This PCB here is a metal detector. The circuit is very easy and I will do my best in this
video to explain to you how it works. We will use real examples and animations so you will
understand everything. The PCB is interesting because it already has the coils on the board
made with the copper tracks, so we don’t need external coils made with copper wires. I will
share with you the schematic for this project and the GERBER files in case you want to order
this same PCB and start making tests with it. All the links are below. So guys, what do you think?
Will this PCB be capable of detecting metal and if yes, how good is it? So, let’s get started.
What’s up my friends, welcome back. This is the PCB I’ve designed for today project,
so let me explain you all the connections, how the PCB was made and why I’ve made in that way,
and also explain you how this simple circuit should work and detect metals.
The schematic for this PCB is this one and you could download it from below
if you want to use the same values. Also download the GERBER files for the PCB,
go to PCBWAY.com, upload the zip file on their website and select the settings such as color,
materials, amount of PCBs and so on. Order the PCBs and make this same project
an learn something new. The PCB is made out of two parts. The coils, which is the upper part,
and the circuit which is below here. The coils are winded on both sides of the PCB
so the same coil starts here and continues to the other side with a total of around 50 loops.
You can’t add copper area to the PCB because that would affect the inductance of the coils,
so leave only the tracks without any copper around them.
According to my tester, the coil has a resistance of around 12 ohms and an inductance of 80uH.
Together with the PCB, to make this project we need a buzzer that will emit the sound when metal
is detected. We need 3 BJT transistors, two PNPs and one NPN. Some capacitors,
some resistors and a potentiometer. And we need a power source which could be a simple battery.
The components list is very short. Before we mount the PCB I should explain you how this simple
circuit could detect metals and then we can test it and talk about the sensitivity of such PCB. So,
as you can see on the schematic, we have two coils.
One is of around 50 loops, and the other one is just one loop and is called a feedback coil.
If we take a closer look at the PCB we can see that we have one coil that starts here on C6,
makes the loop around and it goes till here.
And we have another coil that goes from here, makes all the loops to the middle of the PCB, and
then to the other side it makes all the loops and it finishes here where we have all the components.
So let’s use some animations and better understand this circuit. Let’s go step by
step and first we remove the second part of the circuit and only be left with these components.
Voltage is applied here so we have ground and VCC. Through this 220K resistor will pass enough
current in order to turn on a little bit this NPN transistor. So now, since the Q1 transistor is ON,
a small current could pass through the main coil of 50 loops. As we all know about inductors,
when current is passing through them, that will create a magnetic flux around it.
This magnetic flux is rising and it will reach the other coil, and once again as we know about
magnetic induction, that will induce a small current into this second coil. This current will
sum up to the current from the 220K resistor so now the Q1 transistor will open even more.
That will also increase the current through the main coil as well. Again, this will induce more
current into the second coil and this process will go till the Q1 transistor is fully ON.
At this point the magnetic flux is not rising anymore so it will be a static magnetic flux.
By the magnetic induction theory, we know that only oscillating magnetic fluxes could
induce current. So, this now static magnetic field won’t induce current into the second coil anymore.
So the current at the Q1 base will drop so this transistor will start to turn OFF.
At the same time the current through the main coil will start decreasing, and this will once
again induce a current into the secondary coil but this time the current is reversed.
So now the current will be subtracted from that 220K resistor
so the transistor will go to fully OFF. The magnetic flux will be stable once again and
this process will repeat over and over again creating a very high frequency oscillating signal.
Here I have the circuit mounted on my breadboard and connected to the coils.
One loop for the feedback coil and a lot of loops foe the main coil. When I supply the circuit, we
can see on the oscilloscope that we get around 30 KHz oscillating signal. But look what happens when
I get a metal close to the loops. The oscillation stops or the amplitude is getting smaller.
Why is this happening? Let’s learn why. Well, now we add the second part of the circuit.
While we have an oscillation created by the first part of the circuit on this point here,
which is the base of the Q2 transistor, that will switch this transistor ON and OFF very fast.
But when Q2 is ON, together with this capacitor here it will maintain a value
of VCC at the base of the third transistor. Since this third transistor is a PNP type,
with VCC it will be OFF so no current could flow towards the buzzer so no sound is emitted.
But when we place a metal near the coils, part of the magnetic flux will pass through the metal
get lost, so now the oscillation amplitude here at the Q1 base will be smaller. It will get so small,
that the Q1 transistor won’t amplify the signal enough, so the oscillation will die out.
Without oscillation, Q2 will stay always OFF so we don’t have VCC at the gate of the
third transistor anymore. And why is that? Well because now the 2.2K resistor here
will pull down the Q3 base and turn it ON.
So when it turns on it will supply the buzzer and that’s when we hear the sound of metal detected.
That’s how this circuit works, it’s pretty simple. Now let’s assemble the PCB and test it out.
Soldering the components is not that difficult since they are all trough hole. So just follow
the values on the schematic and solder them one by one. Be careful with the transistors and not
solder them backwards. We have a nice silk-layer on the PCB so we can know where each component
will go and also the polarity. All the components are now soldered. i remove the buzzer protection.
Now we need to adjust the oscillation point. We do that using this potentiometer. If you
remember this was connected at the emitter of the first transistor so by changing the potentiometer
value we change the amount of current that is passing trough the coil so with that we adjust
the amplitude and by that, when the oscillation starts. Supply the circuit with around 5V.
You will hear the buzzer beeping. Rotate the potentiometer till the sound stops.
This is the where we have the oscillations but amplitude is almost OFF. Now let’s test it.
I get it close to a metal and it beeps,
so it works.
Even through some material, it can detect the metal,
that’s why they use such detectors in airports and detect metals inside people pockets.
Now as you can see, the detection distance is not that big.
Only a few cm so this circuit is not that sensible. The problem is with the coil on a PCB.
Because on the board, the loops are placed side by side so the coil is occupying a lot of area.
A coil made with wires could have the same amount of loops but on a smaller area. So with the PCB,
when a metal is placed in the middle, only a few turns are near the metal.
Making this circuit with external coils made out of copper wire might increase the
sensibility. So you can use the same circuit we have here on the PCB but you add your own coils.
The inductance of the coil might affect the oscillation speed and so on,
so have that in mind. For example, my PCB coil has a resistance of around 13 ohms and
an inductance is of 80uH according to my tester here but I’m not sure how precise this tester is.
Is also very important that you use an active buzzer.
That means it has internal electronics that will make it sound with 5V DC voltage. Usually, passive
buzzers need an analog signal in order to beep, but an active buzzer will beep with DC voltage.
We need this because the circuit is applying DC voltage to the buzzer.
Instead of a buzzer you can also use an LED so instead of sound alarm you can have a light alarm.
So guys, you can get the schematic and the GHERBER files from below
if you want to make the same tests. I hope that you have learned something new and if so,
consider subscribing and maybe give a like to my video. Thanks again and see you later guys.