Jonathan Thomson's web journal

7400 Competition — Flickering Jack-o’-lantern October 21, 2011

Filed under: Electronics — jethomson @ 12:43 am

The dollar store sells some cool, toy jack-o’-lanterns that have a small incandescent bulb with a bimetallic strip which causes it to blink and a removable battery holder with an on/off switch. They are very easy to disassemble. They’re a great deal for just a $1! Here’s a picture of it’s insides before I added my custom board.


Any or all of the six inverters in a 74HC14 can be used as a relaxation oscillator by adding a feedback resistor and a capacitor from the input to ground. The board below produces four square waves with frequencies somewhere around 2.5, 6.25, 9.19, and 15.24 Hz. Combining them together with a simple resistor mixer results in a seemingly random, non-periodic waveform, which when used to drive an LED causes a fast, eerie flickering effect. I placed one blue and one red LED in circuit such that when one was lit the other would be dark and vice versa. The red LED has a lower turn-on voltage so it’s the dominant color seen. The LEDs are covered in hot glue to diffuse their light.



Using a resistor mixer like this wastes power because whenever two inverter outputs are high and the other two are low then both LEDs are off and the only load is 1 kohm. However, the mixer has the advantage of allowing the square waves to both add to and subtract from one another meaning that an LED will put out different levels of brightness as well as being on and off for different durations. The red LED has three levels, bright, dim, and off, but the blue LED with it’s higher turn on voltage only turns on and off. This mixer also makes it very easy to control two LEDs by sourcing current to one and sinking current from the other. If you don’t want dueling LEDs you can use the outputs to drive transistors in an open-drain arrangement to save power and get a wider range of lighting levels. It’s also possible to make a simple persistence of vision device by connecting different colored LEDs to separate oscillators. For a more candle-like flicker effect I suggest using resistors in the 1 to 10 Megaohm range. Ultimately I settled on the values I used through trial and error until I found an flickering effect I liked.


Watch closely and every once in a while you’ll see the jack-o’-lantern turn completely blue with no red for a brief instant. It looks like a creepy after image. The blue flash happens so fast that you’re not quite sure what you saw which gives this jack-o’-lantern a very spooky quality. Unfortunately the flickering is a bit too quick for my camera so you will see some screen tearing in the video.



Here’s a link to the RGB pumpkin I did last year.


3 Responses to “7400 Competition — Flickering Jack-o’-lantern”

  1. Little C. Diagram please?



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