why can't i can't connect a small light to an audio output and get light?

Question

connecting headphones to a small battery gives (a rather unpleasant) sound, how come it doesn't work the other way? (my goal is to get some LEDs to flash to music) and for my testing i didn't use LEDs, i just used a small test lightbulb

Answer 1

Light bulbs have a long lag in change of light intensity with change of voltage input. LEDs will change rapidly and can follow high pitched sounds.

Your other potential problem is that you may not be providing enough power to light the lamps. They should come on at reasonable brightness when you have the volume at half scale. That will give them enough dynamic range to change with the music. You might want to look at little kits that are made to modulate lamps based on sound input just to see how they do it.

Answer 2

ordinary light bulbs have a very low resistance until heated sufficiently to glow. your amp may think you have a short on the output and be shutting down for protection or it may not see enough loading once the bulb is lit. you didn't say what your "small" test lamp is, a flashlight bulb has a cold resistance of less than 1 ohm. Similarly, if you are using a 120 volt lamp of 60 watt class, it would have a resistance of 240 ohm when lit, so your amp may not have output at such loading. Most audio amps, particularly the IC types need a 10 ohm resistive load to work properly.

As to LED, they will fire at about 3 volts so a 1 watt audio IC will just barely work, but audio is not a DC voltage and the D in LED means diode. The LED fires in the reverse current connection. If you put two LEDs back to back, the conduction of the diode part starts at 0.7 volts, so the reverse current never gets high enough to start the light function in the other LED. If you use one LED the diode part will clamp the output of the IC at about 1/4 the voltage needed to fire the light function unless a current limiting resistor is put in series. The closest you will get to your desired circuit is to use 2 LEDs, in series and reverse connected and could work with a 2 watt class IC amp.

However the results will not be very satisfying as it will take a lot of audio just to get the LED to fire at all, and then it will quickly run to to full brightness. A better approach is to not use an audio amp, but a NE555 timer set to modulate pulses to the LED based on average DC of a rectified audio sample. That way the LED will "appear" to have varying brightness based on duty cycle of the pulses and can perform over a wider range of audio levels.

the difference between a battery and audio amp is a battery will deliver some current into any load and an audio amplifier only works into narrow range of load resistance.

Answer 3

Make sure that the energy being emitted by the headphone jack is actually enough to power the bulb. And that you aren't shorting the system by having the wires cross.