# Low battery indicator circuits



## tangent

I've been thinking about low battery indicator circuits lately. 

 So far, the only thing I've seen on these forums is ppl's zener/CRD circuit, used in his personal amps and then adopted into the PPA and PIMETA. Just shutting the LED off when the battery is low is kind of weak, though: it isn't an indication, it's a lack of indication. You can infer that the battery is low, or you can infer that there is no power at all.

 Last night I came up with this circuit:







 It uses a bicolor LED (just two matched LEDs wired back-to-back) set to show green when the supply is over a certain voltage, and red when it drops under that voltage. The comparator (LM311) might oscillate when the midpoint of the voltage divider is right at the zener voltage, in which case you'll get orange or yellow, depending on the LED. There's a low-power version of the comparator (LP311) that would be ideal here; you could probably tune it so the entire circuit draws maybe 2mA more than the LED alone.

 Bicolor LEDs are available in all possible combinations of red, yellow and green. Sadly, there are no bicolor blue+X or white+X LEDs yet, probably because the voltage drops are too dissimilar, so matching the two chips for brightness would be a problem.

 You could easily modify the comparator circuit to just turn on a low-battery LED, and have a separate power LED. This would open up the full range of LEDs to you, since it doesn't depend on a bicolor type.

 Another variation I've seen runs the LED normally while the supply voltage is high, but flashes the LED when it gets low. This one uses just a single LED, but it does need more active parts than any of the above circuits.

 A zener plus a CRD is about $2, so all of the above circuits actually are about the same price or cheaper. They all take more board space, so there's little choice in a compact circuit like the PIMETA. But if space isn't an issue, which would you rather have? Are there any other cool low-battery circuits you've seen that you like better?


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## GWN

How about this one? Another variation on the theme. The led is off until it hits a threshold low voltage then comes on telling you its time to change batteries. You can adjust the threshold by varying R2.


http://www.designnotes.com/CIRCUITS/lowbat.htm


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## comabereni

DELETE: Wrong thread.


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## Garbz

a bit off topic but thanks for explaining how the whole 2 colour led thingy works. I was about to build something with one but using a not logic gate to reverse the power, this will save considerable effort


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## thrice

Quote:


  Originally Posted by *tangent* 
_Sadly, there are no bicolor blue+X or white+X LEDs yet, probably because the voltage drops are too dissimilar, so matching the two chips for brightness would be a problem._

 

Never say never:

http://www.lsdiodes.com/multi-color/


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## flecom

Quote:


  Originally Posted by *thrice* 
_Never say never:

http://www.lsdiodes.com/multi-color/_

 


 those are 3 pin LED's


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## thrice

oh yeah, I guess you would need two pin leds huh...sorry.


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## aos

A comparator shouldn't really oscillate - that's what the histeresis is for. I.e.
 there is a difference between low-high and high-low thresholds. I am not
 familiar with that particular chip though.


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## flecom

what about something like this, recycle the existing circuit and just add a transistor and a second led... this would let you use nice 3 pin leds like white blue, or even better blue red!! 
	

	
	
		
		

		
		
	


	









 it would light one led like the circuit is supposed to, and then when the led turns off it will light the other... or so i think im not to great at this


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## aos

Somehow I think that the bottom LED will always be lit - D1 will always conduct enough current for the transistor to turn on.


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## flecom

then put a diode or a resistor or something so that it wont be enough to trigger the transistor


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## setmenu

For something to give one a little more info as to battery level, the circuit
 I made here is usefull.

http://www6.head-fi.org/forums/showthread.php?t=76412

 It could be made smaller with a surface mount chip and smaller resistors.

 Setmenu


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## hadron

This is what I built a few months back for my DacKit 12V 10 cell NiMH battery.

 >12V green
 11-12V orange
 <11V red


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## aos

Hadron, I like your solution. It isn't much more compllicated or expensive or size-consuming than tangent's (except that you'd have to get rid of potentiometers which is trivial) yet it gives a valuable addition of
 low battery indicator (not just dead battery indication).

