Need homemade sparrow-free feeder plans

[FourFussyHens]

I have heard that there are plans somewhere for a homemade chicken feeder that only opens when the chickens step on a pressure plate that makes the lid open.

The design keeps sparrows and doves out of the feeder.

My Girls usually attack strange birds, but they are allowed to range free during the day, and the pest birds wait until they're away from the pen, and then hit the feeder.

Any help here?

Sue