Suggestion for a lack of light warning unit when in motion
Use a dynamo to power your circuit.  This is how old bike lights used to work before batteries became more popular.  This way motion detection is passive and all you need to do is have a buzzer sound when a light sensor reports darkness.

You can still buy small clip-on bike dynamos cheaply and you can just modify one of those into a single device that has the buzzer and light sensor built in.

http://www.ebay.co.uk/itm/BICYCLE-BIKE-DYNAMO-LIGHTS-SET-HEAD-REAR-LIGHT-Cycle-Power-Retro-Classic-/262145176729?hash=item3d0911f499:g:DqoAAOSw~CRTpzDo

These devices simply have a shaft with a friction wheel on one end that presses against the tyre and spins powering a small generator.  Simple and effective.