Wednesday, 3 February 2016

Proximity sensing with dedicated IC chips - making breakout boards

Although the bluetooth dartboard scoring thing isn't actually in place at Nerd Towers, the BTLE4 app is already written (using Unity) and sends each players scores as ascii to the 3-way 7-segment displays


Shonky coder art and "unobtrusive" debug messages? Of course it's not finished. But it works!

But a few different projects have come up recently that require a bit of RnD. So what better excuse to crack out the ferric chloride and show yet more photos of filthy ferric-chloride fingernails?

Here are some proximity sensors that we're playing with


The one on the left, with the silver package and two lenses is pretty much what it looks like - an IR led and photodiode in a single package. The soic chip and the (much smaller) sot-23 chips are Microchip's own versions of capacitive sensing ICs but with built-in, programmable sensitivity.

The eight-pin thing being pointed at is very interesting. It's a full-colour proximity and RGB sensor. So it can detect levels of red, green and blue placed under it. That's really exciting! We've used sensors for detecting RGB colours a number of years ago. Then, we flashed a red, then green, then blue LED and detected the relative amount of light reflected, to decide whether a card was red, green, blue or black. It worked. But it was a bit clunky. And didn't do so well differentiating between, say, a yellow and a green card, or a cyan and a blue one. And a grey card was sometimes detected as black. But then again, sometimes a white one was too. And while that was enough to tell the difference between red, blue, green and black, it wasn't very good to tell the difference between black and white!

All that's an aside though. What we have here are a number of different proximity sensors, which we need to work out how to use, so we can choose the best performing one for the current project. But potentially having access to an RGB colour sensor has got us thinking about a few other ideas that have been on hold for a loooong time.

In the meantime, we need to get these things mounted onto some breadboard-friendly breakout boards. Let's etch....


Normally the sight of a PCB with just the traces remaining and no ground plane or copper pour filling the blank spaces makes us scream. But in this case, because we're working with such tiny pitch components, and want to be sure there's no chance of bridging between pins under the chip, we etched away far more copper than we'd normally do, with a board of this size.

Luckily, the pitch of the silver IR proximity sensor is the same as the SOIC chip, and the pitch of the RGB sensor - although tiny and fiddly - is the same as the sot-23 chip. So all we need to do is etch a couple of breakout boards for each pitch, and get soldering!


A few minutes later (and, ok, a couple of re-tries after messing up the tiny sot-23 components) we had our chips soldered onto the pcb(s). Perhaps the hot air gun might have been useful here, but even on the tiny sot-23 stuff, a line of paste and then raking it out with the tip of the soldering iron was quick and easy,


After breaking the board apart (panellising pcbs isn't such a good idea when they are this small and we've only a junior hacksaw to cut them apart!) our sensors were ready for testing on the breadboard


Tomorrow we need to digest the datasheets and try to get some data out of them. We only need the proximity function for the current project (it's not exactly top secret, but it is for a commercial commission, so we'll stick to generics here for now) but we have a project of our own that we're looking forward to trying out the RGB sensor on, and will get a full write-up when we get time!