Detailed breakdown of a digital pregnancy tester shows how it "reads" the same paper strip you can buy cheap

This was a tweet that went viral last month:

My wife had been checking if she was pregnant, initially with test strips, which can cost as little as 20 cents each. She got some positive readings, so wanted something "more accurate", a digital test.

Here it is. $12/ea, and it literally just reads a test strip. What a scam. pic.twitter.com/JGFnytbwFd

— Xtoff (@CrunkComputing) August 10, 2020

And here is a very detailed breakdown of a similar device:

The one I got: pic.twitter.com/kuTzN5A3AV

— foone (@Foone) September 4, 2020

It's a surprisingly complicated chip. You might think it's very limited because it's only got 64 bytes of ram, but it's actually using a pipelined architecture to operate at 1 instruction per cycle, giving it quite good performance for a 4mhz CPU. pic.twitter.com/pGZaEzo3MT

— foone (@Foone) September 4, 2020

There is value added, though:

Pregnancy tests are 99% accurate in the lab, 75% accurate in the wild due to misreads- mistakes which are highly dependant on education and socioeconomic status. No, it is not stupid or wasteful to use a hardware interface to help women with this.https://t.co/BFbM36D8l1 https://t.co/rg4ncKLtke

— Naomi Wu 机械妖姬 (@RealSexyCyborg) September 4, 2020

And an explanation for why the strip shows a line:

The lines are actually antibodies attached to small gold particles. They fix to hormones in the piss, and are carried until an area where another line of antibody capture the hormones, and the gold-antibody conjugate.

— Sebastian Worms 🌐 (@sfsworms) September 4, 2020

IF you make that second line of antibodies in the shape of a YES, you'd run into issue that (imagining it flows from bottom to top) the first line of the E would capture all the antibodies, leaving non. The E would look like a _.

— Sebastian Worms 🌐 (@sfsworms) September 4, 2020