Lesson video

Hi there, everybody.

My name is Mr. Butt, and welcome to your design and technology lesson for today.

It's wonderful you could join me.

We have got a fantastic lesson for you today.

What we're gonna be looking at today is how you can virtually and physically test your systems and your circuits.

We're gonna be using multimeters both in Tinkercad in the virtual world and also in the real world physically.

This lesson is part of the systems approach to design sustainable futures unit.

Today's outcome: by the end of today's lesson, I want you to be able to virtually and physically test a control system and a circuit.

We've got four keywords for you today.

The first is multimeter: an electronic tool that measures voltage, current and resistance, and actually quite a few other things as well.

Now today we're gonna be using multimeters to measure voltage and current both in Tinkercad and also in the physical world.

The next keyword is voltage: the measure of the push from the battery that causes charge to be transferred around a circuit.

And of course this is measured in volts.

We then have current: the rate of flow of that charge in a circuit.

And of course this is measured in amps.

And finally, test: this is where we check quality, check function and check reliability.

And in this case, we're gonna be looking at our systems and our circuits.

We've got two learning cycles today and the first one is all about virtual testing.

So let's take a look.

It's important to test the function of an electrical circuit or control system.

Really important.

This testing is often referred to as fault finding.

You might have heard that before, fault finding.

Now we can use a multimeter to measure a number of things.

That includes current, voltage and resistance.

Now it will measure other things as well.

So for example, you can do polarity tests and some multimeters we'll even do things like temperature.

But what we are gonna look at today is current and voltage.

Now control systems can be simulated using CAD software, such as Tinkercad.

And we can use virtual digital multimeters in this simulation when we simulate those circuits to measure voltage current or resistance at different points in the circuit or the system.

Now this is Tinkercad's multimeter, and you can see here it looks very similar to what you might have in the real world, but it's not got as many settings.

But it will still measure current, there you go, in amps, voltage in volts, and resistance, quite confusingly, it uses an r rather than the oms, omega symbol.

Now Izzy has created this circuit and code in Tinkercad for her greenhouse system.

Now this greenhouse system is to measure temperature.

You, of course, might have developed a completely different one and that's absolutely fine, but we're gonna use this code to be able to test our system both virtually and also in the physical world.

Let's have a quick look at what this code actually does.

So Izzy says, "If the temperature is less than or equal to five degrees centigrade, then the blue LED will emit light.

If the temperature is greater than six degrees, the green LED will emit light.

And if the temperature is greater than 30 degrees, the red LED will emit light.

The three LEDs act like a traffic light system and could be used to monitor the greenhouse temperature." So that's a great system to use as part of the greenhouse.

Now Izzy has calculated that she should use a 220 ohm resistor with each of the LEDs to protect them, and she would like to carry out some virtual tests of this circuit to make sure the operation and everything else is working as expected.

So that's a great way to fault finder to test this system.

Quick check for understanding.

What is the unit symbol for current? Is it a, A; b, C; c, V; or d, omega? Pause the video now.

Have a go at this and come back to me when you've got your answer.

It is of course a, amps.

We measure current in amps.

Now when measuring current in a circuit, the multimeter probes should be added in series.

This is really important.

So here we have Izzy's original circuit, a nice simple circuit with her traffic light LEDs.

And then what we're gonna do is we're gonna add that multimeter, in this case, for the blue LED.

And you can see we add it in series.

So what we've done is we've actually broken that link between the resistor and the LED and we've added the multimeter probes in between that.

What we also need to do is make sure we select current in amps on the multimeters settings.

Now when measuring voltage in a circuit, it's slightly different.

The multimeter probes should be added in parallel.

So once again, here's the original circuit with no multimeter added and here's one with the multimeter added in parallel.

What we're actually doing is we're checking the testing and measuring the potential difference across that LED.

So we need to add our probes as such.

And once again, just make sure you select voltage from the digital multimeter settings.

Now Izzy would like to carry out a test to make sure the current does not exceed 20 milliamps for each LED when the circuit is operating.

She's checked the data sheet for the LED and she's discovered that 20 milliamps is actually the maximum operating current.

So that's a really good test to carry out.

So first of all, she's gonna insert the multimeter probes in series between the resistor and the anode, which is the positive leg of the LED as you can see there.

And then what she's gonna do is she's gonna start the simulation and of course set the multimeter to A to make sure she's carrying out the right measurements.