 I'm trying to make a much simpler one with two diodes in style that flecom tried but I haven't had much success yet.


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## aos

I finally managed to get it to work (I have been testing them on protoboard).
 I used two 3mm 1.6V red LEDs but you can use others or even a 3-pin bicolors.
 You might need to adjust resistor values in that case though.

 Basically you add two transistors and four resistors (and of course a LED) to
 the ppl's low battery indicator. The other LED will turn on when the battery
 is low and the first one will turn off, as usual.

 Here it is. Note: I've updated it since I had LED in a position that would prevent use of 3-pin bi-color LEDs.


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## flecom

cool see i kinda had a good idea that time


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## sneared

I've had some good results using Panasonic voltage detectors such as in this circuit:

http://www.discovercircuits.com/PDF-FILES/undervol.pdf


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## aos

Yeah, this is a pretty common problem so there are bound to be ICs to address it. These are actually power management, meant to issue reset signals, but they work for this just fine too. You don't have to use the oscillator part if you don't want flashing, just have the monitor chip trigger a transistor and light up a LED. Or you could have two transistors (PNP and NPN) to light up alternate LEDs, or different sides of 3-pin LEDS as in previous schematics. It's not completely trivial because you have to take care to set the monitor to the voltage you want to trip at (e.g. by setting the VDD of the chip to a voltage divider).


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## tangent

Quote:


 The led is off until it hits a threshold low voltage then comes on telling you its time to change batteries. 
 

Gee, that looks familiar.... 
	

	
	
		
		

		
		
	


	




 In fact, I "invented" that one, too...I added the comparator's output pullup resistor after finding that it never entered the green state without it. I considered the one-color behavior a bug, since I'd gone to the trouble of using a bicolor LED. 






  Quote:


 I was about to build something with one but using a not logic gate to reverse the power 
 

Yes, I briefly considered using digital logic, but analog seems a lot better suited to the problem.

  Quote:


 A comparator shouldn't really oscillate - that's what the histeresis 
 

I wasn't aware that comparators with hysteresis built in existed. The LM311's datasheet specifically recommends adding hysteresis if you get oscillation. In my case, oscillation would be a minor feature, so I ignored the issue.

  Quote:


 For something to give one a little more info as to battery level, the circuit I made here is usefull. 
 

Indeed. I guess that chip was made to be used with bargraph modules. Interesting use of discrete LEDs instead.

  Quote:


 Hadron, I like your solution. It isn't much more compllicated or expensive or size-consuming than tangent's 
 

The use of a voltage regulator as a reference is an interesting tradeoff. A zener's adequate for the job, but it requires 1-2mA so it's not too "soggy". That regulator probably only requires 0.5mA in the adjustable version. But you do pay for it! The adjustable one is only available in 8-pin packages, and it runs nearly $3. Me, I think I'll burn the current and save the cash. I might change my mind if I needed a highly accurate voltage set point, which zeners can't provide.

 It's a good circuit, hadron. A little excessive for my immediate purposes, but still interesting.

  Quote:


 I finally managed to get it to work 
 

Very nice. Economical. It's probably lower current than my design as well. I like that it doesn't blink at three-leg parts, too.

  Quote:


 I've had some good results using Panasonic voltage detectors such as in this circuit: 
 

I had that in mind when I wrote about blinking indicators above. My specific problem with those Panasonic monitor ICs is that they don't come in very high voltages. But, you could combine the blinker circuit with some of the others in this thread.


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## aos

Yes, just the monitor chip is more expensive than all the parts for my version - even though it's only a buck; transistors and FETs are dirt cheap and common. To be honest though, it's just a classic inverting combo, I've used it before. I
 was trying to figure out a way how to use a single extra transistor only but I couldn't get any of them to work. Doesn't mean it's impossible, it probably is.

