Hi, guys, welcome to electrical projects channel, and today, in my laboratory I am going to
build a security alarm device with a reset button.
This device is relatively simple and doesn’t require any expensive components.
Well, for building such alarm device we will need: 4 transistors BC547B, six 10 Kiloohms
resistors, and two 1 Kiloohms resistors, and also we’ll need a small button, for reset,
and an active 12V buzzer.
And for triggering this alarm, I will use a reed switch and a magnet.
Also, we’ll need wires.
Now we are ready to start soldering.
but first let’s look at the schematic, and at first, for some of you, it may seem complicated,
but actually, it is not, especially when you understand what each part of this circuit
does.
In the center, you can see a regular flip-flop circuit, this part of the circuit has some
memory, and maybe word “memory” is too strong in our case, because it only remembers
one bit of the information, one or zero, in our case it will remember whether the alarm
was triggered or not.
And I’ll show you how it works later in this video.
On the right side you can see Transistor Q4 it Controls the active buzzer, and works like
a buffer, between flip-flop circuit and the load.
Transistor Q1 on the left part will trigger the flip-flop circuit, I’ll explain why
we need it a little bit later.
To make soldering and troubleshooting a little easier, at first I am going to build only
a part of this circuit and test it, and then we’ll go further.
I will not use any PCB board, so it will be easier for you to see
how everything connected.
Well, First I am going to connect and solder a black wire to emitter leads of transistor
Q2 and Q3,
Then, let's connect two 10 Kiloohms resistors,
according to the circuit, so each resistor should be connected from collector lead of
the first transistor to the base lead of the second transistor.
And Now let’s solder 1 Kiloohms resistors to collector leads of the transistor Q2 and Q3.
Also let’s solder 10 Kiloohms resistor between the collector lead of transistor Q3, and the
base lead of transistor Q4.
Emitter lead of the transistor Q4 should be connected to the negative wire, which in our
Let’s solder everything.
And now we can solder the buzzer.
Let’s solder the minus lead of the buzzer to the collector lead of the transistor Q4.
And this part of the circuit is almost ready.
Now let’s solder the red wire to the plus lead of the buzzer, and to the 1Kiloohms resistors.
And before we go any further, let’s test this part of the circuit.
For the testing, we need to connect this circuit to a power source.
I’ll use a 12V battery.
But it is not all, in order to test this device I’ll connect a 10 Kiloohms resistor to this
yellow wire, and connect this wire to the plus lead of the battery.
Now let’s test it.
Now when I touch the base lead of the transistor Q2 it activates the buzzer, and when I touch
the base lead of the transistor Q3 it turns the buzzer off.
As you can see everything works fine, which means that we’ve soldered everything correctly.
Now let’s solder the button to the wires.
After that let’s solder the first wire to the base lead of transistor Q3, and let’s
and let's solder the second wire to 10 Kiloohms resistor.
After that 10 Kiloohms resistor should be soldered to the positive red wire.
And Now you can test this circuit again with a wire and 10 Kiloohms resistor, but it is
not necessary.
As you can see, you can already trigger this alarm and reset it with a button.
And you may wonder why we need one more transistor.
And it all because of how reed switch works.
Let me show you.
As you can see without the magnet the reed switch doesn’t conduct any electricity,
and only starts to conduct electricity when I bring the magnet to it.
And if I just install this switch with a magnet on a door, it will trigger the alarm when
I close the door, and not when I open it, which is not what I need, and that is why
we need one more transistor.
So, let’s solder it and I’ll show how it works with one more additional transistor.
Let’s solder the emitter lead of the transistor Q1 to the base lead of the transistor Q2.
Then let’s solder 10 Kiloohms resistor between the collector lead of the transistor Q1 and
the positive wire.
And let’s connect one more 10 Kiloohms resistor but this time between the base lead of the
transistor Q1 and the positive wire,
Now let’s connect two wires that will go from this device to the reed switch.
Let’s solder the first wire to the base lead of the transistor Q1
and let’s solder the second wire to the minus
And this device is ready, let's test it again.
Now if I disconnect these wires it will trigger the alarm.
And even if I connect the wires together again, it will not turn the buzzer off.
You need to reset the alarm with this button.
Let’s see how it works in real life.
I’ve installed the reed switch on this door, and connected to our device, let’s see how
it works.
Everything works.
And that is all for today, I hope this video was interesting.
Thanks for watching, and see next time!