Now to test the blue LED, Izzy needs to reduce the temperature to five degrees or less.

So what she does is she uses the little slider you get there in Tinkercad when you carry out a simulation, she reduces that temperature and then she checks the readout on the multimeter, which we can see there, and of course it's 93.

2 microamps.

It's important that you check that very carefully, which if we do the conversion equals 0.

0932 milliamps, which of course is less than 20 milliamps.

So we're all good there.

Quick check for understanding.

The negative leg of an LED is the anode.

Is that true or is that false? Pause the video now.

Have a go and come back to me when you've got your answer.

It is of course false, but do you know why? Once again, I want you to pause the video, come back to me when you've got your answer.

Well it is of course the anode is the positive and the cathode is the negative.

Well done.

So now Izzy's gonna repeat the same test but for the other LEDs.

So you can see here, she's doing it for the green LED.

So what she needs to do is, of course, set the temperature again, but once again she can see that she's got the readout.

The readout for this is 3.

52 milliamps, which again is less than 20 milliamps.

So that works.

And then repeated for the red LED, and again, connected in the correct way.

And it and the digital readout reads 4.

92 milliamps, which once again is less than 20 milliamps.

So here we have a table showing the different units of measurement for amps.

Now it's important to note the difference with these.

Now you will mostly work in milliamps.

That's what you tend to work as in school.

But as you've seen from Izzy's readout, you sometimes get microamps.

So it's important to know the difference between those.

You very rarely and you shouldn't really be working in amps.

An amp is actually quite a lot of current.

So you can see there 20 milliamps is the same as 20,000 microamps or 0.

02 amps.

Quick check for understanding.

Which of these current measurements is less than 20 milliamps? So we have a, 0.

01 amps; we have b, 0.

025 amps; we have c, 15,000 microamps; and d, 25,000 microamps.

I want you to select all the ones that are less than 20 milliamps.

Pause the video now.

Have a go at this and come back to me when you've got your answer.

So it is of course 0.

01 amps which is of course 10 milliamps, and 15,000 microamps is 15 milliamps.

So those both are less than 20 milliamps.

Well done.

Now Izzy would like to carry out a test to make sure that the voltage is correct for each LED.

Most LEDs have an operating voltage between 1.

2 to 3.

6 volts.

So she wants to check whether each of her LEDs is operating in between those two values.

Another great test to carry out.

So first of all, Izzy inserts the multimeter probes in parallel across the anode, which is the positive, and the cathode, which is the negative of the LED.

You can see there.

Then again start the simulation and set the multimeter to volts, the V.

To test the blue LED, the temperature, once again, it needs to be less than or equal to five degrees so we know that we need to drop the temperature using the slider.

Once we've done that, we can then check the voltage on the multimeter readout and it read 3.

27 volts, which of course is greater than 1.

2 volts but less than 3.

6 volts.

So we're all good there for the blue LED.

Now of course Izzy can then carry out for the other LEDs.

It's exactly the same.

I'm not gonna show you those because I'm sure you understand how to do that now.

So we're now onto your first task.

For your circuit and code, I would like you to carry out tests using the virtual digital multimeter.

Those tests will depend on your circuit and of course your code.

For each test, record your results and explain the outcome.

Pause the video now.

Have a go at this and come back to me when you've completed it.

Okay, so we've got the first one which is for your circuit and code, carry out tests using the virtual digital multimeter.

Well, Izzy said, "I tested the voltage across the LEDs to make sure it was sufficient for them to emit light.

I also tested the current when each LED was on to make sure it did not exceed 20 milliamps." I then wanted you for each test to record your results and explain the outcome.

Well, of course, we had the voltage test across the LEDs.

The maximum was 3.

27 volts.

And as Izzy said, "Well, the voltage for each LED did not exceed 3.

27 volts and it was greater than 1.

3 volts.

All LEDs emitted light," great.

Then it was the maximum LED current.

This ranged from 0.

0932 milliamps all the way up to 4.

2, 4.

92 milliamps.

Now the maximum current measured was 4.

92 milliamps, the red LED.

This was less than the maximum operating current of 20 milliamps.

Brilliant, well done.

So we're now onto the second learning cycle, which is all about physical testing.

Now Izzy has built a quick prototype of a circuit using a breadboard and a microbit.

You can see the microbit there.