 You can combine most of the circuits here to get what you want, i.e. you can add blinking circuit to Zener/FET combo instead of what I did. Using 3-pin bi-colour LED plus blinking would be perhaps the nicest (since blinking attracts hell of a more attention than the steady-on, even of different colour, though lot of people get annoyed by blinking lights).

 I've updated the schematics as I had LED in collector instead of emitter, making it impossible to use bicolours. Now both LEDs can share the same cathode. Note that circuit is somewhat sensitive to resistor values and choice of transistors; I haven't tested any others yet. I'll see if I can add the blinker as well.


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## hadron

Quote:


  Originally Posted by *tangent* 
_The use of a voltage regulator as a reference is an interesting tradeoff._

 

Yeah, a 78Lxx would be cheaper and is available in a wide range of voltages, the LT1121 is just what I had in the parts box at the time, and a zener ref will be the cheapest. Though I would probably supply the opamp directly from the battery in that case, this would cause some variation in the LED drive voltage over the useful battery range.

 And of course, using fixed resistors instead of pots would knock down both the price and size.


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## tangent

Quote:


 a 78Lxx would be cheaper and is available in a wide range of voltages 
 

Even better, an LM317L, adjustable version. If you're going to spend more than a zener, I think you should get finer adjustment than 1-3V between steps, which zeners do provide.


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## hadron

Quote:


  Originally Posted by *tangent* 
_Even better, an LM317L, adjustable version. If you're going to spend more than a zener, I think you should get finer adjustment than 1-3V between steps, which zeners do provide._

 

I don't see the advantage of an adjustable, the only reason to use a regulator is for constant opamp supply voltage (and even this could be done with a zener shunt), and hence a constant LED drive. Since the comparator ref voltages need to be divided to bring them between the rail voltages it doesn't really matter what the value of the ref is. An adjustable just ups the part count with no benefit that I can see.

 BTW, the 78Lxx is available at all integer steps from 3 to 10V, plus a few other values inbetween.

 [edit for typo]


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## morsel

If you are going to make room for a comparator, why not implement a clipping detector instead? This has the advantage of indicating insufficient battery voltage level for the chosen listening level, rather than an arbitrary low voltage level.


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## aos

Because once you are past that "arbitrary" (depends on number of cells etc.) level the batteries are pretty much going to be dead in a short while - so whether it clips at that point is pretty much irrelevant. That is not to say that a clipping detector is not useful as a full-time ocupation in an amp.


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## hadron

A clipping dectector would not be good in my case (DAC). NiMH batteries have a "safe" range of at least 1.0V/cell to keep any one of the cells from going into deep discharge, even at this "dead" level I still have a volt or two above the regulator dropout level.


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## hadron

Here's a minimalist version of my tri-color comparator circuit, should be able to put this together for around $2 (including the LED, Lumex SSL-LX3059IGW is $0.75 from DigiKey).


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## tangent

Quote:


 why not implement a clipping detector instead? 
 

I can hear clipping. I don't need an LED to tell me the amp is clipping. I want an LED to warn me that clipping will occur at some point in the proximate future. If you tune the circuit to give you that warning based on supply voltage, the difference between the two is just a matter of perspective.

 If you had a true clipping indictor, it would be in the audio path, and I have a feeling that capacitors would be involved. Think "sample and hold", in order to keep the LED on for more than a fraction of a second on brief peaks.

 Even that aside, it seems to me that a true clipping detector makes more sense with amps running on marginal supplies (like the Airheads), and in sound reinforcement applications, neither of which applies to most amps you see here.

  Quote:


 I don't see the advantage of an adjustable 
 

Sorry, I was unclear. I was thinking about such things in terms of my circuit, where the reference voltage is linearly related to the clipping point. In yours, you use adjustable dividers to scale that, so the exact voltage isn't all that important.


