Tutorial 13: Wailing Siren

Created on: 4 August 2012
Updated on: 18 January 2023

Build a wailing siren circuit 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.

The wailing siren circuit is built on an electronic breadboard in this electronics tutorial for beginners.

The following video shows the finished wailing siren circuit project built on a breadboard. A rising and falling pitch can be heard in the video. When a capacitor is removed, the tone changes.

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.

Wailing Siren Circuit Prerequisites

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

Wailing Siren Circuit 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

Wailing Siren Circuit Diagram

Click the wailing siren circuit diagram below for a bigger image. The pinout for the PN2907 transistor and and 555 timer ICs used in the circuit follow.

It may be useful to open the circuit diagram in a separate browser tab while building the circuit.

Transistor Q1 in the circuit is a PN2907 PNP transistor, shown below.

Pinout of the 555 timer IC is shown below. It uses the normal 8-pin IC pin numbering convention.

Building the Wailing Siren Circuit

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.

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

For example, 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 9V battery, the circuit will produce the wailing sound. For a different sound, remove capacitor C3 from the circuit as shown in the video.

The images and video above show the circuit powered from a battery pack that consists of six AA cells. These are rechargeable cells, which means that they are 1.2V each. The voltage that is supplied to the circuit is therefore 1.2V × 6 = 7.2V. If six 1.5V cells are used instead, then the voltage is 1.5V × 6 = 9V. Alternatively, a single 9V battery can be used to power the circuit.