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so I added an LED power indicator...but...
so I added an LED power indicator...but...
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Old 9th March 2019, 11:14 PM   #1
tedrick is offline tedrick  United States
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Default so I added an LED power indicator...but...

Hi Guys - so I am putting together a little lm386 amp using a nice guide at:

Build a Great Sounding Audio Amplifier (with Bass Boost) from the LM386

I wanted to add a power indicator so I found spot on the perma proto board I'm using -

What I'm finding is with the led and resistor I have a big drop in voltage to the circuit -- is this because the led and resistor are drawing power/changing the circuit?

I'm using a 500 ohm resistor - not sure what draw of the led is --

without the resistor/led in the circuit Vs out of the LM386 is about 7V but with the resistor/led the Vs is about 4V --

is the droppage because of my power indicator LED?
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Old 9th March 2019, 11:19 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by tedrick View Post
is the droppage because of my power indicator LED?
Must be. Even just 1mA should light up the LED, so instead of 500 ohms, use 5k.
See if the voltage stays at 7V now.
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Old 10th March 2019, 01:32 AM   #3
avtech23 is offline avtech23  Australia
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so I added an LED power indicator...but...
Are you connecting the LED in series with the Pin 6 of the LM386 or parallel?
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Old 10th March 2019, 04:03 PM   #4
turk 182 is offline turk 182  Canada
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is this running off a battery or a wall wart supply?
and just where is the indicator tied in the circuit?
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Old 10th March 2019, 07:11 PM   #5
tedrick is offline tedrick  United States
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Quote:
Originally Posted by avtech23 View Post
Are you connecting the LED in series with the Pin 6 of the LM386 or parallel?
Thank you - I think that's what I did wrong - I think I wired it in parallel -- so is that causing it to draw more power?

the led is getting + voltage from the rail at a separate point than where the IC is getting +

I'll try adding the LED at the same point as pin 6 of the IC -- as shown here:

http://www.generalguitargadgets.com/...tle_gem_sc.pdf

Thanks guys!
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Old 10th March 2019, 07:18 PM   #6
Mooly is offline Mooly  United Kingdom
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so I added an LED power indicator...but...
Connect your LED and resistor in series and then connect the series combination across the LM386 using pins 6 and 4. Make sure to get it the right way around.

If you have allowed the LED to pass to much current then it could be impaired/damaged. Rayma's suggestion of 5k sounds reasonable for your supply voltage.
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Old 10th March 2019, 08:40 PM   #7
radtech is offline radtech  United States
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With a 9V battery an LED with a 500 ohm series resistor will draw maybe 15 mA. If that is loading down the voltage that much you either have a bad battery or a bad connection. A good 9v should be able to handle 15 mA. For battery power you'll want to use as little current as possible for the LED and still have it bright enough so you can try higher resistances and possibly a high efficiency LED. The first thing I would try though is a fresh battery.
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Old 10th March 2019, 09:19 PM   #8
turk 182 is offline turk 182  Canada
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any chance we can get a picture of your handiwork with this build?

this may interest you LED Current Limiting Resistors - SparkFun Electronics
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Old 10th March 2019, 11:33 PM   #9
tedrick is offline tedrick  United States
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Thanks for your help on this guys -
I've attached in the led and r as radtech recommend and I'm getting better measurements

here's a couple pics of the project -- oval highlighted was my original position of the led leads, the rectangular highlighted shows recommended series config --

was my first setup (oval highlight) technically in parallel?

rectangular
https://i.ibb.co/KyRSBVD/IMG-20190310-181144338.jpg
oval
https://i.ibb.co/6wXGZz9/IMG-20190310-181133426-HDR.jpg
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Old 11th March 2019, 12:03 AM   #10
turk 182 is offline turk 182  Canada
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can't say for certain as to the series or parallel thing without seeing what's connected on the under(foil)side of the board...
the led is in series with the current limiting resistor but is in parallel with the ic power pins, don't know if that helps your understanding.
series parallel combination circuits can be confusing if your not familiar.
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