3.4 Designing a sensor circuit
Figure 17 is a screenshot taken from Multisim Live, showing a circuit with four parts. On the right is a light-emitting diode (LED) and a 470 Ω resistor, . On the left there is a device called a light-dependent resistor (LDR). This is labelled and forms a voltage divider with a fixed resistor . The resistance of the LDR has been measured as 380 Ω in full ambient light and 1.5 kΩ in the dark. We want the LED to switch on when the environment begins to darken and the resistance of is 680 Ω or more. (Note that is the label that Multisim Live gives to the op-amp.)
The resistors and form another voltage divider, which will provide a ‘reference’ signal. Both and have resistance 100 kΩ. Battery provides 9 V, so the reference voltage is
If is set to 680 Ω and variable resistor is also 680 Ω, the voltage at A will be the same as the reference voltage, because
As it gets darker, this voltage will increase.
This circuit is shown implemented as a breadboard in Figure 18. When it gets dark and the sensor receives less light, the LED illuminates as required.