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.
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.