Spooky Fading LED Eyes Without IC's (updated: steps 3 & 4)

First of all, let me say that this is my first instructable. Secondly, I did this project a few years back, so I apologize for the lack of step-by-step pictures.

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

This circuit is called an astable multivibrator. It is basically the same thing as a 555 timer. The resistors and capacitors determine the frequency of the output signal. There is plenty of documentation on the web that describe how this circuit performs and how to calculate the values of the components to get certain characteristics. The values I listed on the previous page are what I used to make the circuit in the video on the final page. You may need to tweak some part values to get the exact timing you want. Don't be afraid to experiment!

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.

Comments

[Llamarama]

Very well written, very informative and generally a very good Instructable! I especially like how you labeled the key parts in the circuit diagram! Keep it up!

[Xombee]

I have a question about the schematic. Does the first capacitor have the positive side to the left, the second one to the right, and the third one pointing up? I wanted to make sure before I applied power to it. Thanks for your input!

[xenthuin]

Xombee --- Sorry for my VERY late reply... for some reason I didn't get emailed when you commented... anyway, your description of the direction of the capacitors is correct. I hope you got it to work!

[gschoppe]

if you fill the eye sockets with hot glue (even in a thin layer), you get an awesome diffusion that makes this effect much less obviously led based... awesome execution

[xenthuin]

That thought crossed my mind while I was writing the instructable. I've seen lots of examples on here for diffusion techniques, I may give it a try when I get some time. Thanks for checking out my instructable!

[Lftndbt]

Oh that is spooky!! Nice one!! My LeD Spidies would love a set of these. (check my profile for link, I don't like dropping links to my stuff everywhere) Thanks for your contribution. Very well written might I add.

[xenthuin]

Thanks for the comment. I love those Spidies on your profile!