This instructable will show you how to make a "spooky fading eyes" circuit that can be used for Halloween decorations. All of the components should be available in any electronics hobbyist's box-o-parts, but if for some reason you are missing something, all parts are available from Radioshack.
Below is the summary of parts you will need. When I first started this project I came across several examples on the web that used OPAMP's, microcontrollers, or 555 timers. I am fond of designing circuits that do not require the use of any IC's, so here is what it takes to do that:
1 - 9V battery
1 - 9V battery clip
1 - 9V battery connector
1 - SPST toggle switch (single-pole single-throw)
1 - 100 ohm resistor
3 - 10kohm resistor
1 - 33kohm resistor
2 - 330kohm resistor
1 - 1Mohm resistor
2 - 10uF electrolytic capacitor
1 - 220uF electrolytic capacitor
3 - 2N3904 NPN transistor (or equivalent)
2 - LED (1.7V forward voltage, 20mA)
1 - Small perforated circuit board
1 - Breadboard (if you want to experiment or don't like to solder)
X - Screws (depending on what you mount the circuit to)
X - Wire
Step 1: The Circuit
The output of the astable multivibrator is slewed into a triangular shaped wave to control the final output transistor. This gives the fading in/out effect since the current through the base of the final transistor will transition slowly (i.e. not a square wave). Again, you may need to experiment with the value of the 220uF capacitor to get the best fading effect.
Step 2: Building the Circuit
Start with a solderless breadboard... make sure the circuit works before you solder!
For the perforated circuit board, make sure you can fit all the components on before you solder!
Also make sure you can mount the circuit board into your project.
If you are mounting the LED's into something (like a skull as I did), drill the holes into a piece of scrap wood first to make sure you have the right size drill bit!
You don't need to mount the LED's on the perforated circuit board directly. In some cases it might be easier to solder wires to the board and get the LED's into position that way.
Make sure you put the (-) side of the electrolytic capacitors towards ground!
Remember, the short end of the LED goes towards ground!
Step 3: The Circuit (rev. 2)
Step 4: Breadboarding
Please note that some of the values do not match the schematics (I just threw this together with parts that were close to the original values). The changes are:
R4 & R5 = 270k (were 330k)
R3 = 27k (was 33k)
C3 = 2 X 100uF (was 1 X 220uF)
And obviously I'm only driving 1 LED...
Video of the breadboarded version: