Daftmike's low battery indicator guide

[Basement_Modder]

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 thought that if you adjusted the circuit's transistor/ resistor values you could make one that made the red LED come on at 3 or 2.9v or something. Not an identical circuit, but one that is assembled the same way with different parts.

 

[daftmike]

The circuit uses the forward voltage drop of the green led to monitor the battery voltage.
Because the leds are in series when the low battery led comes on, it needs a battery voltage of at least 4v to light a green and red led, 6v or more when using blue and white leds.

A similar circuit using just one led to indicate a low battery could be made that might work with a 3v supply but i wouldn't be confident in it's precision, the variances in component values would affect the circuit too much to be really useful at such a low voltage.

Also the way that alkaline batteries discharge doesn't really lend itself well to this type of low battery detection, when an alkaline cell is 80% discharged it will be at only 1.1v or so, this means the gameboy battery might only have 2.2v when the led needs to come on, this is too low to work with this type of voltage detection.

 

[Basement_Modder]

Perhaps you could use a ti voltage booster card to supply the 4v for the modified circuit to be used in a Gameboy color.

 

[daftmike]

When I say it wouldn't work I really mean that it wouldn't work.

If you used a boost regulator then the circuit couldn't tell anything about the battery because it would be being fed a regulated voltage. Also the thought of using a $15 smps in such an application makes me cringe.

The main point of my original circuit is it's simplicity, it uses as few components as possible. A low battery indicator could be designed for a gameboy, a decent voltage reference and a comparator would do a good job but the applications are too limited and it isn't really something I'm interested in doing.

 

[N00BModdr]

A low battery indicator could be designed for a gameboy, a decent voltage reference and a comparator would do a good job but the applications are too limited and it isn't really something I'm interested in doing.

Ah...is there anybody willing to take the time to do this? I would be very grateful...

 

[Basement_Modder]

Hmmmmm...

Well what about feeding 4v batteries into the circuit and using a ti step-down card to supply the 3v to the Gameboy? Then all you would have to do is find out how low the batteries could go before they die, then have the circuit come on, say, .2v beforehand.

 

[N00BModdr]

Hmmmmm...

Well what about feeding 4v batteries into the circuit and using a ti step-down card to supply the 3v to the Gameboy? Then all you would have to do is find out how low the batteries could go before they die, then have the circuit come on, say, .2v beforehand.

I posted a thread in handhelds about this. Anyone who can help please take a look.

 

[daftmike]

@Basement_Modder
I concede that this circuit would indeed work if you used a 2-cell li-ion battery to power your gameboy, but then again that is exactly the type of application I designed the circuit for in the first place. :rolls:

 

[Basement_Modder]

@Basement_Modder
I concede that this circuit would indeed work if you used a 2-cell li-ion battery to power your gameboy, but then again that is exactly the type of application I designed the circuit for in the first place. :rolls:

Well then, I shall add this to my "projects I'll get around to someday" pile.

 

[daftmike]

Updated the first post with my mk2 circuit.

 

[roody]

A similar circuit using just one led to indicate a low battery could be made that might work with a 3v supply but i wouldn't be confident in it's precision, the variances in component values would affect the circuit too much to be really useful at such a low voltage.

Also the way that alkaline batteries discharge doesn't really lend itself well to this type of low battery detection, when an alkaline cell is 80% discharged it will be at only 1.1v or so, this means the gameboy battery might only have 2.2v when the led needs to come on

 

[cfc_12]

Re: Low battery indicators

Here is a small circuit for a low battery indicator.

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

cam you re upload the picture? looks like it doesnt exist anymore