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Old 14th October 2008, 02:26 PM   #1
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Question simplest LED circuit driven by mp3 player headphone amp

Hey guys, what is the simplest circuit I can use to get an LED to light up every time the headphone amp of my iRiver mp3 player gets up to, say, -24dB?
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Old 15th October 2008, 12:42 AM   #2
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try something like this. first set the bias threshold to where the LED barely begins to turn on, and back off 1/10 of a turn or less. then play a tone into the circuit at the desired signal level and adjust until the LED is barely lit,
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Old 15th October 2008, 05:01 AM   #3
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sweet, but check it out. i'm a total noob so is that arrow on the bottom a ground? since this gonna be portable and isn't actually gonna be able to touch a ground, would that be the negative terminal on the 9 volt battery? do I also connect the ground from my headphone amp to the same place?

I have 2N2222 transistors. how would i modify this to use that part?
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Old 15th October 2008, 11:29 AM   #4
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you could use the 2N2222 transistors, but your bias voltage will be about 0.7V instead of between 3 and 5V, so you will want to replace the 100k resistor (R3) with a 10k or 20k. R1 would connect to the collector, ground to the emitter and the junction of R2,R3 to the base.

on a 2n2222 if you look at the transistor with the flat (numbered) side toward you and the wires pointing down, the wires are from left to right Emitter Base Collector. -28db is about 2millivolts, not sure if a 2N2222 will have enough gain by itself to be a good indicator driver. maybe the MOSFET would suffer the same problem. you might need to make the two 2N2222's into a darlington where you connect the collectors together, connect the input transistor emitter to the base of the output transistor, and the emitter of the output transistor goes to ground. this creates a compound transistor known as a darlington. the gain of the transistor is the gain of the first transistor multiplied by the second one, the base is the base of the input transistor the emitter is the emitter of the output transistor and the collector is the two collectors tied together the bias will now need to be 1.4 volts.
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Old 17th October 2008, 05:28 AM   #5
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cool. so do the negative battery terminal and the headphone amp ground both connect at that ground - type symbol?
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Old 17th October 2008, 11:26 AM   #6
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yes. ground symbols are somewhat iconic (they somewhat resemble what they are, or were when the symbol was invented) here are several different symbols for ground some are for "EARTH" ground some are for chassis ground or "common" you may even see more than one used in a schematic or the same symbol but a letter like "C" or "E" or "G" or other letters like "A" for analog and "D" for digital.

a couple of the symbols are iconic of a stake driven into the dirt, a couple are iconic of attachment to a metal frame (once in a while you will see "frame ground" referred to), one is iconic of attachment to a busbar..... you get the picture? (pun intended)
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Old 18th October 2008, 05:32 AM   #7
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Ok, i set up the circuit like you said with the 2n2222, but I'm having a rough time getting it biased with fixed resisters (i don't have any pots and i need this thing to be as robust and compact as possible). am I right in assuming that the total voltage drop across R2 and R3 should be 9v and I'm looking to get around .7 across r3:


bias voltage / R2 voltage drop = R3/R2 = .7v/8.3v = 10kOhm/R2

R2= 10kOhm*8.3v/.7v = 83kOhm/.7 = 119kOhm


does that look right? my circuit seems to be intermittently validating it, but it just died, so i wonder if i blew another transistor.
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Old 19th October 2008, 05:05 AM   #8
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just an "off the top of my head" calculation. make R2 30k, and R3 2.4k. the reason for the high resistor values with the mosfet, is that the gate was being biased by voltage only. a bipolar needs a bias current.

with the 1k resistor in series with the LED, and with other values shown in the drawing or suggested in this message, you shouldn't be burning transistors out, especially with a 9V source. the transistor ideally should be biased just at it's conduction threshold so that adding signal will turn it on
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Old 19th October 2008, 07:31 AM   #9
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There are 2 main types of level detectors.
* peak
* average

a peak detector will make LED light as soon as only one short signal reaches a certain voltage
as such sginal peak can be very short, the human eye will not be able to see it
* so peak detectors needs some storage
usually the short peak will load one rechargable capacitor
and fed by the capacitor the LED will light long enough for human eye to see

average level detector
this will light when the average of the music reaches a certain level
* by simply adjusting one potentiometer
we can adjust so that eye can see the LED light at one average level
(even if the peaks are not visible)
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Old 19th October 2008, 05:06 PM   #10
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the level detector in question is just a "can you hear me now?" indicator, nothing fancy....
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