I'd like to hook up an LED to a cell phone's headphone jack. When there's audio, turn on LED, when there isn't, turn it off. Is the best way to go about this just using a npn transistor? Any links to a complete circuit? I know I probably will have to supply external power using 5v.

  • 3
    \$\begingroup\$ If you want to have the LED flash in the rhythm of music, take a look here for some ideas. \$\endgroup\$
    – AndrejaKo
    Feb 18 '12 at 0:13
  • \$\begingroup\$ I have used an earplug set from ebay that pulses light to sound on the connecting cable. To activate it I call the phone and it flashes. \$\endgroup\$
    – ElsieW
    Dec 23 '16 at 14:28

Vin through R1 through D1 through R2 to base Q1
C1 from D1 - R2 junction to ground
+5V through R3 through LED1 to Q1C

Q1 any NPN higher beta better. Emitter to ground.
R1 about 100R - just stops cct loading Vin much.
C1 say 19 uF - 100 uF.
D1 = 1N4148 or anything.
R2 = 10k
R3 = 220 ohms.
Q emitter to ground.

Whoever may feel free to draw up circuit.
Vin R1 charges C1 via R1 & D1.
Vootage on C1 turns on transistor via R2.

Vin_peak ~~~= 4 x sqrt(power) for 8 ohm system.

Try this - just maybe.
Here the LED is powered BY the audio sugnal directly.

Vin through R1 through D1 through LED to ground.
C1 from D1.LED join to ground.

D1 = 1N4148 or anything. C1 say 10 uFto 100 uF. R1 SOT( select on test) say 470 ohms to start.

  • \$\begingroup\$ I hope you can help me out with these questions so I better understand this circuit: 1. What do you mean that R1 stops circuit from loading Vin? Doesn't D1 stop any current from flowing back to Vin ? 2. What is the reason for having R3 before the LED ? Thanks \$\endgroup\$
    – M Sleman
    Feb 21 '12 at 18:33
  • \$\begingroup\$ @MSleman - Input "sees" whatever load you attach - input is AC so diode loads half cycles and distorts sound if allowed to. R3 controls LED current. No R3 = very bright and very short lived. Order of R3 and LEd unimportant - can go R3 - LED or LED-R3. \$\endgroup\$
    – Russell McMahon
    Feb 21 '12 at 22:00
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    \$\begingroup\$ Got it. I had a chance to build the first circuit but the led is still flashing while playing a file with constant loudness. I tried 10, 50, and 100 uF capacitors. Any other ideas to make the led light steady ON or OFF? \$\endgroup\$
    – M Sleman
    Feb 23 '12 at 6:36
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    \$\begingroup\$ Also, hooking up to my pc headphones works. Doesn't work to my cell headphones. Seems like headphones on cell only output 0.6 mA / 0.1v \$\endgroup\$
    – M Sleman
    Feb 23 '12 at 7:03
  • \$\begingroup\$ Try R2 = 100k. Having a high current gain transistor will be necessary as R2 increases. I find BC327-40is excellent. Using a MOSFET for Q1 will allow longer time constants. \$\endgroup\$
    – Russell McMahon
    Feb 23 '12 at 8:49

I can give an answer to half of your question. Basically connect anode of the LED to microphone port and cathode to the ground. You should have a LED lit, note that LED will be constantly on.

  • \$\begingroup\$ No current-limiting resistor? \$\endgroup\$ Apr 22 '13 at 9:05
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    \$\begingroup\$ If you are up for impedance matching for the maximum power transfer, You can also add a resistor in series but note that either ways power wont be the possible maximum on LED. \$\endgroup\$
    – kunguz
    Apr 22 '13 at 10:26

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