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# 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.)

Figure 17 Circuit diagram for an open-loop op-amp switching an LED (screenshot from Multisim Live)

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.

Figure 18 Breadboard circuit for the open-loop op-amp: (a) in light conditions, the LED is not illuminated; (b) when it gets dark, the LED is illuminated
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