Oct262019

Why switch debouncing is so important...

Published by paul at 1:58 PM under General

Here's a graphic illustration of why mechanical switch debouncing is so important:

 
 
It's the output of a push-button switch when I press it. The metal contacts inside the switch bounce against each other, creating two pulses when I expected just one. If I'd been debugging something sensitive to pulses, like using the output as a clock pulse, this would have messed up the debugging.
 
There are various methods of debouncing switches, and the most common is to use an S-R latch


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Oct252019

Smoothing capacitors on power lines

Published by paul at 3:42 PM under 555 | 8-bit computer | General

Well, the best laid plans of mice and men, and all that. Now I've got a long-term project to finally get back into electronics and blogging after an almost 10 year gap.

I'm building an 8-bit computer on breadboards, based on the kit and videos by Ben Eater - see here.

The first thing I want to blog about is something new I learned today. I'd always known how putting small ceramic capacitors across power lines helps to smooth out logic transitions and prevent spurious problems, but I didn't know about using larger capacitors for the same purpose.

Below is a scope plot of the rising edge of the output from a 555 clock circuit, with a 1us timebase, and no smoothing capacitors:

 
 
You can see that when the transistors in the 555 switch on, there's a big pull from the power supply and the output voltage spikes momentarily.
 
This next plot is with a 0.01uF ceramic capacitor across the power lines: 
 
 

You can see that the voltage spike isn't so high, but then oscillates down to a steady state over a few microseconds.
 
Finally, here's a plot with a 10uF electrolytic capacitor across the power lines:
 
 
 
You can see that the rising edge is super-smooth as the capacitor provides all the necessary power without the big pull from the power supply.
 
You learn something every day!
 


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