Optical Sensor?

I'm building a vending machine. Basically, I've designed a way that make sure the coin you put in is a quarter, but then I need an optical sensor. I can put electrical tape on the other side of the sensor, so there is very little light and the sensor will capture very little voltage. However, when a shiny quarter passes through, I need an optical sensor to send enough voltage to maybe a solenoid, which will momentairly let a little candy out of the opening. I have photo diodes, but would this work? Is there somewhere I can get a sensor and get it done with shipping for $10?

In addition, could I find a really BIG capacitator so it takes three quarters to store up enough energy?

Thanks

You need a reflective sensor like this. Available from DigiKey for about $3.50. They have lots of others, too.