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## aos

Actually if you want a clipping indicator, it's far neater to just implement a VU-meter, like including the chip someone else listed on this thread. That way you can see if it's clipping (i.e. it's all the way to the top) plus it shows the real-time volume level. Complexity of both designs would be about the same (you'd need some extra LEDs though), but the vu-meter has far greater feature value.


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## Wooderson

Here's one I found while scoping out the DDAC 1543 as a possible (and likely) next project.







 Original link here.


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## Excalibur

If conserving battery current is a real issue, you may want to replace that NE555 (draws 7-15 mA) with a 7555 CMOS type (draws a few hundred uA). It will be a drop-in replacement for this application.
 The LM311 draws about 5 ma. and even lower curent opamps are available.


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## aos

Blinking solutions using two transistors such as presented earlier in this thread take less space and are cheaper, though.


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## tangent

Quote:


 The LM311 draws about 5 ma. and even lower curent opamps are available. 
 

From my original post, "There's a low-power version of the comparator (LP311) that would be ideal here..."


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## tangent

Quote:


 LP311...would be ideal here 
 

Scratch that. The LM311 and LP311 are *horrid* comparators. I tested my circuit with LM339s, which is what I had on hand at the time, and that worked fine. I got some of the '311s in, tried them today, and their output voltage is extremely soggy. No good at all for the purpose. I dropped an OPA227 in and it worked perfectly.

 Bottom line, if you build my circuit, you need a low-voltage op-amp that swings its output to somewhere near its rails.


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## tangent

Quote:


 Note that circuit is somewhat sensitive to resistor values and choice of transistors 
 

aos, I just tried your circuit with substantially the same resistor values, changing only to 2N3904s, and it worked. However, I found that there's a dead zone of over 1V where neither LED is on. 

 Thinking about it, it looks like this is normal: it seems to be due to the softness of the zener knee plus the fact that the high LED can still have current going through it yet not have enough current to light. So, the "current robber" transistor still has base current, keeping the low LED's transistor shut off.

 I was right about the current requirements: aos's transistor circuit only takes a fraction of a milliamp above what the LEDs require. The comparator circuits are rather more hungry.


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## tangent

Hadron, I tried your second circuit, and found that zeners aren't appropriate with it. The zener voltage just isn't stiff enough at low voltages. Low-voltage zeners perform badly.

 Instead of going back to the voltage regulator idea, because that's either expensive or has high Iq, I found a cheap voltage reference: LM385, $0.50 for the TI version at Digi-Key. It will run on as little as 20uA, and it's available in a 2.5V version. Low voltage gives more flexibility in setting up this circuit. Here's my cut at it:







 I don't have that reference part on hand, so I don't know if it works any better. I'll get some in and report back.


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## aos

Yes, Zeners are rather non-ideal as I found out looking at datasheets while making this cricuit (I first tried with higher wattage Zenner and that didn't work - it's just too soft). You could increase transistor hFE to compensate (e.g. use Darlington) but I'll see what I can do otherwise.


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## setmenu

I recollect making some sort of 2or 3 led indicator a while back using schmitt
 triggers set at varying trigger levels, it was had to be very precise as the level differences were small, worked a treat, and used little power.
 Can't remember what I did with the circuit or schematic now though, so
 not much use really 
	

	
	
		
		

		
		
	


	







 Setmenu


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## aos

Tangent, shouldn't Vbd be actually tied to the reference?


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## tangent

Quote:


 You could increase transistor hFE to compensate 
 

How would that help? The problem is that the robber transistor still has base current past the point where the LED turns off. Increasing hFE is fixing a non-problem.

 What is needed is a more precise way of cutting off the LED power.

  Quote:


 shouldn't Vbd be actually tied to the reference? 
 

I suppose it could be made to work either way. I like it this way.


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## aos

How can it work this way? The second comparator is comparing voltages from two voltage dividers. That means it's only comparing voltage ratio, not the absolute values. You cannot say it will trigger at 7V or any other voltage then.

