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Circuit Design Spells: Crystals and Resistors

They say that any sufficiently advanced technology is equivalent to magic. And while this may conjure up images of wizards in ivory towers, the truth is that there are rows of hedge mages waiting in the wings. In electronics, our hedge mages are makers without a electronics degree, wielding irons and running wrapping wire cowboy-style in our quest to make shit work.

Hedge Mages

It’s a rite of passage for a makeshift magician to learn a few rules of thumb; little tricks to use over and over again when designing hardware. I’ve collected a veritable menagerie of spells and here I will share them in hopes of bettering my fellow hedge mages. Engineers operating pick-and-place machines may laugh at their flaws, but 90% of the time they will get the job done.

Pull Up / Pull Down Resistors

Look at a electronics forum and you’ll find heated discussion surrounding what values to use for pull up or pull down resistors. You could do the proper calculations given the signal level voltage and the threshold level of your DAC, or you can follow this simple spell:

If your line is busy you use 4.7k, but if your line is quiet then 10k will do!

If your line is running a signal protocol like I2C or SPI, you want to make sure that it asserts high or low so you don’t waste time trying to troubleshoot protocol errors. Who cares if you drain a little more power through a 4k resistor, it’s not worth the hassle!

Loud Versus Soft

On the other hand, if you have a “quiet” line, like the reset button for an IC, then 10k should do the trick. No math required, just plug them in and focus on making something neat!

Crystal Capacitor Values

One common stumbling block for would-be sorcerers who want to interact with time is choosing a working value for capacitors in use with crystal oscillators. To achieve oscillation, the crystal needs a surrounding network (this is the capacitors surrounding the crystal) that will provide a 180 degree phase shift from input to output. This combined with the on-board amplifier in the IC will provide the 360 degree shift required for oscillation.

Crystals

Well that sounds confusing. Ignore all that noise! You only need to know one value to find the right capacitor values, and it’s usually listed right in the component name. That value is the load capacitance of the crystal. Once you know the load capacitance value you can use this simple spell:

C1, C2 = 2 * CL - 10pF

Using this to calculate your capacitance values will keep your crystals humming and your ticks on time!

Hedge Mages Forever

Don’t let a lack of experience or knowledge stop you from taking the plunge and trying your hand at circuit design. It’s truly a special moment when the first board you design yourself lights up without a puff of that magic smoke.

If this guide was helpful to you, or if you have comments about this article, please feel free to reach out. I want to hear what you think! If you want to see more content like this, view the archive, subscribe to my newsletter or support me on Ko-Fi.