We then have the jump wires to the pins, which will be connected also to those crocodile leads which will connect to the microbit.

We have the LEDs, and you can see Izzy obviously didn't have a blue LED, so she's used an orange one.

That's absolutely fine.

She there you can see she's got her 220 ohm resistors.

The breadboard that she's built her circuit on.

The ground, which obviously goes to the ground of the microbit.

We've got some jump wires on there, which are all laid out nice and neatly to make sure that the negative leg of each LED is connected to the ground.

And now of course we need to look at multimeters.

So Izzy would like to conduct physical tests of her prototype, testing the same functions as in the virtual tests.

To do this, she will need to use a digital multimeter.

Now what's really important is I'm showing you an example of the multimeter I've got.

Your multimeter might be different and it's important you understand the functions of it.

So you need to read those instructions carefully.

Let's have a look at how you can navigate around a digital multimeter.

So first of all, we have the display screen and this is for displaying the measurements that you are carrying out.

We then have this area here and this is for DC voltage.

And this is selected to measure DC voltage in volts.

And you can see you've got a number of different numbers on there, which we'll talk about in a second.

We then have the off.

And I'm sure you can understand that that will turn the multimeter off and it's important to do that once you've finish your test to make sure the battery doesn't run out.

We then have voltage AC.

Again, probably not gonna be using that whilst you're in school, but it's there and that's selected to measure AC voltage.

And then of course we have our amps on there with again lots of different numbers, and this is selected to measure current in amps.

Quick check for understanding.

What is this part of the multimeter fall? That part highlighted with the board around it.

Is it a, displaying the measurement; b, measuring DC voltage; c, measuring current; or d, measuring AC voltage.

Pause the video now.

Have a go at this, come back to me when you've got your answer.

This one is of course measuring DC voltage, well done.

So let's continue navigating around our digital multimeter.

So now we have some sockets.

This is the amp socket which is the 10 amps current socket, which again you're probably not gonna use.

We then have our voltage milliamp socket, which again is for measuring volt, milliamps and ohms as well.

We then have our COM socket, which is the common socket.

You'll use that a lot.

And then we have also on the dial, back to the dial, we have the continuity, and this is for measuring continuity or some people call this the diode test.

And then we have ohms. So if you wanna measure resistance, you select that and you'll measure your resistance in ohms. So it's good to know the standard settings and sockets to use for some of the most common measurements we'll carry out in school.

So first of all, if you're gonna measure a voltage less than 20 volts, this is what you need to do.

So you can see the dial is set to DC volts and it's set to 20.

So that means anything under 20 volts it can read.

And we're also using the volts, milliamps, ohms socket and the common socket you can see down there.

If we then want to do a measurement on current less than 20 milliamps, once again we're gonna set the dial to amps.

We're gonna set to 20 m, we is 20 milliamps and we're gonna use those same two sockets.

And then also it's similar for if it's less than 2000 ohms, then what we do is we set it to our ohms section 2000.

And again, it's the same two sockets.

It's as easy as that.

Quick check for understanding.

Which multimeter dial is set up to measure current? Is it the one at a, b, or c? Pause the video now.

Have a go at this and come back to me when you've got your answer.

It is of course b.

You can see it's there.

It's set in the A section, the amp section.

So Izzy would like to carry out a test to make sure the current does not exceed 20 milliamps for the red LED when the circuit is operating.

So first of all, she sets her multimeter to read less than at 20 milliamps and attaches the two probe leads to the correct sockets.

Then, what she needs to do is insert the multimeter probes in series between the resistor and the anode positive of the LED.

And you can see she's done this by actually unplugging one of the legs of the resistor and connecting the crocodile clip to that leg.

And then what she's done is she's inserted another jump lead that yellow one you can see into the positive leg of the LED, and she's connected that to the other side.

And if we look at what these are connected to, you can see this lead is the vaults and milliamp socket and this lead is the common socket and that way she can read, she can read the current really easily.

To test the red LED, the microbit temperature needs to be greater than 30 degrees centigrade.

So that's quite difficult.

What is she gonna do to be able to heat up the the microbit? Well, what Izzy does is she uses a hair dryer very carefully and only on a warm setting, not a hot setting.

And she gently heats up the microbit until the red light comes on, which means it's over 30 degrees.

Soon as Izzy hit that red light comes on, she then takes the reading from the digital screen, and in this case, it was 2.

