Following the logical steps of the 'Design Process' it would be sensible to decide on the details of the circuit we intend to use. The circuit given here is a possible starting point that will simply trigger the relay when a light shines on the LDR - There could be several input sensors around the buggy that would make it reverse on contact with a wall or stop following a sharp sound. However it is only the basic light sensitive circuit that will be considered here.
Modelling the circuit using a software package (such as 'Crocodile Clips') will allow the values of the components to be chosen. It would be possible to calculate what is needed, but using modelling software allows the circuit to be developed - and be seen to work, before construction begins. Screen shots or print-outs of your developments of the circuit will show that you have experimented and learned about the process of improving the circuit. The circuit diagrams shown on the next pages are produced on 'Crocodile Clips' and allowed a series of steps to be followed in making the sections of the circuit easier to follow.
RECAP on the theory- Once a pulse of light has triggered the transistor circuit a voltage is present at the 'BASE' of the transistor. The level of this voltage is set by the variable resistor (R2 in the potential divider) and so the buggy sensitivity can be adjusted to different light conditions. The transistor allows the charge from the capacitors to turn on the relay - and of course the relay will remain turned on until the voltage from the resistors has dropped to a level low enough that the relay reverts to its 'off state'. The buggy then stops and requires recharging. Clearly then, the length of time of the run is dependent on the value of the capacitors. The circuit highlights the way capacitors act when they are in parallel ( see the diagram below) and so to get a higher capacitance value - and a longer time - the components must be wired in parallel.
A matrix board can be used to test how long the motor will run. The propeller is attached to make it easier to see the motor output. ( Incidentally the propeller could be used in a differently designed buggy that was driven forward by airflow rather than a direct drive to the axle.
This use of a capacitor illustrates the principle of calculating the 'Time Constant' for capacitors and resistors.
Whilst designing the buggy you should be aware of the basic concepts we have covered so far and how they relate to the project in which you are involved.
A battery will supply the VOLTAGE to turn the motor. Remember the VOLT is the unit of electrical pressure. The higher the voltage the greater is the force available to cause the CURRENT to flow in the circuit. The current is measured in AMPS - This is the rate at which current flows in a circuit. The POWER available in the circuit is measured in WATTS. This is calculated by VOLTS x AMPS in electronic circuits. It isn't only batteries that can supply current in circuits - the CAPACITOR can also do this. Capacitors come in several different packages as shown here. We could use them to store a small temporary charge which would last only for a short time.