Back when I was manufacturing and shipping soldering projects, I had many boxes with stock. I dreamt of a small device that I could leave in each box and update every time I take an item in or out of it. It needed to be cheap, keep state for a long time, be able to count to the thousands up or down, and have a fast- and slow-count mode for usability’s sake. I designed the circuit shown in Figure 1. It uses a “555” to generate a clock and a “4020” 14-bit ripple-counter chip to keep state. I didn’t mind a binary count; it kept costs down by not requiring a digit display and it exercises the mind. Being able to choose whether the count is up or down dynamically seemed too complex for the scope of the project, so I opted to design the circuit to be able to count up or down depending on how one built and by using solder-blob jumpers.
 
Binary counter circuit
Figure 1
I’ve set the 555 to output a fast or slow frequency depending on a choice of a double-state momentary pushbutton — in this case a wonderful Snaptron dome — that connected two different resistors to the feedback. Another pushbutton dome controlled the availability of power to parts of the circuit, and so minimized power consumption. Pressing that momentary button would allow the LEDs to turn on and enable the 555. If that button is not pressed, the 555 is in reset and the LEDs don’t have a way to be on. Finally, there’s a “reset” pushbutton dome that resets the count of the 4020.
 
(After building the circuit I discovered a minor issue. If I press the count button, the count changes even though I’m not pressing the power button. This is because the 555 will change the output state from where it was left off when reset got activated.)
 
BINCO
Figure 2
Theoretically, while the “power” button isn’t pressed and at room temperature the 74HC4020D consumes 8 μA and the TLC555 consumes 10 pA while in reset. Rounding up to 10 μA with a CR2032 capacity of 200 mAh we get over two years of running. But what consumes most is the 555 when it’s running: 250 μA (plus about 10 μA for the LEDs and counter), so that has a real impact on longevity.

I like this circuit, but I’m not sure it’s practical for what I intended it for, since in practice it won’t last long enough and I’d just waste batteries (and lose the count). A pencil and paper in the box and some discipline would meet and exceed the specs I outlined above! But maybe you could improve this circuit?
 

Circuit Design, Counters, and More

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