reading-in-bed limiter

At a certain age, children are often packed off to bed with the final admonition: `All right, you can read in bed for a quarter of an hour, but then you must turn off the light and go to sleep`. However,. as most parents will know, the children tend to suddenly loose all sense of time in this situation ... When a member of the Elektor design team was faced with this problem, he started looking for an electronic solution. The final circuit, as published here, has proved extremely effective.