[cctalk] Re: Relay computers

Dwight wrote: Like minds, I guess. I did the same kind of thing when I was a kid, too. Except, we were pretty poor, and couldn't afford Radio Shack relays. I had to make my own, out of 1/4" plywood scraps for the base, tin snipped from coffee can lids, nails for pivot points, wire-wound nails for electromagnets, and tacks for contacts. I made both usual relays was well as relays with two coils, one to set the relay and another to "unset" it (latching). The wiring was all telephone wire, from a 50-pair cable that my Dad found somewhere and gave to me. My sequential combination lock was for a "burglar alarm" I made for my bedroom door. I made my own keyboards using similar materials, but mine were decimal, encoded with diodes that a neighbor who was into electronics gave to me. The combination was four digits that had to be entered in the correct order on the keyboard outside my door. I also had a keyboard next to my bed that allowed me to override the alarm if someone knocked to come in, as well as to reset the password at any time. The password was stored by sixteen of those set/reset latching relays. The four digit password had to be entered entirely, then an "ENTER" key pressed, and if there was an error anywhere in the password, as soon as ENTER was pressed, the alarm would go off, which was a ringer salvaged from a telephone that someone gave me. The only way to shut off the alarm was to enter the correct code. If someone opened the bedroom door when the alarm was set (by entering the code) the bell would start ringing. The transformer from my train set powered the thing. It worked reliably, but my Mom grew tired of it quickly. Not easy to enter the code with a basket-load of clothes in her arms. I learned a lot from making that thing. The hardest part was making the sequencer that would step as each digit was entered, then compared the entered digit against the corresponding digit of the stored code. That took quite a few relays. I got quite good at making the relays so that they were reliable and consistent in their behavior. It was a total rats nest of wiring on a piece of larger plywood that leaned up against a wall in my room. A few years later, I built a four-bit binary adder using a bit-serial ALU and sequencer made with old telephone relays. The two numbers to be added were entered using a keyboard I made from a salvaged touch tone telephone keyboard. Then, an ADD (#) button was pressed, and the sequencer (which was made with a motor that turned a drum with contacts made from sheet metal screws) would step through selecting each group of two bits, adding them, saving the carry, and then moving onto the next two bits. The result was displayed on five #47 lamps. It was not particularly fast; the fastest I could make it go would give a result in about 1/2 second. Trying to run it any faster resulted in errors, probably due to issues with the design rather than the relays themselves. The thing covered a tabletop, again wired together with telephone wire. I tired of it pretty quickly because it just added numbers. I had visions of somehow making a memory out of relays and making some kind of programming method, again probably using a drum with contacts that would close when a screw was screwed into the drum, such that it could run short programs, but I didn't have the space to build out the rest of it, so, I eventually took it apart so I could have my table back. What I'd give to be able to have all that free time during the summer when school was out. Virtually no real responsibilities (mow the yard, keep my room "clean", do dishes) - just limitless time to dream up projects to build with limited resources. I had no training in logic design, I just kind of figured it out as I went along. -Rick