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Old 27th March 2013, 05:53 PM   #1
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Default amp power indicator

In the Feb. 1984 (I know, old) Speaker Builder mag., there was an article on how to make a cheap and easy amp peak power LED indicator. I have one (it still works) but want to make some more for other amps I have. Has anyone updated the parts by any chance (e.g. the LM3915 is obsolete now)? I have tried to substitute new parts (an NTE 1509 for the IC) but haven't had any luck getting it to work. Thanks for any help.
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Old 27th March 2013, 06:21 PM   #2
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What makes you say the LM3915 is obsolete?
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Old 27th March 2013, 06:47 PM   #3
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Poor choice of words...after I saw your response I went online and whadda ya know, found it. Should have done that before. Now that I have your attention, the circuit uses 7 mini- LEDs which aren't specifically identified in the SB article. Can I just pick any LEDs from Mouser ( or whoever) or is there a certain type required by the LM 3915? I'm not a EE, so I am floundering here...Thanks for your help.
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Old 27th March 2013, 08:21 PM   #4
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You can use any type of LED. Yours just happens to use the mini 3mm size, but 5mm or larger, or rectangular LEDs, will also work. Any color.
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Old 28th March 2013, 12:30 AM   #5
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Thank you Sofaspud...In terms of "any kind of LED" I was thinking about whether LEDS are differentiated by voltage ratings, etc. and if an LM 3915 requires certain LEDs with specific ratings.Is it correct I don't need to worry about this? Thanks again for your help.
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Old 28th March 2013, 02:09 AM   #6
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The main LED specification & differentiation is the wavelength of light emitted. This includes everything from ultraviolet to infrared. Different semiconductor materials create the different wavelengths, and the material properties also create a different forward conduction voltage (i.e. the voltage where the LED turns from off to on). So green LEDs will all tend to have a similar voltage, all red a similar voltage, etc. Other than that it's mostly just the current rating and package size of a particular LED.
The latest LM3915 datasheet states it will drive any LED, and while that may not be strictly true, it should be true for any of the common indicator-type LEDs suitable for your project.
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Old 28th March 2013, 08:54 PM   #7
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LEDs tend to have a characteristic voltage drop, but the thing they care most about is the current through them. Those LM39xx chips have the LEDs wired to power supply on one end, then they bring the output pin on the IC to a logic low to light each one. It won;t care if the Vdrop on the LED is 1.2v or 2.1v.

Here is the data sheet on the IC, and if you just google LM3915, a number of VU meter circuits pop up.

I like to strip old dead gear down to the last screw. I find it good mindless activity when I need a break. I find stereo receivers and old cassette decks often have LED VU meters, and in many cases the whole meter is one little board. SO I save them, probably never use them for anything, but what the heck, a box of VU meters. Point being there is a ready made circuit and display, for free. Usually a four-pin connector: Power, ground, L signal, R signal. Hook it up on the bench, works like a charm.

http://www.ti.com/lit/ds/symlink/lm3915.pdf
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