I want to build a counter to track completed cartridges with an optical sensor. I found a kit for the counter that will count up every time the ground and positive are connected. I have a two wire optical sensor that would continuously connect the ground and power except when a cartridge breaks the light. I need a circuit that would use the sensor, but only bridge the outputs when the sensor is triggered.

These are the parts I found:

QKits Electronic Kits: QK129, 4 Digit Up/Down Counter kit (http://store.qkits.com/moreinfo.cfm/QK129)
Invalid Request (http://www.digikey.com/product-detail/en/OPB819Z/365-1262-ND/1015177)

I am open to other solutions

You could wire in a normall-closed relay to connect the counter's clock pin to ground, and use the sensor to apply voltage to drive the relay. The N/O relay will provide an open circuit to the counter when it is powered, and close the contact when the power from the sensor is cut, thus incrimenting the counter. So when the sensor provides voltage (normal condition) no counting occurs, and when it stops applying power the relay closes and the counter's CLOCK pin is grounded.