#555 timer

Sherman, set the WABAC machine to summer of 2013.  It was late July, and my mom and I (at the encouragement of her friend) had started watching the new Doctor Who, and we were over a season into it.  On an outing, we just happened to stop at a B&N bookstore, where I saw this cute light up TARDIS, and happily bought it.  It’s about 3″ tall, including the light, and 1.5″ on a side:

This is with the white LED, of course; the blue one looked MUCH more out of place.

I was initially disappointed to discover that its illumination was a) with a blue LED, and b) static, either on or off with the switch on the bottom.  So I opened it up and, finding a sufficient amount of extra space inside, started planning a hack.

I didn’t actually get around to serious planning until a month later, when I figured out how I might build an LED “fading” circuit with minimal component and power requirements.  So I did a Google search for circuit ideas, and what I came up with was this simple 555 timer circuit:

Yes, I drew this.  One of the perks of growing up with an EE is learning electronics and schematic drawing.

Besides the ever-useful 555 chip, only a few other components were needed: a capacitor to store charge (which helps create the “fading” effect); a potentiometer (variable resistor), to adjust the frequency (the speed of the fading); a general-purpose transistor; and another resistor for the LED (the blue LED came with one connected near the switch, I think, but I used a different value).  Finding all of these parts, except one, didn’t even require leaving the house (yet another perk of living with an EE), but I did have to find a 3 mm white LED.  (At Fry's, probably.)  This is the finished circuit, which I put together that Labor Day weekend:

The part number is vague, but the chip is definitely a Philips/Signetics NE555.  It was a numbering scheme often used by Hewlett-Packard for outsourced parts, so this was probably among the old parts salvaged by my dad from his work.

Assembling and soldering the circuit took me several hours, mostly because I am a perfectionist was being careful to not make mistakes.  Then there was the difficulties I had with the battery—or rather, batteries.  The toy TARDIS came with a single CR1632 3-volt lithium cell, with a metal contact on the side for the (+) and on the bottom for the (-).  To operate the 555 and LED, I needed at least 5 volts; the simplest solution to this was to use two CR1616 3-volt cells in series.  The problem was that the tab on the side (for the + terminal) would contact both batteries, making a parallel circuit.  So, I had to fabricate a new terminal that goes over the top of the cells, without touching the sides: