GeekNet Inter-Office Telegraph System

Here at Columbia Data Products, we take very seriously the threat of natural disasters and nuclear war. Therefore, we have constructed our own inter-office telegraph system that we can use in case of an emergency that wipes out our internal computer network. That way we won't have to walk to each other's office to tell each other that civilization has ended.

Here is Bryan in his office with his telegraph station.

And here is Don in his office with the heart of GeekNet on his desk.

GeekNet schematic
This schematic shows how to construct an optical-only GeekNet, requiring only 3 wires be run from station to station. This is for the sake of simplicity, to get the general idea across. The 3 wires shown are +4.5VDC, signal, and ground. The voltage level +4.5VDC is chosen because the blue LEDs we use work best at that level. For the power supply, I used a battery holder that holds four AA alkaline batteries, only I soldered a wire across one of the battery slots so that it actually holds three AA batteries. If you are building your own GeekNet, feel free to choose whatever voltage level makes sense for your adapted design.

Each LED/key pair (arranged in columns in the schematic) belongs to one of the GeekNet participants. When any participant presses his telegraph key, all of the LEDs light up simultanously, because they are all wired in parallel. In essense, this is a telegraphic "party line". Therefore, it is important to establish a protocol for one person to call a particular other person on the network. We do this by assigning a unique calling code to each person on the network. For example, Bryan's code is a single "dit" repeated every few seconds, and Don's code is a pair of "dits" repeated every few seconds. When Don wants to call Bryan, he repeats Bryan's calling code until either Bryan answers, or Don gets tired of trying to call Bryan. If Bryan answers, he does so by holding down his key for a few seconds, causing all LEDs to stay on. Then the caller (Don) identifies himself using his own calling code (in this case, two dits). Then Bryan confirms that he is the intended recipient of the call by sending his calling code back a single time. This final step in the protocol might sound redundant, but it is useful in the case where someone other than the intended recipient is answering the call. Of course, spoofing is trivial, so the entire protocol operates on the honor system.

In practice, it is a lot harder to receive Morse Code by looking at flashing lights than it is by hearing a tone being turned on and off. Therefore, we also have a single shared audio oscillator circuit whose DC power inputs are connected in parallel with all the LEDs. From the oscillator's output runs a fourth wire, called the tone line.