This timer uses a minimalistic display. The decimal point blinks for the number of tens of minutes. There are four buttons. Button 1 resets the timer. When the timer is reset it goes into Helter Skelter mode:
If you push another button, the timer increments to one minute and starts counting down. Helter Skelter time *does* count against the first minute and wraps, so you have to avoid Helter Skelter after you push reset to set the timer to be accurate to one minute. Button three increments the minutes, and button two increments the tens of minutes. So, to set the timer for 31 minutes, push reset, push button four (leave "Helter Skelter") and push button two until there is a 1 with three blinking decimal points. Here is a picture of 21 minutes. You can't see the blinking dot, but you do see the dot, and you must trust me that this picture is truly 21 minutes. I used a regular PC speaker, since I have a bunch them floating around from clones over the years. The really cool thing about this timer is that it has a very loud, creepy, two tone alert.
I used an 8048 for it because I have a bunch, and it is my all time favorite piece of electronic coprolite. The 8048AH version of the 8048 seems to work better initially after applying power. The speaker is driven by an LM386.
Here is an disassembled listing of the binary
Here is a binary of what is in the 2716 program memory
Here is a PCB file of a single PC board
Here is a jpg of a tinned pc board after etching
Here is an encapsulated postscript of the schematic
Here is a fig (xfig) file of the schematic
Here is a pdf of the schematic
Here is a png of the schematic
Here is a postscript file of the PC board
Here is a PCB file of two timers (fits on one page of film to make two at once)
Here is a postscript file of a two timers PC board
Timer circuit board partly populated
Timer circuit board populated
Circuit Board Wiring Notes
Jameco still has 8035s for sale, which you can use for the CPU. They have 2716s for sale also. The 8048 is in old XT keyboards and other devices from the early 80s, so you might be able to scrounge one up.
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