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LED peak level indicator

post #1 of 6
3/6/11 at 3:03pm
Thread Starter 

So I'm a big fan of blinking LED's and so on, so I was thinking it would be pretty cool to have a red and green (right and left) LED inside my preamp that would illuminate when a desired level was reached on the input. I searched around the web and couldn't really find what I was looking for so I thought I would come here for help. The closest I've found is this:

 

 audio_indicator2.gif

But it is stated to work more on speaker outputs (>1000mV), and what I want is something for line levels. Also, I cant seem to find any 2n2222's (I know they are somewhere, but where...) But I did find a baggie of 2n5089 tranies.

 

So, can someone help me with a simple simple circuit that could do what I want, preferably run on 12-15v and use 2n5089 transistors?

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post #2 of 6
3/6/11 at 3:12pm

You should go for an led level meter based on a national semiconductor lm3914 or similar. Would be way cooler than just a peak detectorbeerchug.gif

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post #3 of 6
3/6/11 at 3:23pm
Thread Starter 

I've thought about that and dont really have the room. A simple small 2 led indicator would be pretty fun anyway!

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post #4 of 6
3/14/11 at 4:52pm

Comparator circuit. That will do it.

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post #5 of 6
3/14/11 at 7:07pm
Thread Starter 

I googled it and found many different circuits. any specific links? I dont really want to use a specific IC, i would rather something build one diodes, transistors, resistors and capacitors. 

 

Any help would really be great guys!

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post #6 of 6
3/14/11 at 8:08pm

The best I was able to find is on the following link. If you look at page 10, it has a circuit called a "Limit Comparator". You actually only need one half of this circuit for what you want to do.

 

You basically set some reference voltage on one of the pins, and feed your signal to the other. When your signal exceeds your reference, the output of the comparator changes. This output will most likely need to dive some type of transistor, as comparators can't always source enough current to light an LED. The comparator looks just like an op amp, only it has no feedback. It basically sits at one rail or the other. An excellent book with some comparator applications is the Sedra & Smith book. It's a little math heavy, and a bit boring, but it does give you the basic idea:

 

http://www.amazon.com/Microelectronic-Circuits-Electrical-Computer-Engineering/dp/0195338839/ref=sr_1_2?ie=UTF8&qid=1300158384&sr=8-2

 

http://www.national.com/ds/LM/LM193.pdf

 

Some people have used op amps as comparators, it is usually poor performing though, and you need lots and lots of hysteresis to pull it off.

 

The comparator packages come in amazingly tiny packages, so there won't be a huge size constraint for getting one into your circuit. If fact, the transistors are bigger than the comparators in some cases.

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