Evil Pumpkin

My kids had a big plastic golf ball made like a jack-o-lantern with a pumpkin stem on top they got from somewhere. It had a 4W incandescent light bulb in it powered from the electrical outlet that provided the light source. They either started fighting over it, or did not pick it up when I told them to clean up the living room. Either way, I confiscated it.

For whatever reason, I decided to pull out the light bulb and replace it with red LEDs as a light source. I wanted the light to fade on, stay on for about 10 seconds and then fade off, and then repeat the cycle about 10 seconds later. While that is not terribly novel, I think how I did it is relatively unusual in this day of microcontrollers.

In the description below, refer to the schematic at the bottom of the page.

A 555 timer chip along with R1, R2, and C1 creates a (almost) square wave output with about 10 seconds on time and about 10 seconds off time. The 555 chip I used was an ancient TTL chip with a 5V operating voltage limit and an unknown current drive limit. Because of its questionable current drive ability, its output goes to Q1 which effectively increases its current drive.

When the 555 timer turn off transistor Q1, capacitor C2 charges through R4 and R5. When the 555 timer turns on Q1, C2 discharges through R5 and Q1. With the capacitor resistor combination used, it takes a couple of seconds to fully charge and discharge.

As C2 charges some current is “diverted” through R6 and Q2 which turns Q2 on which turns on the LEDs. Because the capacitor charges slowly, the current through R6 increases slowly which means Q2 turns on slowly. As C2 discharges slowly, R6 current slowly decreases which turns off Q2 slowly. As a result, the LEDs are slowly turned on and slowly turned off.

Video of it in action is at https://youtu.be/OtLGioqxVX8

The schematic is shown below.