45 milliamps, which once again is less than 20 milliamps.

So we're all fine there.

Izzy would now like to carry out a test to make sure that the voltage is correct for the red LED.

As we know, most LEDs have an operating voltage of between 1.

2 and 3.

6 volts.

So once again Izzy needs to set the multimeter correct.

So she sets it to the 20 volts and also attaches the two probe leads to the correct sockets.

Now this one is slightly easier.

She inserts the multimeter probes in parallel across the anode, the positive, and the cathode, negative of the LED.

And all she does here is she simply just touches them on the legs of the LED.

And you can see that one there is obviously going to the vaults milliamp socket and that lead there is going to the common socket nice and easy.

Once again, she's got to make sure that temperature exceeds 30 degrees C.

So again, using a hair dryer on a warm setting and very carefully, she gently heats up the surrounding area around the microbit until that red light turns on.

Soon as that red light turns on, she takes her reading, which is 3.

15 volts, which is greater than 1.

2 volts and less than 3.

6 volts.

So once again, we're all fine here.

Now Izzy's got a question for us.

"Why do my virtual test results differ from my physical test results?" Now I want you to have a think about this, maybe have a chat with the person next to you.

Pause the video and come back to me when you think you got an answer.

Did you get an answer? I'm sure you did.

Well, the reason for this is usually 'cause the tolerance of physical components.

Physical electrical components are manufactured to a tolerance.

Virtual electrical components are the exact value stated.

There's no tolerance in there.

Now some simulation packages can, of course, build that tolerance in.

But with Tinkercad, you're probably gonna get what would be a perfect reading.

In the physical world, you're not gonna get that 'cause of the tolerance.

The multimeter will also have a tolerance of measurement as well.

And you might even find using different multimeters, you get slightly different results.

Now if we look at the resistors that Izzy used, the 220 ohms resistors in the circuit, it's got four color bands.

Now these signify the value in ohms but also the tolerance that we're working with.

So the first band is red and that equals two on the resistor color band chart.

The second band is red, that also equals two.

The third band is what is known as the multiplier.

So you basically times whatever number you've got there, which is 22 by 10, which of course will equal 220.

But the fourth band is the tolerance, and in this case, it's gold.

So that means the tolerance of plus or minus 5% of whatever the value is in this case 220 ohms. So yes, the resistor value is 220 ohms, but it could be 209 ohms or it could be 231 ohms with that tolerance taken into consideration.

So we're now onto your final task, task B.

The first thing I want you to do is build a prototype of your circuit or at least a part of your circuit.

I then want you to carry out tests using the digital multimeter.

For each test, record your results and explain the outcome.

And then finally, explain why the physical results differ to the virtual results.

Pause the video now.

Have a go at this and come back to me when you've done the task.

So, how did you get on? Well, your circuit might look something like this or it could look very different to this depending on what your greenhouse is actually doing.

I wanted you to carry out the test using the digital multimeter, and for each test, record your results and explain the outcome.

So this is Izzy's.

So the voltage across the red LED, when the temperature is greater than 30 degrees was the test.

The result was 3.

15 volts.

The voltage for the red LED was 3.

15 volts.

This is greater than 1.

3 volts and less than 3.

6 volts, which is exactly where we want it to be.

The next test was the maximum red LED current when the temperature is greater than 30 degrees.

The reading was 2.

45 milliamps.

And the explanation, the maximum current measurement was 2.

45 milliamps to the red LED.

This is less than the maximum operating current of 20 milliamps.

Well done Izzy, and well done you.

And finally, I wanted you to explain why the physical results differ from the virtual results.

Well, this is due to the tolerance of physical components.

Physical electrical components are manufactured to a tolerance.

Virtual electrical components are the exact value stated.

The resistors value is 220 ohms, but it could be 209 or 231 ohms depending on the tolerance.

The multimeter will also have a tolerance of measurement.

So that brings us to the end of today's lesson.

Let's have a quick summary.

Testing electrical circuits and control systems is often referred to as fault finding.

A multimeter is used to measure current, voltage, resistance in both virtual and physical circuits and control systems. Resistance and voltage are measured in parallel, but current is measured in series.

This determines how the multimeter probes are placed to take the measurement.

Virtual and physical test results will differ due to the tolerance of physical components and test equipment.

You've been absolutely fantastic today, well done.

And I look forward to seeing you all next time.

Goodbye.