It's been a long time since I've posted an update, you can thank school for that. Now that the semester is over time to get to work!

The core of this project is the sensor input stage. It's a fairly simple circuit. Just a comparator and a NPN Fet. I chose a single supply rail to rail comparator (MIC7221) . This keeps it nice and simple. However the input voltage can't rise above the positive rail with out risking damage. A good portion of the circuit is built around this rule.The sensors I am using can go well above this limit. To control them I placed a standard diode in it's input path. This keeps the negative ac portion out letting in only the positive part. After the diode there is a 5.1 volt TVS diode to keep the voltage levels withing spec. The TVS diode cuts off all portions of the wave above 5.1 volts.

Another thing I have to aware is what happens when the circuit is off? In other words the positive rail is now at 0 volts. Remember the input can't be above the values on the rails with out risking damage to the comparator . To protect the chip I put a 2N7000 in the signal path with the gate attached to the positive rail. Now when the circuit is off nothing can reach the comparator. I can also adjust the refrence voltage for working with different type of darts. The vigilon disks need a different reference to work right.

Probing the signal out put with my scope shows an okay wave form. Some parts are a bit nosiy but they are long. An avr's interrupt pin should have no problem picking it up. The out put is in yellow and the blue is the raw sensor signal. You can see the effect of the TVS diode in limiting the positive swings.

With this approach I can chain multiple sensors onto a single input stage if I want. I'm thinking the final dart board will have 20 segments. Weather or not they will all be independent or not is undecided. Now I need to put 20 of these circuits on a board and send it off to osh park!