 As for the other problem, higher hFE means the circuit (which acts like a switch) will be more sensitive (comparator should always have high gain). It is quite possible though that you'd need to play with the values of resistors to make it work with a particular LED and particular transistor (depending on its turn-on point, Vbe, hFE etc.) which is obviously very much not desired. I'll try to make it better. But the other appealing solution is similar to yours, yet it uses a different chip. I ordered it from Digikey as well as your regulators so I'll try it tomorrow.


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## tangent

Quote:


 That means it's only comparing voltage ratio, not the absolute values. You cannot say it will trigger at 7V or any other voltage then. 
 

Sorry, some bad algebra here. 

 I'll get around to changing it later.


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## aos

I've just stumbled onto what looks like to be a simple, compact and cheap solution, yet high precision and uses only about 1.1mA or so (and that only until the battery is "empty"). I have parts at home so when I get there I'm going to try it and post it if it works...


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## aos

Here is a new circuit. I tested it and it works (with a three-pin bicolor as well). I don't know how big is the transition voltage range as I don't have a variable power supply (I tested by removing one 1.5V cell from a pack) but I doubt it will be very large, given that a regulator is used.

 The circuit is based on the previous one, plus one of the examples from the regulator's datasheet. It's the already well-know TL431. Only $0.6 Canadian.
 The rest is just transistors and resistors, dirt cheap.

 If you want a tri-color (i.e. add orange color (red and green active together) for between red and green) you can double the circuit and remove the third transistor, then change the resistors to set up threshold voltages to overlap.

 This circuit is set to detect a 10AA battery pack from falling below 11V (i.e. 1.1V per cell) - because that's what I needed. To change the voltage, set the voltage divider R1/R10 so that it's at 2.5V when the battery voltage is as desired. Set the R11 resistor so that the current through the regulator is at least 1mA when the voltage of the battery is minimal where you still want to have the LEDs operating properly (otherwise it'll shut down and it will revert back to what was "normal" power, though if you set it low enough there will be no power in the battery left to light the LED anyway). Set the R3 and R4 for desired LED intenstity.

 Now if you'll excuse me, I'm going back to Star Ocean.


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## hadron

tangent, I am sure you are correct, the second version of the circuit I put out there was untested. I agree that a bandgap ref will work much better than a weak-in-the-knee zener; while most cost significantly more than standard zeners, the LM385 is one of the few competitively priced ones out there.


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## tangent

This is very similar to hadron's second circuit, but with one of the dividers in a different position to make the arithmetci easier, and using the band-gap reference instead of a zener. I built it, and it works beautifully. 

 Simply set the dividers equal to 2.5 divided by the desired trip point. My values are for 7 and 8V, so the dividers are set to 0.357 and 0.3125, respectively.

 I'll be building your circuit next, aos.


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## tangent

I built your most recent circuit aos, and tested it with a variable bench supply. There seems to be no gap either way: no point where both LEDs are on, no point where neither is on.

 It takes the same number of resistors and LEDs as the hadron/tangent circuit, so the only part difference between the two is the active parts. The aos parts cost about 1/2 that of the hadron/tangent parts, the two probably take roughly the same board space, and the aos circuit has a small edge in current consumption. The resistor values in the two schematics above won't give equal brightness, but this still applies after correcting for that.

 The only advantage remaining to the hadron/tangent circuit is the three-state feature.

 aos, I don't understand what you mean by "double the circuit and remove the third transistor".

 I also don't understand how you knew the voltage across the TL431 would go to ~1.9V when it begins conducting in this configuration. No circuit in the datasheets I've seen shows this trick, and I don't see an explanation in the datasheet that explains it. Do you just have to study the TL431 internal schematic to know this?


