Sunday, 9 February 2014

Hall sensor tester

One of our board sections seems to have a fault.
No matter how many times we introduce a playing piece over just one square of just one board section, there's no data coming back from it.

It's not a design fault, because other, identical, board sections work just fine - it's a problem particular to this board. We thought it was going to be an easy fix: after all, there are only three pins on each SOT-23 hall effect sensor. Simply reflow the solder on each of the pins, and we're done!

Except it didn't quite work.
We tested continuity between the ground pin and our ground plane - that was fine. We had continuity between the power trace and pin 1 on the  sensor; there was even continuity between the output lead and the PIC input pin. Everything looked fine, it just didn't work!

Removing tiny little SMT components from homebrew boards can easily result in knackered traces (even with professionally made boards, it's easy to lift the pads by overheating them, but homebrew boards are particularly susceptible). So before we even take the "broken" sensor off, we need to make sure that the sensor we replace it with is working.

So we knocked up this quick hall sensor tester:

It's nothing more than a 3v battery supply and an LED. The "output" pin of the hall sensor is connected to the cathode (ground) pin of the LED and pulled up to 3v through a 1K surface mount resistor. When a hall sensor is held down on the SOT-23 pads and a magnet introduced, the hall effect sensor pulls the output pin low, and the LED lights up.

Here's our LED lighting up as the magnet on the tweezers triggers the hall effect sensor. These are super-bright LEDs - it's just the flash from the camera severely dampens the effect!

So at least now, when we come to replace our "broken" hall effect sensor, we can be sure that we're replacing it with a known working one (and we also have a way of testing to see if the other one really is broken, or there's some other problem stopping it from working) .

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