How to Add a Low-Battery Light to Your Portable

[Kyo]

i doubt that would work. They'd just dim. Besides, the ones with weaker resistors would always be on. You can add a switch in there, but that kind of takes away the point. Would you honestly flip a switch to see whether an LED lights up or not? It's lot like it's eating that much power

[timmeh87]

You'd just need a transistor for each LED.

Also about resistor wattage: instead of guessing about it just do the math .

Lets make the incorrect assumption that the transistor acts like a short circuit when its "ON", and that your battery is 7.5V. In that case, the most voltage the 1k resistor will see is 7.5V, but probably less.

Watts = V^2/R = 57mW

So as you can see, you can use an 1/8W resistor and it would still be perfectly fine.

As a general rule, if your voltage is less than 5v and your resistors are more than 1k, you can always use 1/4w resistors without worry. (max resistor power is exactly 250mW)

[superdeformed]

You should not need a 1W resistor, and using one isn't likely to change how much heat is produced by this circuit. Even if you're using a 12V battery, the most current that can pass through the resistor is 12mA (assuming the transistor is a perfect switch, so even this is a little high), which means your resistor is dissipating 0.144W. Things heat up when you run power through them, this doesn't mean it's dangerous though.

As for power consumption, 40mA is a pretty big current draw for a power indicator. Ideally you would want an indicator circuit to use as little power as possible when you're running off of a battery. As is, you're burning a significant amount of current through the zener and base of the transistor and relying on their imperfections to keep the circuit from smoking itself, which is generally not a good idea. Using the "tuning" resistor will significantly decrease the current draw and as you said it will also increase the voltage that triggers the LED to turn on, but you can compensate for this by using a lower voltage zener.

Another way to build an indicator would be using an op-amp (an LM741 would probably work fine). You could easily switch between two LEDs, and the only current you'd really have to worry about is how much is going through your LEDs. If you only want one LED to light when the voltage gets low, you could just use a comparator IC and it would use even less power (you could of course use a comparator to do two, it would just increase the number of components in the circuit).

[Kyo]

I thought that 1W thing seemed weird. The rest you said (specifically about the op-amp) is also very interesting. I won't have a go at this in my Atari (as it will have interchangable AAs), but once I use Li-ions or some other form of internal battery, I will definitely look into that.

[bacteria]

So guys, I have bought the components Mario suggested: using a transistor, two 3.6v Zener diodes to make the LED come on at about 7.2v (the 7.4v Li-ions start at 8.3v) and the 1k 1 watt resistor.

Is it going to work as it should, be reliable, not get hot and be nice and safe? Comments please!

[Kyo]

well, it depends on the voltage of the battery, so it's relatively unreliable for interchangable batteries (i.e. AAs), but if you tune it right for your battery, it should be no problem. In case it goes on too early, just throw in a resistor

[Mario]

So guys, I have bought the components Mario suggested: using a transistor, two 3.6v Zener diodes to make the LED come on at about 7.2v (the 7.4v Li-ions start at 8.3v) and the 1k 1 watt resistor.

Is it going to work as it should, be reliable, not get hot and be nice and safe? Comments please!

According to information posted a bit earlier, even a 1/4 resistor will be fine. I said a 1w just to be safe.

Also, the two-LED one does not work yet, I need to do a bit of experimentation. Just go with the 1-LED one for now.

[Rekarp]

I agree with superdeformed on this one. 40mA is way to much for a low battery indicator. The use of a op-amp would bring it to about 10-20mA.

[Mario]

Then could we have a tutorial on how to do that, please?

[Rekarp]

something similar to this

http://www.ece.uvic.ca/499/2001b/group0 ... kt_low.jpg

I would replace the voltage divider at that is tied into the inverter pin of the opamp with a potentiometer so you can adjust when the LED goes off

[hy phrend]

I also found a similar schematic using a quad op-amp.

http://ourworld.compuserve.com/homepage ... #meter.gif

Gives four different voltage level indicators.

[Rekarp]

another good link

http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm

[wajimba]

Back to bacteria's question:

So, if you put 4 x 0.25 watt 1k ohm resistors in parallel would that give 1K ohm resistors at 1 watt? (like amperage works on batteries)?

Like Kyo said, resistors in parallel add up as 1/(thier resistance). However, you could achieve this using 4 x 250 ohm resistors in series. Then it would give you an equivalent 1K ohm resistor at 1 watt.
(It would be the other way round for caps btw)

[grayfox126]

Mario, you are awesome! XD Im definitely going to place this in my DCp

[Virtual Boy]

Does a 1/4W 75ohm resistor work?
http://cgi.ebay.com/75-ohm-Axial-Lead-C ... .m14.l1262