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## aos

What I meant is, you can make two circuits like this, which nets you total of 4 LEDs. You can then set each to different threshold. In one of them keep only the first transistor/LED combination in each (the other one is just an inverter on top of the first one). This LED will be active when the voltage drops below certain level; let's set it to level where you want the LED to be "orange" (i.e.to the red part of the tri-pin bicolour LED). Then make 
 another, full circuit, except that you can replace the first LED with a 
 resistor and just keep the second one. Tie this one to green LED and let the
 threshold of this circuit to be so that it extinguishes when you want
 the LED to be "red". That way you need 1 1/2 circuits to achieve 
 three colour state. The cost is a bit larger size and extra 1mA of consumption. One benefit though is that the builder doesn't have 
 to populate the extra parts on the board if all he/she wants is a 
 two-state light, and also you could actually go for two separate 
 LEDs, blue one for good power, and bi-colour orange(red+green)-red 
 for the low battery indication (you'd need two full circuits for that).

 Since TL431 is TO-92 and so are other two transistors, wouldn't it take less space than a DIP-8 chip and a TO-92 reference? You can opt for vertical
 mounting of resistors to make it smaller - drawbacks of vertical mount
 shouldn't be of concern in a circuit like this.


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## mono

IMO, this is a lot of expense and work for a simple LED. Better to put time and construction costs into the amp itself? I mean, it's neat and would make the whole-amp better but portables are big enough already?

 I'm in favor of KISS. Based on LED v-drop, use resistor that will shut it off when voltage gets too low. Sure, it will consume a few (couple?) more mA at full battery voltage (unless you like to run batteries down to nothing and hear the clipping, but then all these added components will consume a couple mA too, right?

 On the other hand, what I'd find more useful is if the amp turned OFF when voltage got too low, like if left on and unattended. Can than be accomplished simply by putting a zener + transistor in series on, to break the power input?

*Edit*: Not to be misconstrued, I think the work being done by Tangent et al is great. Educational, inspiring, and if you don't cut it out my office will look like radio shack before long!


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## JDAPJ

And...I would be really interested if u would take the time and explain it in detail..I'm sure there are more people than me interested in this..
	

	
	
		
		

		
		
	


	



 Works great with a distinct turn-over between led's, which I prefer, it all depends on where u decide to let the "red" led lit doesn't it? (a third color isn't that all important)

 /Peder


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## aos

I was asked to repost the image since the original has disappeared. I don't seem to be able to attach anything to either old or new post even though it's only 5k so here's a link.
   
http://www.aoselectronics.com/gfx/web/lobatt2.png


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## Avro_Arrow

You have to use the "insert image" button now...
   
  Circuit looks good, I'll have to try it out.


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## qusp

i'm all for discrete cct's for stuff, but there are all manner of chips from Ti and linear tech that do all of this and much more. one from linear in particular LTC2953
   that includes a momentary driver, LED driver, fuel gauge and voltage monitor. uses a resistor network to sense voltage and at a level of your choosing will trigger a response, be it a warning LED, logic level signal to an MCU for house cleaning, or initiate low current operation etc, then at the second low level it will trigger a shutdown which can be pirated to pull the regulators low and latch until battery goes over the set level again. all sort of possibilities. I prefer the linear stuff as I love their LDO's and this works very well with them. pretty low power consumption too. for all this it requires fairly minimal parts count also, probably less than above


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## noveltone

Thanks for your idea qusp. I checked around and it appears this chip is only available in SMD 12-pin, $5. 14uA current draw is attractive, however. I'm the one who asked aos to repost his circuit...thanks again!  I'm trying to shoehorn a bicolor circuit into the scrathpad of the Pimeta V2 board, which is spaced for through-hole components. Browndog has an apadter for $6 but in the end I think I'll do the simpler Tangent/Hadron or aos path. If some more ambitious person takes a crack at it, I'm all ears, however. As it is I'll be doing some 2-1 Y soldering in the air to make it work.
   
