This is the sixth part of my series on my experiences while repairing electric and electronic devices. Here’s the fifth part. Recently my child purchased a cheap (10 €) step counter watch gimmick (shown below after the repair). Unfortunately it was dead on arrival, as it could not be powered on. So, dutifully (and with a little joy¹), I retrieved my screwdriver set from my locker and opened its case.
Inside I found a battery holder for a LR1130 coin cell with a nominal voltage of 1.5 V. Whipping out my multimeter I was able to confirm that the coin cell was fully charged. I did, however, notice that the contacts of the coin cell holder had a lot of wiggle room. In fact, there was so much wiggle room that the step counter’s electronics did not get any power. I fixed this by gently bending the cell holder’s metal parts towards the coin cell.
However, the most interesting part is how the „step counting“ is actually implemented. In your smartphone you do have MEMS-based accelerometer ICs, most of them with dedicated internal logic to derive steps from raw accelerometer measurements. Not so for this, China-made device: The step counter is a cantilevered, current conducting weight with a tiny spring which closes a circuit (with the vertical metal rod) when the step counter is shaken. Manufacturing cost: next-to-nothing. Accuracy: also next to nothing.
Studying this design, I feel conflicting emotions. On the one hand, it takes quite some ingenuity to come up with such a cost-effective way of implementing a step counter. On the other hand, it’s clearly garbage, and adding to our world’s accumulating piles of non-recyclable electronic waste.
¹ One does derive quite a lot of joy from repairing stuff, especially for children 🙂