Introduction: DIY an Air Raid Siren With Resistors and Capacitors and Transistors

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This affordable Air Raid Siren DIY project is suitable for researching self-oscillation circuit composed of just resistors and capacitors and transistors that can enrich your knowledge. And it is suitable for National Defense Education for Kids, in the meantime, it can also be used to demonstrate how do we use resistors and capacitors to generate periodic waves to drive a speaker to make sound in Science and Technology lessons to engage the student to keep their minds on learning and exploring.

The necessary materials:

1 x 2.7k

1 x 20k resistor

1 x 56k resistor

1 x 103 ceramic capacitor

1 x 47μF electrolytic capacitor

1 x 9014 NPN transistor

1 x 8550 PNP transistor

1 x switch button

1 x 4Ω 2W Speaker

1 x header pins

Step 1: Solder the Resistors to the PCB

Resistors do not have polarity, insert them into the corresponding position on the PCB. Image ① is showing the 2.7kΩ resistor inserted in the position of R3, image ② is showing the 20kΩ resistor in the position of R1, image ③ is showing the 56kΩ resistor in the position of R2. How do we know the correct value of each resistor? There are two approaches to figure it out. One is to use multimeter to measure it out and the other is to read the resistance value from the color band printed on its body. For example, the resistor on image ⑥ is with 2.7kΩ. How do we get the 2.7kΩ as a result ? As we can see that the first color band is red which represents the digit number 2, the second color band is violet which represents the digit number 7, the third color band is red which represent 100 as a multiplier. OK, Let us connect them together and we get 27x100=2700Ω=2.7kΩ. For more details of reading resistance value from the color bands please refer to the blog on by right clicking your mouse to open the page in a new tab on your browser.

Step 2: Solder the Electrolytic Capacitor to the PCB

Please note that the electrolytic capacitor has polarity, the leg near the white band should be inserted into the hole in the shadow zone on the PCB.

Step 3: Solder the Switch Button Into the PCB

Set the switch button on the place as shown in image ⑨ and solder it as shown in image 11.

Step 4: Solder the NPN and PNP Transistors and Header Pins Into the PCB.

For the PNP transistor in this project there is a model number, S8050, carved on the flat surface of itself. For NPN transistor there is a model number, S9014, carved on the flat surface of itself. Both of the NPN and PNP transistor should be placed by putting the flat surface on the same side of the diameter of the semicircle on the PCB. The 8550 PNP transistor should be soldered to the VT2 on the PCB while the 9014 NPN transistor should be soldered to VT1 on the PCB. The header pins should be soldered to the J1 on the PCB, leaving the long part for outer connection with the power supply device such battery holder and voltage source etc.

Step 5: Solder the Speaker to the PCB

Before we do the job, we should use a wire cutter to carefully rip off a small part of skin of the wire and make a little solder wire on the exposed wire by the soldering iron, just as shown in image 14. And please follow the image 15 to image 18 to solder the speaker to the PCB.

Step 6: Analysis

As we can see from the above diagram that VT1 and VT2 is connected to work together as a Direct Coupled Amplifier, or DC Amplifier. The R3 and C2 is conducted as a Positive Feedback to the amplifier circuit. The generated frequency is determined by the values of C1, R1 to R3 and C2. C2 is also playing as a role of coupling that blocks the DC signal. When we press down the switch button, or SB, the circuit starts working, C1 is charging and VT1 is conducted, VT2 is conducted in sequence, the generated frequency of this circuit is rising from 0 to about 1.7kHz in a period of time, when the frequency reaches its maximum it will not keep rising even you still keep the switch button pressed down. During this process the sound making by the speaker that driven by the changing frequency is growing from small to loud.

When we release the switch button, C1 plays as a role of battery that starts discharging to supply energy to the circuit, the generated frequency begins falling from about 1.7kHz down to 0Hz gradually, the sound making by the speaker goes weakening gradually.

This project is quite simply but contains lots of knowledge of the fundamental circuit that it is ideal for study purpose. The DIY materials are available at