Each of the pods in chaodependant has:
* a [[wp:Hall Effect]] sensor
* a high power LED.
* some magnets
I've kept as much as possible of the electronics in each pod, so that they are modular, and to reduce the distance that the unamplified hall sensor signal has to travel.
== Amplifying the Hall Sensor ==
We're using surface mount hall effect sensors, which are tiny 3 pin packages - power, ground and signal. They give 2.5v with no magnetic field, and then are linear at +/- 5mV/Gauss. The signal that comes out is pretty small (until the magnets get really close to them!) so se need a way to amplify it. Also, the sensor is going to be in a strong magnetic field to start with, due to being mounted in the pod. So, we need a biasing amplifier, in order to set the bias voltage to match the basic output level, and then amplify around that. We used TL072 opamps to do this, with the second half providing a low pass filter for smoothing.
Since the opamp is a differential amplifier, it amplifies the difference between the two inputs. So, to bias it, simply apply a voltage to one of the inputs. In this case, we used a 2k4 variable resitor, connected via 33k fixed resistors to ground and Vcc, to get a usable bias voltage within the range we needed.
(post re-dated to when it happened)