The sensor (LDR) is obviously the trigger for the buggy to set the system in operation and for the buggy to move off its starting line, but to generate enough torque there must be enough current flowing in the circuit to give the necessary power.
Remember : Volts x Amps = Watts     …   and the Watt is a measure of power since its definition is 1 joule per second  ( 1 Js^-1 ).   This is a measure of the energy available since 1 joule is the energy lost by 1 kilogram falling through 1 metre under the force of gravity.   Imagine a 1 kilogram mass falling on your toe from a height of 1 metre !
The capacitor cannot supply enough current to give the power we need so we will use a separate battery circuit.  If you check the rating on the side of batteries it will show not only the voltage but also the current capacity of the supply ( measured in Ah   - Amp hours )   The relay is the link between the two circuits.  It requires very little power to operate it and so will work from the small current available from the capacitors on the front of the buggy - We charge these from an external   PP3 ( 9 volt) battery) to 'arm' the vehicle ready for the start.  The variable resistor makes sure the car doesn't move in the light conditions in which the race takes place - before the starting 'flash' goes off.

Be sure you know how the potential divider controls the current flow in the circuit.  Make up a light sensitive circuit using the matrix board before you go on and complete the rest of the circuit on the PCB shown here. 
       
Potential Dividers are fundamental to the study of Systems and Control and Electronic products.

Wednesday, February 11,