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The Lord of Time (a.k.a. NE555)

A project log for 74xx Discrete Clock

A retro-style digital clock based on 74xx discrete logic chips

stephanStephan 05/19/2020 at 17:331 Comment

What is the most elementary part of a clock? The clock source itself! Here we need some kind of oscillator with a stable and well-know frequency.

There are a lot of oscillator circuits out there, but I wanted something simple, easy to build ad precise of course. I found some 10MHz oscillators ins my shelves, but the clock divider circuitry would be way too large (too much ICs). I also found a 32.768kHz crystal from an old kitchen clock. Just add a 15-bit counter (4x74161) and you get a nice 1Hz clock signal. Sounds good so far! I tried building a simple oscillator for the crystal based on an inverter and some resistors and tiny capacitors. Unfortunately, I was not able to get it oscillating (maybe a breadboard is not a good playground in this case). I had no dedicated oscillator chips for the Quartz available... so what do to?

NE555 to the rescue! Oh dear, the good old NE555. I have used it in so many (not very time critical) projects. So why not use it for a clock?

I am using a straight forward oscillator circuit for the NE555 here, but with an additional diode to have identical HIGH and LOW times. These times are defined by the capacitor (2x1µF Wima foil capacitors) and two potentiometers. I wrote a simple microcontroller program to measure the HIGH and LOW times so I can adjust the potentiometers to try to come as close as possible to 1Hz. Well, it worked - somehow. I fixed the potentiometers with some fancy nail polish. Of course the precision is not the best (e.g. thermal influence on the capacitors and resistors).

Schematic:

Pictures:


Discussions

Ken Yap wrote 05/19/2020 at 23:26 point

Well done on the project and being able to reuse a whole bunch of chips from the grab bag. Did you encounter any faulty chips in the bag I wonder?

As for the 32.768 kHz crystal oscillator, it's very sensitive to stray capacitance. Have a look at #Crystal tester for some details. If you have some CMOS inverters in that bag, sure you you can make an oscillator with the crystal.

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