybe even change the transistor. I'd have to check with daftmike on which ones, though.

 

[daftmike]

This circuit will not work for a gameboy, it is specifically designed for the type of batteries you might use in a portable, eg. 7.4v or 11.1v li-ions or 7.2v ni-mh etc.

For those struggling with the resistor calculations I have updated my first post with a link to a spreadsheet that will do all the maths for you.

 

[eurddrue]

This circuit will not work for a gameboy, it is specifically designed for the type of batteries you might use in a portable, eg. 7.4v or 11.1v li-ions or 7.2v ni-mh etc.

For those struggling with the resistor calculations I have updated my first post with a link to a spreadsheet that will do all the maths for you.

I knew it.

And, are you really a robot designed to help us n00bs out with all of our electronic needs?

 

[neverused]

It seems there is a password needed for the dl, but I cannot find it... :huh:

 

[N00BModdr]

Yeah, there is a password needed...

 

[daftmike]

Sorry about that, should be ok now.