Low battery indicator circuits

[flecom]

cool see i kinda had a good idea that time

 

[sneared]

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

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

 

[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).

 

[tangent]

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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.

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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.

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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.

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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.

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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.

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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.

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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.

 

[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.

 

[hadron]

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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.

 

[tangent]

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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.

 

[hadron]

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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]

 

[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.

 

[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.

 

[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.

 

[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).

 

[tangent]

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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.

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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.

 

[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.

 

[Wooderson]

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

Original link here.