Hello~
A ball which emitting 100Hz Red light is used in a soccer game. I'm designing the visible light sensor to detect the ball. For the optical device, I have two choice: Photodiode and LDR. An AT89C2051 as the filter to detect the 100Hz red light emitted by the ball.

Here comes the problem:
The AC source of here (Macau, next to Hong Kong) is 50Hz. So the frequency of the lighting device (bulb, or "mercury bulb"?)in the playground is 100Hz. It's the same to the ball's frequency.

It has been working fine, because I used the photodiode to detect the ball. But here is another problem: The two goals on the playground is mounted with two IR light sources, which help the robot to distinguish which is its own goal. So I'm designing a new receiver with the photoresistors. But the photoresistors DO receive the 100Hz from the AC bulb... so what can I do?

Thanks to all kind of help~

Stanley