Giving the Gift of 'The Internet' for Christmas

While on holiday break this year, I decided to have some fun dusting off my analog electronics skills with a fun build of "The Internet" from everyone's favorite show The IT Crowd. If you aren't familiar, here's the clip:

Now I've seen this project done a few times before, but I wanted to try my hand at it with the simplest analog circuit of discrete components that I could think of.

Naturally, the first circuit that comes to mind is a simple RC time constant circuit, where a capacitor is charged with a given voltage source based on the current supplied to it via a resistor. The values of the resistor and the capacitor determine the rate at which the capacitor is charged up to the same potential as the given voltage source.

To discharge the capacitor, the voltage source must be removed and the circuit shorted to ground. The circuit will take the same amount of time to discharge as it did to charge.

The LED on the prop box in the clip above appears to be blinking at about a 1 second rate (one second on, one second off). To achieve this, a 100uF capacitor is needed with a 10kΩ resistor feeding it.

In order to combine the charge and discharge of the capacitor into the same circuit, an astable multivibrator uses two NPN transistors with their emitters grounded and their bases cross-coupled with each other (the base of the first transistor is connected to the collector of the second transistor and vice-versa). The resistor and capacitor of the RC time constant circuit are then placed between the cross-coupled transistors such that when one transistor is on, it charges the capacitor on the opposite transistor until it turns on which removes the voltage source from the first capacitor until it discharges.

I did a quick simulation in LT Spice to verify the circuit. Since there only needs to be the one LED flashing, the second LED in the circuit can simply be removed and replaced with a short.

To build the circuit, I breadboarded it first to get my general layout of the components before transferring it to be more permanent on a perf board with the LED on long leads to reach the top of the prop box to give me the freedom to mount it wherever inside the box.

A simple AA battery holder with 4 batteries did the trick nicely to power the little circuit for quite some time to come. For the switch (since I'm not the most mechanically inclined) I used a latching push button that's accessible via a hole drilled in the side of the project box.

I attached the battery holder and the circuit board to the inside of the box using velcro strips for easy swap out in the future.

And just like that, the gift of "The Internet" is made simple! This was a fun little holiday break project that was simple yet kept me from getting bored since the holidays this year weren't as eventful as usual!