Single LED VU/signal meter - LM386? - diyAudio
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Old 15th January 2012, 01:05 AM   #1
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Default Single LED VU/signal meter - LM386?

What is the simplest way to control a single LED to indicate the presence of a signal coming into an amplifier? Essentially a VU meter but only using a single LED. This is quite common on the front of amplifiers used in pro audio & I want to replicate something similar for some of my DIY builds (or at least experiment with it).

Can I just use a LM386, connect the signal to pin 3 & the LED +ve leg to pin 5? I'm not expecting the result to be accurate in any way, just to vaguely increase the LED's brightness as the level of the signal increases.
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Old 15th January 2012, 01:21 AM   #2
wwenze is offline wwenze  Singapore
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Well... increasing the current through the LED doesn't really increase its brightness once it is bright enough. There's a photo here:

LED center How is LED brightness related to current?

So what signal/clipping indicators usually do is have the green orange and red LEDs turn on (or get bright enough) at different input signal voltages.
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Old 15th January 2012, 01:35 AM   #3
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I was thinking about something like the 'signal present' LEDs on the front of a JBL MPX 1200 (a single green LED per channel) which alter their brightness visibly in response to different levels of input - I presume then it must be something more complex than just an LED & an op-amp if the difference in brightness if you just change the current is so little as in your link?
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Old 15th January 2012, 02:39 AM   #4
wwenze is offline wwenze  Singapore
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If the LED is controlled via PWM then you have more control over the perceived brightness. A low-frequency class-D amp with a modified output filter might actually work.
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Old 18th January 2012, 09:28 PM   #5
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I just soldered together a little test circuit using a Tip31 & it achieves the desired effect. Completely inaccurate of course, but a suitable indication that there is a signal present.
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