# Say It With Me: Root-Mean-Square

If you measure a DC voltage, and want to get some idea of how “big” it is over time, it’s pretty easy: just take a number of measurements and take the average. If you’re interested in the average power over the same timeframe, it’s likely to be pretty close (though not identical) to the same answer you’d get if you calculated the power using the average voltage instead of calculating instantaneous power and averaging. DC voltages don’t move around that much.

Try the same trick with an AC voltage, and you get zero, or something nearby. Why? With an AC waveform, the positive voltage excursions cancel out the negative ones. You’d get the same result if the flip were switched off. Clearly, a simple average isn’t capturing what we think of as “size” in an AC waveform; we need a new concept of “size”. Enter root-mean-square (RMS) voltage.

To calculate the RMS voltage, you take a number of voltage readings, square them, add them all together, and then divide by the number of entries in the average before taking the square root: $\sqrt{\frac{1}{n} \left(v_1^2 + v_2^2 +...+ v_n^2\right)}$. The rationale behind this strange averaging procedure is that the resulting number can be used in calculating average power for AC waveforms through simple multiplication as you would for DC voltages. If that answer isn’t entirely satisfying to you, read on. Hopefully we’ll help it make a little more sense.

If you live somewhere where summers are hot and dry, you can instantly tell which homes don’t have automatic sprinklers installed. Or they may have them installed, but like the blinking “12:00” on that VCR of yore, the owners may not have mastered the art of programming the controller. To be fair, the UI on most residential irrigation controllers is a bit wanting, which is the rationale behind letting Google Calendar tell your sprinklers when it’s time to water.

Granted, someone who is mystified by setting a digital clock is not likely to pull off [ClemRz]’s build. It’s still pretty simple stuff, though, centered around an ESP8266 as it is. And calling the result an “irrigation system” is a little bit of a stretch, given that it could only support a single zone with a solenoid valve harvested from a defunct sprinkler timer. But as a proof-of-concept, or to water a small area, it hits all the marks. The ESP8266 drives the latching solenoid valve through an H-bridge chip after reading your Google Calendar and looking for upcoming events to open or close the valve. The Google Script and the ESP8266 code default to failsafe so that a mistake doesn’t leave the valve open and run up your water bill or drain your well.

It’s easy to see how this can be expanded to control a multi-zone irrigation system and support a smartphone UI for instant control of the valves. Overrides based on weather forecasts would be a nice feature too. Or you could just read the soil moisture levels directly with backscatter sensors.

# Defeat The Markup: Iphone Built By Cruising Shenzhen

[Scotty Allen] from Strange Parts, has just concluded a three month journey of what clearly is one of the most interesting Shenzhen market projects we have seen in a while. We have all heard amazing tales, pertaining the versatility of these Chinese markets and the multitude of parts, tools and expertise available at your disposal. But how far can you really go and what’s the most outrageous project can you complete if you so wished? To answer this question, [Scotty] decided to source and assemble his own Iphone 6S, right down to the component level!

The journey began by acquiring the vehemently advertised, uni-body aluminium back, that clearly does not command the same level of regard on these Chinese markets when compared to Apple’s advertisements. [Scotty’s] vlog shows a vast amount of such backings tossed as piles in the streets of Shenzhen. After buying the right one, he needed to get it laser etched with all the relevant US variant markings. This is obviously not a problem when the etching shop is conveniently situated a stones throw away, rather simplistically beneath a deck of stairs.

Next came the screen assembly, which to stay true to the original cause was purchased individually in the form of a digitizer, the LCD, back-light and later casually assembled in another shop, quicker than it would take you to put on that clean room Coverall, you thought was needed to complete such a job.

[Scotty] reports that sourcing and assembling the Logic board proved to be the hardest part of this challenge. Even though, he successfully  purchased an unpopulated PCB and all the Silicon; soldering them successfully proved to be a dead end and instead for now, he purchased a used Logic board. We feel this should be absolutely conquerable if you possessed the right tools and experience.

All the other bolts and whistles were acquired as separate components and the final result is largely indistinguishable from the genuine article, but costs only \$300. This is not surprising as Apple’s notorious markup has been previously uncovered in various teardowns.

Check out [Scotty’s] full video that includes a lot of insight into these enigmatic Shenzhen Markets. We sure loved every bit of it. Now that’s one way get a bargain!