  Collin
  
  Quote: 





qusp said:


> LTC2953


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## aos

You pretty much need to design a circuit board for that, though. You also have to order the chip from the supplier. While you could just hack the analog circuit as well as buy parts in your local electronics store. And it is probably cheaper, too. I agree though, if I was designing something I'd probably use the chip like you mentioned. But if you are building somebody else's design it's much easier to add a small perfboard with a simple analog circuit than open Eagle, spend an hour in it and then wait 3 weeks for the boards to arrive (at high cost)...


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## Avro_Arrow

Here is the schematic from aos.
  I added some notes to help people adjust the values.


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## holland

Note down a PN for VR1, as Q1 and Q2 are noted as well.  The circuit looks easy to perfboard.


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## Avro_Arrow

Done
  
  Quote: 





holland said:


> Note down a PN for VR1, as Q1 and Q2 are noted as well.  The circuit looks easy to perfboard.


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## noveltone

Thanks for posting aos's design with additional notes.  I'd like to ask something. I had bought previously the parts required for Tangent/Hadron's circuit, and he's listing resistor values in excess of 100k.  Since I was going to use that circuit I'm resulting in R1 and R4 values of 470k and 511k (if R2 is 100k). Pretty high values and I'm concerned these will introduce needless noise to the Pimeta's power supply.  Aside from more current draw, is there any reason I can't tone those down to values more like those shown in aos'? 
   
  Collin


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## Avro_Arrow

Here is a little board I drew up for the circuit.
  1 inch square. I'm sure it could be smaller if
  you tombstone the resistors.


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## noveltone

Great! Thanks for drafting this out.


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## Avro_Arrow

The resistors won't add noise, they are not in the signal path.
   
  Quote: 





noveltone said:


> Thanks for posting aos's design with additional notes.  I'd like to ask something. I had bought previously the parts required for Tangent/Hadron's circuit, and he's listing resistor values in excess of 100k.  Since I was going to use that circuit I'm resulting in R1 and R4 values of 470k and 511k (if R2 is 100k). Pretty high values and I'm concerned these will introduce needless noise to the Pimeta's power supply.  Aside from more current draw, is there any reason I can't tone those down to values more like those shown in aos'?
> 
> Collin


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## qusp

Quote: 





aos said:


> You pretty much need to design a circuit board for that, though. You also have to order the chip from the supplier. While you could just hack the analog circuit as well as buy parts in your local electronics store. And it is probably cheaper, too. I agree though, if I was designing something I'd probably use the chip like you mentioned. But if you are building somebody else's design it's much easier to add a small perfboard with a simple analog circuit than open Eagle, spend an hour in it and then wait 3 weeks for the boards to arrive (at high cost)...


 


  agreed, not the sort of thing you would have just sitting in your parts bin,well unless you are me  and you do need a PCB really, suppose you could get creative. PCBs are not so hard to do yourself these days though, but does require a new level of knowledge and skill. that said the app note circuit is pretty small and simple. I like your cct as well and would probably do something like that if I was as you say; adding it to an existing device.


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## Avro_Arrow

Here is a schematic I was messing with a few months ago...
  I just thought I would post it for interests sake.
  I think it had about 0.2mV of hysteresis.
  You could sub the 7805 for any voltage reference.
   
  Sorry for the TINA drafting...


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## Reintz

Avro_Arrow, thanks for the schematic at page 4. I changed the layout a bit and made myself an indicator pcb measuring 14,9x15,6mm (which is something like 0,587x0,614inch). It fits in my CMoy perfectly.
   
  My CMoy before

   
  and now

  The smallest pcb is tle2426.


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## Avro_Arrow

Thanks Reintz.
  Thanks also to aos for the original schematic.
   
  Looks good, I'm putting one in my next amp also.


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## holland

Damn, that is a sexy little thing.  I wish I had the equipment to etch...


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## Avro_Arrow

Reintz got me thinking to try and make this smaller.
  I switched to SMD 0805 resistors and SOT23 devices.
  Here is the result. Same schematic as post #56.


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