I didn't think it would happen to me

A project log for Tardygrade

3D printed quadruped with a tiny BOM

Dejan RisticDejan Ristic 08/12/2021 at 00:030 Comments

I'm now officially a victim of the component drought of 2021. But I will persevere.

The PCB for Tardygrade is as simple as can be. The most exotic components on it are four surface mounted hall switches. They detect the position of a magnet embedded in the main gear, which lets the uC know the position of the robots legs. 

When designing the first PCB prototype I had whittled the choice of hall switches down to two options:

These were both hand solderable and not too expensive when bought i bulk. Since the magnet is fairly far away from the sensors (almost 9 mm) I went with AH9246, which is branded as having "ultra high sensitivity".

Two weeks later I received the PCBs and after having added some botch wires everything ended up working as expected.

I then started working on a new improved PCB. I browsed Mouser for components, and noticed that the stock of AH9246 was down to 3000. 

Didn't there use to be around 8000 last time I checked? And factory lead time was 18 weeks. Hmm, better stock up before they run out. 

Naah, I'll do it tomorrow. 

Next day it was down to 0.

I checked on my second option AH1815. They were running low too so I cleared out Mousers remaining stock of about 1000 units. They were a bit cheaper anyway and required fewer external components. But at this point I had somehow forgotten all about the difference in sensitivity and just assumed that the AH1815 would work as a drop in replacement for AH9246.

Well, turned out it didn't. The AH1815 wasn't even close to detecting the magnet at that distance. And at the time of writing factory lead time for the AH9246 is almost a year ( ! )

But I think the situation can be salvaged with a minor redesign of the main gear and PCB.

Instead of being embedded inside the gear body, the magnet will be slotted into the upper cam of the gear. This puts it right up against the ceiling of the chassis, and much closer to the hall switches. Seemingly close enough for the magnet to get detected.

There's and issue with that solution however: 

With the old magnet placement the hall switches could be placed along the front, rear, left and right edges of the board. But a magnet placed in the cam will be at a diagonal of the board for each of the four relevant leg poses. This, in combination with the fact that the magnet now traces a smaller circle, means that the hall switches would overlap the uC pin headers, using the current layout.

So what I may need to do is to rotate the headers for the uC boards, to make just enough room to fit the four hall switches on the bottom of the PCB.

Well, it's not the worst thing in the world. Might even look cool. And at least I'm now stocked up with hall switches for the foreseeable future.