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Tutorial 13: Wailing Siren

Created on: 4 August 2012

In this tutorial you will build a wailing siren that plays a tone that increases and decreases in pitch. The circuit is built from two 555 timer ICs and some additional electronic components. The wailing tone is played on a loudspeaker.

This video shows the finished project:

Can't see the video? View on YouTube →

This is the first tutorial in this series that does not include step by step photos of the circuit being built. It is important to start learning to build circuits from the circuit diagram only.

Prerequisites

Complete Tutorial 5: 555 LED Flasher before attempting this tutorial. Read about diodes.

Components

In addition to a breadboard, wire links, 9V battery and battery clip, you will need:

Qty Part Designator Notes Type
3 100k R1, R5, R6 1/4W, 5% or better Resistors
4 10k R3, R7, R8, R9
1 1k R2
1 100 ohm R4
1 100uF C1 Electrolytic capacitor Capacitor
2 10uF C2, C3
1 100n C4 Non-polarized
2 555 U1, U2 555 timer ICs (usually marked LM555 or NE555) Semiconductor
1 PN2907 Q1 PNP transistor
1 1N4148 D1 Diode
1 8 ohm loudspeaker SP1

Amazon.com

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Circuit Diagram

Click the circuit diagram for a bigger image.

Wailing siren circuit diagram


555 timer IC pinout

555 Timer IC Pin Numbering



PN2907 PNP Transistor Pinout

PN2907 PNP Transistor Pinout



Building the Circiut

The circuit is really just two 555 timer circuits joined together. Start by building the left part of the circuit around U1 (the left 555 timer). A photo of the complete circuit is shown below.

The complete wailing siren circuit on breadboard

Click the photo below for a larger image of the completed circuit.

Close-up view of the siren circuit





When building the circuit from the schematic diagram only, always carefully check the circuit after building it. The best way to check it is to take one IC at a time and check from pin 1 through to pin 8. Check that each item and connection shown in the schematic is present on your breadboard circuit.

E.g. in the wailing siren circuit, start with IC U1 and check that it is connected to the GND rail of the breadboard. Now check that pin 2 is connected to pin 6 of the IC and that capacitor C2 is connected from pin 2 and 6 to GND. Also check that C2 is connected with the negative lead to GND. Check pin 3 of U1 next – see that it connects through R6 and C3 to GND. Carry on checking the rest of the pins of U1 and then move on to U2.

Operating the Circuit

After connecting the battery, the circuit will produce the wailing sound. For a different sound, remove capacitor C3 from the circuit as shown in the video.

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