Step 1: Safety Precautions
Step 2: Materials Needed
-9 Volt Battery Connector (if using a battery)
-An Audio Splitter
-Speakers or Headphones
-An MP3 Player (or computer)
-1 or more LEDs of any color
-1 NPN Transistor (this guide uses the 2N3904)
-1 100-400 ohm resistor
-An audio Y Cable
-A few alligator clips
Step 3: Calculating Resistor Values
resistance(R) = (power supply voltage - LED voltage drop) / LED current
The typical recommended LED current is 20mA, our power supply voltage is 9V, and the LED voltage drop for a clear blue LED is 3.0V. This results in:
Of course this doesn't need to be exact, a 330 ohm resistor will work fine here. If we plug 330 ohm back into our formula we get
(power supply voltage - LED voltage drop) / resistance (R) = LED current
(9-3)/330 = 18 mA
18 mA is a good value to power our LED. If the current is too low, the LED will be dimly lit, and if the current is too high, the LED will become very bright and may explode.
Step 4: Powering the Breadboard
2) Insert the wires into the respective positive and negative rails of the breadboard
Step 5: Powering the LED
1) Insert the negative end of the LED into the negative rail, and the positive end into an adjacent hole.
2) Insert the 330 ohm resistor in series with the LED, connecting the positive leg to Vcc (9 Volts).
The LED should be illuminated. If not, check the orientation of the LED and that all connections are inserted into the breadboard fully.
Step 6: Adding the Transistor
1) Insert the NPN transistor into the breadboard so that the 3 different pins are in separate columns.
2) Using a jumper wire, connect the emitter pin to ground.
3) Reorient the LED so that the negative leg is connected to the collector pin.
4) Connect a jumper wire to the base pin. (we will use this for our audio signal later)
Now our switch is almost ready, but the LED will not turn on yet, because we haven't connected anything to the base.
Step 7: Connecting the Audio Y Cable
1) Connect an alligator clip to each of the RCA pins.
2) Connect one alligator clip to the ground sheath on one of the channels. (IMPORTANT: ensure that the clips do not touch)
3) Connect the other ends of the two signal-carrying alligator clips to the base pin of the transistor.
4) Connect the other end of the ground alligator clip to a jumper wire inserted in the ground rail of the breadboard.
Now our circuit is almost complete. We are done with the breadboard, so go ahead and plug the battery in again.
Step 8: Seeing the Flashing LED
1) Connect the audio jack end of the Y Cable to one of the audio splitter inputs.
2) Connect the headphones or speakers to the other audio splitter input.
3) Connect the other end of the audio splitter to the MP3 player or computer.
Step 9: Enjoying the Result
2) Ensure that the battery is connected.
3) Start the music and enjoy the show!
Step 10: Additional Modifications
-Adding additional LEDs
-Filtering out treble, bass, and band frequencies and blinking different colored LEDs
-Separating out the left and right channel to control different LEDs
-Changing the input to be driven by a microphone
-Inserting the circuit into a set of speakers.
In the above video you can clearly see that the quieter tones aren't picked up very well if at all by the circuit, this issue could be alleviated by using an operational amplifier to amplify the signal. Unfortunately, this would require either a dual supply or a rail-splitter, both of which are beyond the scope of this tutorial. If you are interested in reading about amplifiers, have a look here: http://en.wikipedia.org/wiki/Operational_amplifier_applications#Inverting_amplifier
Below is an example of a similar circuit I made that makes use of high-pass and low-pass filters to separate out the treble and bass signals. To achieve ideal results in a filtered circuit, it is recommended that an amplifier be implemented. For more information about filters, have a look at these pages: