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Greetings,
It's really annoying that you can't tell when you battery's gonna die. For DMG there's an indicator sold by Kitsch-Bent.com so no problem here. For GBC or MGB it's more problematic. One option is take it from GBA SP but hell I hate it when it comes to trashing one console to mod another. So does anybody have a working schematic or something?
Thanks 
Last edited by friendofmegaman (Jan 17, 2014 10:03 pm)
The circuit could be modified for use with a GBC/MGB, I'm pretty sure. I personally would have done the circuit a little differently. It now connects just to the battery terminals. I would have used three terminals, ground, regulated +5V Vcc and the sense voltage. The sense voltage trigger point could then be set to something suitable for either a DMG or a GBC/MGB (something using only two 1.5 V batteries).
The problem then becomes fitting the board inside a GBC/MGB. And speaking of that, unless that board is single-sided, it could be made more space-efficient by placing components on both sides of the board. Though that might be problematic when you're going to mount it, but yeah, might be beneficial for the smaller 'boys. But this is just turning into a technical review of the batt_dmg.
You could likely not use an indicator from a GBA SP, as it's probably made using a Li ion battery charging chip which has an internal voltage reference made for that battery chemistry. This mean the threshold for a low voltage is likely set higher than the voltage of two AA/AAA, ie two AA/AAA will always detect as being low. Not to mention that it's easier to design a simple circuit from scratch than it is to reverse engineer the GBA SP's. 
Same with the GBA's, which is using AA batteries, but the indicator circuit is likely integrated into the voltage converter.
The lack of a proper voltage indicator is really a shame. That's what I've thought too, when I started to work on my DIY frontlight. At the moment, the work on the frontlight isn't done, due to the thickness of the Endlighten T. However, the voltage indicator is working. So If some people are interested only in the circuit and the program for the PIC controller, then I can upload both.
Last edited by Thor17 (Jan 22, 2014 4:11 pm)
The lack of a proper voltage indicator is really a shame. That's what I've thought too, when I started to work on my DIY frontlight. At the moment, the work on the frontlight isn't done, due to the thickness of the Endlighten T. However, the voltage indicator is working. So If some people are interested only in the circuit and the program for the PIC controller, then I can upload both.
Oh, hey! I've been following your frontlight thread for a couple of months. Awesome work you're doing, please keep going!
So you use PIC for voltage indicator... interesting I haven't considered that. I though more of a zener diode or purely transistor based circuit. Anyways if you could share your voltage indicator that would be sooo much appreciated. I'm pretty sure lots of people want it, but everybody (like me) tries to find a shortcut, lol Speaking of which https://www.sparkfun.com/products/11087...
Of course a Zener diode or simply a comparator would do the job. If I wouldn't work on a frontlight, maybe a small comparator circuit would have been my choice. However, the Nintendo DS is also using a bi-color LED to indicate it's battery state, if I remember right. So I thought, it would be cool to integrate such a function to the GBC because actually, the red LED is COMPLETELY USELESS.
I tested, what's the critical voltage when the GBC is shutting down. I used 2300mAh Ni-MH batteries. The Gameboy was running proper with a voltage of 2.0V, it took approximately 5minutes, then the battery voltage drops to 1.8V and then the Gameboy shutt off. Alkaline batteries should also work at this voltage level, but that depends on the load and the batteries. The real critical voltage may differ. Especially if a frontlight is installed. Due to the higher load the battery voltage drops faster when it's running out of juice. So I guess with a frontlight you don't have 5minutes left until the gameboy stops working.
Interesting... well, I think the actual voltage for LED to go off is the matter of testing. In fact I have this guy http://forums.modretro.com/viewtopic.php?f=7&t=3640 set up on the breadboard, but I need to set up the pot and find suitable LED and R1... at this point my knowledge is not enough to do this probably rather simple thing. But that's the pleasure of learning things, right?
So I played a bit with the circuit I mentioned in my previous post (this one). I tried to tune the pot so the LED would go off at 2.5 V (GBC is functional at 2V but we need some time).
The experiment "design":
- I used to AA batteries as power source and emulated low voltage using pot.
- I emulated low voltage using a pot.
Question: As nitro2k01 said it's probably better to connect the guy to 5 VCC, but what the low-voltage point is gonna be in this case?
The result:
1. When voltage reaches around 2.8V the LED begins to light up (the light at this point is barely noticeable)
2. Light increases gradually until it reaches its peak at 1.9V
3. The light is still quite faint even in it's peak (which is not surprising since it gets only 1.74 V of 2.2 needed)
4. I tried 150, 100 and 47 Ohm resistors for LED (R1 on schematics) but there's no difference in how bright the LED is. I am missing something about LEDs.
Problems:
1. LED's light is weak even at its brightest point
2. The brightest point is 1.9V when GB is non-functional already, so it's completely useless at this point.
I need some better understanding of this circuit.
Last edited by friendofmegaman (Jan 25, 2014 12:13 am)
I guess your problem is, that the value of R2 is too high. This circuit is using NPN transistors. For switching they need a current that flows from the base to the emitter. The current that is needed to switch, or in this case to open up, depends on the current gain of the type you are using. If the voltage is high enough, then Q2 gets enough current to switch and held the base of Q1 low. If the voltage drops, then the base-emitter current of Q2 isn't high enough, so there doesn't flow a collector-emitter current anymore. The base-emitter current of Q1 can flow and then the LED lights. Newer LEDs are very bright, so 500µA or max. 1mA should be enough. (2.5V-1.8V-0.2V) / 0.001A= 500Ohm [0.2V is UCE]
You shouldn't use that circuit because there is no trigger, there is no exact level at which the LED lights up. You could use a NE555 (or better a newer CMOS type) if you just want a LED lights up if a voltage drops below a certain value. The 555 is easy to use. There are many sites available that explaining how it works.