Micro Switch sensor replacement

aestechnic

31 Aug 2020, 14:20

Hi all,

I have previously reported on a USB interface for a Honeywell Micro Switch keyboard (viewtopic.php?f=7&t=24122, I should update that with the finished project) and I complained about two dead keys. I managed to replace the Hall effect sensors in those with common 3-pins 3144 sensors and I thought that might deserve a separate post.

These are 4A3S switches. The sensor inside is 4-pins, with an active-low enable input the 3144 lacks. But some of the keys have the enable input hardwired to ground, making them always-on and these sensors I could replace (moving their original, working sensors to the faulty keys). This picture sums it up:
sensors.jpg
sensors.jpg (147.68 KiB) Viewed 313 times
  • Left: The 3144 sensor face up. Pins left to right are 5V, GND, output (open collector).
  • Middle: Another 3144 face down and with pins bent to match the original plate in the keyswitch. The original sensor has been removed with a knife from the plate; the pinout left to right is 5V, output, enable, GND.
  • Right: Finally, a "prepared" 3144 soldered to the plate and ready to go inside the keyswitch again. Again, this works because this sensor will go in a switch with grounded enable pin (3rd from left).
The 3144 is quite a bit fatter than the original sensor, but fits in the keyswitch. All is well and I can't notice any difference in actuation point.

zzxx53

08 Sep 2020, 17:12

Very interesting guide. Should come in handy when I finally get around to fixing my MicroSwitch.

User avatar
Redmaus
Gotta start somewhere

08 Sep 2020, 18:13

Dude nice! I didn't even know these sensors could fail and definitely didn't know you could replace them.

This might come in handy for when I try to convert the two hall effect boards I have.

aestechnic

09 Sep 2020, 20:56

Redmaus wrote:
08 Sep 2020, 18:13
[…] I didn't even know these sensors could fail […]
Yeah, I was also a bit surprised. Although Horrowitz–Hill write (in the Art of Electronics) "…we once had an epidemic of progressive magnet anemia in a Hall-effect keyboard"!

User avatar
DMA

15 Sep 2020, 00:53

Oh wow. I thought it should be possible, but proving it's possible is IMPRESSIVE.
Congratulations on advancing state of the art.

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