An introduction to electronics
An introduction to electronics

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

An introduction to electronics

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, cap r times italic five. On the left there is a device called a light-dependent resistor (LDR). This is labelled cap r times italic two and forms a voltage divider with a fixed resistor cap r times italic one. 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 cap r times italic two is 680 Ω or more. (Note that cap u times italic one is the label that Multisim Live gives to the op-amp.)

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

The resistors cap r times italic three and cap r times italic four form another voltage divider, which will provide a ‘reference’ signal. Both cap r times italic three and cap r times italic four have resistance 100 kΩ. Battery cap v times italic one provides 9 V, so the reference voltage is

nine postfix times cap v equation left hand side prefix multiplication of cap r times italic four divided by open cap r times italic three plus cap r times italic four close equals right hand side nine postfix times cap v equation left hand side prefix multiplication of 100 postfix times 000 postfix times cap omega divided by open 100 postfix times 000 plus 100 postfix times 000 close postfix times cap omega equals right hand side 4.5 postfix times cap v

If cap r times italic one is set to 680 Ω and variable resistor cap r times italic two is also 680 Ω, the voltage at A will be the same as the reference voltage, because

nine postfix times cap v equation left hand side prefix multiplication of cap r times italic two divided by open cap r times italic one plus cap r times italic two close equals right hand side nine postfix times cap v equation left hand side prefix multiplication of 680 postfix times cap omega divided by open 680 plus 680 close postfix times cap omega equals right hand side 4.5 postfix times cap v

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.

Described image
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
T212_1

Take your learning further371

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses372.

If you are new to university level study, we offer two introductory routes to our qualifications. Find out Where to take your learning next?373 You could either choose to start with an Access courses374or an open box module, which allows you to count your previous learning towards an Open University qualification.

Not ready for University study then browse over 1000 free courses on OpenLearn375 and sign up to our newsletter376 to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371