Tak, Engin and I are in the midst of building our media controller. We have built two of our own pulse oximeters to measure heartbeat. We used this design from this site:
Building an earlier prototype, we got a clear enough signal with the one amplifier.
We then soldered together two permanent boards.
Unlike the site referenced above, we get better readings with the emitter/detector right next to each other rather than facing each other.
Engin also experimented with different colored lights for the tank and different color/viscosity fluids to put in the tank.
We ordered two magnets and have them working off a signal from an Arduino (using both analog and digital write signals). The magnet needs 12-14V, which we currently have coming from the DC power supply (hooked into the Arduino through a transistor on a bread board).
Putting all these pieces together, we are able to go from sensor reading to magnet activation. The only problem is that the readings activating the magnet are mostly just noise. This is our biggest challenge in completing the project, getting a clear signal where noise does not hide the actual pulse readings.