TV timer?

Anyone know of a Television timer?
Maybe a token to be dropped into this device or maybe a code punched in which would then turn on the power to the TV for 1/2 hour increments.
The power cord would be locked into a box preventing bypassing. I could rewire the TV, I think easily, but not everone has an electronics degree, or cares to void their warranties.
Or design this into the remote, even blocking certain channels.