Random Dialler for 555 Adding Machine

I threw this dialler board together to demo the 555 Adding Machine for the Wyong Field Day. Each year I take some of my latest projects to exhibit at the ARNSW Homebrew Group table. I had only just "finished" the adding machine, and started building this at 1 am the day of the event - I had a 7:00 am start - it was a long day!

The random dialler board.

The dialler is an Atmel ATtiny13V driving three 2N7000 pull-downs through three resistors. I probably might just as well have used the chip directly, but I figured the external transistors made it more robust and perhaps useful for other projects in the future. A lithium cell (on the back-side of the board) provides power for the unit. The clock is the internal RC at 128 kHz, it pulls 130 uA so the battery lasts a long, long time - but I turn it off by pushing some cardboard under the +ve contact.

The firmware code is very straight forward, busy waiting, AVR libc pseudo-random number generation, nothing fancy.

Very worth the last-minute effort though. I took the charlieplexed xmas tree too, and the blinking lights attracted a lot of passer-bys. I met a lot of old friends and some new "fans" of my site too. Many of the younger attendees asked questions, and everyone marvelled at the back-side of the adder's complex-looking wiring - some even played with the manual dialler when I plugged it in for them. I met one gentlemen in particular who used to work for Telstra, he told me all about the dialling mechanism's history, and could read the date codes, telling me it was from a 1988 production run - one of the last and not quite as old as I had assumed.

My projects on the homebrew bench.

I took a lot of photos and video of the Field Day this year. I'll be posting more about it soon.

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Parent article: 555 Adding Machine.