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Simple 3 Point LED Volt Meter?

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EDY19

Tue Dec 26 2006, 12:53AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I've just got a couple subwoofers im my car, and im trying to make a little circuit in order to detect voltage pulses that go above 30V, 31.62V, and 34.64V which represent approximately 450W, 500W and 600W respectively with my 2 ohm speaker load. This way i can see the peak values and know if i have to worry about blowing the amplifier or subwoofers. I am interested to see if there is a fairly easy way to do this, i tried a couple things with zener diodes, but they seem to stay on when there is still current flowing, and the LED stays on, just slightly dimmed after a hard bass hit. I also saw the LM3915 IC, but I don't need anything all that complicated, and i have no use for A 10 LED meter. Suggestions, or flaws in my thinking? Oh, and Merry Christmas!

...

Tue Dec 26 2006, 01:54AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would use a few op-amps wired as comparitors (no feedback) and a few voltage dividers. All that you would need is 3 op amps (a lm324 would work great), and ~10 resistors, and course the leds. Make a 10:1 divider across the speaker input, and feed it into the + inputs of the amps. then put a variabe resistor (across 0-12v) into each of the - inputs of the amps. The outputs go into the leds.

BTW first post from my shiney new hx4700 that I put linux on

Simon

Tue Dec 26 2006, 02:50AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

... wrote ...

BTW first post from my shiney new hx4700 that I put linux on

Congrats!

I've done this before with an lm324. It works fine.

One lm324, a regulator for a fixed reference and a string of resistors as a voltage divider to provide the other reference voltages. You have the audio input to the non-inverting input and the voltage reference to the inverting. Very simple.

What can you do with that fourth op amp? You could make a filter to get the envelope of the signal or provide some slew.

EDY19

Tue Dec 26 2006, 03:29AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Come to think of it, i may just do a single led to represent when the load is drawing more than 500W RMS, which is what the amplifier is rated for. So if the RMS voltage is above 31.62V, or the peak is above 44.72V, the LED will light up. It seems to me like you could do this with a zener, dropping resistor, 1N4002 and a LED. Is there some way to get this working properly?

Simon

Tue Dec 26 2006, 10:19PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Here's my first thought. I'd drive the transistor to saturation and possibly get rid of R.

Caveat: I haven't tried this; it's just something I thought up.

Marko

Tue Dec 26 2006, 10:31PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

From a quick tought, I don't see why it wouldn't work with zeners.

Arrangement of zener, LED and resistor also needs diode in reverse to prevent LED reverse overvoltage and death.

It may require some fiddling, though, to get zeners properly 'aligned' as power and voltage levels may not be the same in reality and on paper.

Zeners may have somewhat ''smoother'' transition than LM3915 but I don't think of it as a problem with just 3 leds.

Considering your margin of just some 2-3 volts it's a bit hard to make them respond well.

Zeners will also suffer from exscessive dissipation due to high voltage drop, and using a generous series resistor is advised; but I'm pretty sure current will be high enough to drive a LED.
This way is, although, easy and doesn't require external supply.

If you need more power for any reason I suggest the circuit simon drew, or LM324 thing as a bit more-comlex.

Avalanche

Tue Dec 26 2006, 11:08PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I built a project a few years ago called a 'watt watcher'. I think I originally got it from an 'everyday practical electronics' magazine. Anyway, it is pretty much exactly what you are looking for, but i've looked all over and I can't find the photocopy.

Anyway, I can remember roughly what it consisted of, it was a row of 3 LEDs to indicate different power levels, and it used ordinary transistors and zener diodes probably something very similar to what Simon drew. So that approach certainly does work, I'll have another serious search through my old magazines if you're interested.

EDY19

Wed Dec 27 2006, 03:00AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I did find this site, which has exactly what I want, but for the sake of experimentation, im going to try something with zener diodes first

Sulaiman

Wed Dec 27 2006, 09:51AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I have a few thoughts on this topic;
1
If this project is for a learning experience or 'cosmetic' enhancement
then you should detect both positive and negative over-voltage
2
If it is to actually protect the amp/speakers then a speaker fuse would be better
3
A pair of 'inverse-parallel' silicon diodes across the input to the Power Amplifier
will prevent gross overdriving of the amplifier
4
Your ears will detect the onset of clipping quite well
5
Where will you locate the leds/indicators so that it's safe to drive and monitor the amp?
6
Two 39V zeners (in series 'pointing' in opposite directions) with a 12V indicator bulb in series
connected across the speaker wires would work I think.
7
Unless you are very careful with your wiring there is more chance of the monitoring circuit causing damage than damage due to overdriving.

Sorry for the quite negative approach
you may guess that I don't see the value of 'early warning' indicators for this application
other than 'cosmetic'

Simon

Wed Dec 27 2006, 11:11PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

What I don't like about using zeners directly to do this is that they hold their voltage. They don't suddenly become plain conductors.

Say you've got a 12V zener in series with a 1kohm resistor. If you put 12V into the circuit, this will be completely dropped across the diode, there will be no voltage across the resistor and no current will flow. If you have 12.5V, you have .5V across the resistor and .5mA. As you increase the voltage, the current increases. There's no "switch on".

Using the zener like this for a light will sort of work. You just won't be able to say, "Hey, the light's on. It must be this voltage." It'll be more like guessing what the voltage is from how bright the light is. (An LED might be better than a bulb by being more nonlinear.)

That's why I had a transistor and said drive it to saturation. It makes it more like a switch.

Edit: I had a look at that page you linked to. It's almost exactly what we were talking about from the start with a few embellishments. Just ask if you need any explanations.

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