3 - "Input" Subsystem Circuits

In the input subsystem, the LDR and Variable Resistor (VR1) are connected as a Voltage Divider. When their relative resistance values change, the voltage across the LDR (VLDR) changes too. This is useful as we can adjust the variable resistor VR1 to ensure the "switch on" voltage of 0.7 volts appears across the LDR (VLDR) when the required lighting conditions are met.

Move the white arrow at the top to see how the LDR's resistance value increases as it gets darker. You can move the white arrow next to the variable resistor to change its value too. Whilst you are doing this, note how VLDR's value automatically changes. For a 9 volt circuit, these values are governed by the following formula:

Now adjust the pointer at the top so it is in line with the clouds and slowly move VR1's pointer down until the value of VLDR on the meter exceeds 0.7 volts. At this point you should see the LED switch on. Try adjusting the circuit for other lighting conditions and note the resistance and voltage values displayed. Enter these values in the formula to try it out.

In the list of other FastTrack inputs, the voltage divider sensing circuits similarly produce analogue, or varying voltages and the switch triggers produce on/off digital voltages of either 0 or 9 volts.