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Old 11th April 2011, 08:58 PM   #1
graygem is offline graygem  United States
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Default Need audio input indicator

I need an circuit for a simple audio input signal indicator for a small 3 channel mixer. I just want to be able to know when I have a audio signal coming into the channel.
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Old 11th April 2011, 09:04 PM   #2
zdavesf is offline zdavesf  Canada
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look up a comparitor circuit. Can be done with with cheap (or expensive) opamps. but comparitors like LM393 are best. you can also get the LM3915 chips and have a VU meter.

Dave
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Old 12th April 2011, 04:34 AM   #3
graygem is offline graygem  United States
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I'm looking for something simple with a low parts count. Could you direct me to anything specific?
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Old 12th April 2011, 10:56 AM   #4
AndrewT is offline AndrewT  Scotland
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lm391x LED drivers
strip of rectangular LEDS, or discrete LEDs.

Did you try looking up lm3915 after that previous posting?
What's the point of asking if you are not prepared to listen?
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Old 12th April 2011, 01:44 PM   #5
graygem is offline graygem  United States
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The point is I need something cheap, and simple. The lm3915 is too much for what I want, it is also an obsolete chip. All I want to do is know when an audio signal is present on a mixer channel. One single LED. Something like this peak indicator circuit.
Click the image to open in full size.
from Extreme Circuits that I just found.
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Old 12th April 2011, 02:30 PM   #6
LAJ is offline LAJ  United States
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Default You meant LM339

Thats about as cheap and simple as it gets.
But I like the Samsung chip KA2281. It has hysteresis that the LM39xx chips do not have.
It only drives 5 LEDs which makes it simpler to build.

Last edited by LAJ; 12th April 2011 at 02:32 PM. Reason: Added info
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Old 12th April 2011, 05:24 PM   #7
graygem is offline graygem  United States
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I also found this.
Audio LED.jpg

Will this run alright on 12v? Or what should I do to run it on 12v?
Thanks.
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Old 12th April 2011, 05:50 PM   #8
LAJ is offline LAJ  United States
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Surely the transistors will operate at 12V. The pot will probably have to be re-adjusted and/or changed to a higher value.
The advantage of hysteresis in an audio display is that at low passages the LM39xx LEDs will flicker.
Whereas, with hysteresis, the LEDs are either full on or full off. Some people like this display better and professional equipment often uses this technique.
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Old 12th April 2011, 06:17 PM   #9
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How about 3 DPDT relays, with an LED parallel with the coils, rotary switch to switch from relay to relay.
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Old 12th April 2011, 08:21 PM   #10
Itsmee is offline Itsmee  England
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Quote:
Originally Posted by graygem View Post
I also found this.
Attachment 218181

Will this run alright on 12v? Or what should I do to run it on 12v?
Thanks.
Change the 100R to 1K.

The simplest you could make it is this.
Attached Images
File Type: jpg Audio LED.jpg (45.8 KB, 247 views)
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