The basic job of the pedometer is to measure the amount and intensity of your walking or running; there are many on the market.
Older style pedometers had a little metal ball bearing that bounced up and down with your hip movement, every time the ball bearing was forced upwards it would hit a couple of contacts, complete an electrical circuit, and another step would be tallied.
Newer pedometers use electronic accelerometers, which use fine strands of wires swinging in the breeze. On movement, these tip of these strands tends to stay still due to their inertia, but the root of the strand has to move, this forces the hair to bend, which changes its electrical properties and that is what is measured. Think of it like someone with long hair doing some headbanging, the tip of the hair and the root of the hair are moving at different speeds, bending the hair. The mechanism is actually quite similar to how our inner ear works out balance, except the inertia is provided by a small amount of liquid inside a tube. Inside the tube, there are very fine hairs that can feel the liquid moving over it.
Typically, these electronic accelerometers come in triplets, one to measure left to right, another for forward and back, another for up and down. These accelerometers are incredibly small, and are basically everywhere these days, including mobile phones.
The FitBit One is a pretty amazing bit of kit, it’s not much bigger than a couple of pieces of chewing gum, has a battery that lasts a week, stores minute by minute data and uploads it wirelessly over Bluetooth 4. Using the up-and-down accelerometer it even tries to work out when you’re going upstairs, which uses a lot more calories. It also has a sleep mode that I’m using, but I haven’t tried to correlate the data with the sleep monitor yet.
It hasn’t all been rosy with the FitBit though, more in another post on your data freedoms.