While looking at that light on the seat I came up with the thought that if there was a pressure switch on top of the light with a delayed switch off time. You could make the light come on every time the rider sat on the seat. If they got off the seat to pedal, the delay switch would keep the light on.
You wouldnt need a dynamo and you would never forget to turn your light on.
Will this make me milions, or am I destined always to be poor?