Have you ever heard a police siren and wish you had one? you could fire up a siren anytime you want and feel like you are in a fictional, non-threatening, and safe high-speed chase! Well in this Instructable, we are going to be building our own police siren circuit, that you can use anytime you want to feel chasing down bad guys...awesome!

The 555 timer integrated circuit (IC) is a useful integrated circuit used in a variety of functions such as oscillation applications, timing, and pulse generation. In this tutorial, the 555 timer, is the star of the show. We are going to use its astable mode configuration to simulate our police siren.

image credits

Police Siren - Getty Images

555 timer - Wikipedia

Supplies

For this circuit, you are gonna need:

555 timer x 2
1k resistor, color code: brown, black, red
68k resistor, color code: blue, grey, orange
10k resistor x 3, color code: brown, black, orange
10uF polarized capacitor x 2
100nF capacitor x 2
Breadboard
Connection Wires (10 minimum)
Speaker
(5-9) V Power supply

Step 1: Police Siren Circuit

The 555 timer police circuit is a simple yet effective circuit used to imitate the sound of a police siren. It can be constructed from widely, and readily, available electronic components. Making it a popular choice for hobbyist, beginners, and electronics enthusiasts interested in building their circuits, also serving as a tool to understand better the 555 timers.

Step 2: 555 Timer

The 555 timer is an integrated circuit used extensively in electronics for different functions such as timing and oscillation. It is a very popular element due to its versatility in electronics due to its simplicity, reliability, and wide range of applications

The 555 timer can operate in three different modes, these provide different configurations, timing, and functions.

Monostable mode: outputs a single pulse of current during a certain amount of time.
Bistable: it alternates between two stable states when indicated, usually known as a flip-flop.
Astable: outputs in an oscillation/signal waveform.

the 555 timer has 8 different pins:

Pin 1, ground
Pin 2, trigger
Pin 3, output
Pin 4, reset
Pin 5, voltage control
Pin 6, threshold
Pin 7, discharge
Pin 8, voltage input

On this occasion, the 555 timer will be working in astable mode. Pin 2 and 6 are connected allowing are circuit to repeatedly trigger itself on every cycle, giving us a free running oscillators. on each cycle, the capacitor charges up through timing resistors, but only discharges itself through one of them since it is connected to pin 7. The output waveform would be a square wave with a frequency and duty cycle determined by the components used, resistors, and capacitor. We can change the timing intervals by changing the value of the resistors and capacitors, the same as our duty cycle that comes from the ratio between our resistors. Some common applications of a 555 timer in astable mode are timing circuits, pulse generators, LED flashers, and tone generation.

Image credit

555 timer schematic - tutorialspoint.com

Step 3: Capacitors

A capacitor is a device that stores electrical energy in the form of an electronic field. Capacitors are composed of two conductive plates that create this electric field when voltage is applied to them. The energy stored in capacitors is usually discharged when needed in the circuit, which is one of their main purposes, they are also used for timing, filtering, and decoupling.

In our police siren, capacitors are responsible for the conjunction with resistors to determine the frequency and duty cycle of the output waveform, also we will be using two different times of capacitors, "regular" and polarized capacitors. The "regular" capacitors are also known as non-polarized capacitors and are commonly made of ceramic. Meaning that when implementing them in our circuit it does not matter in what direction they are connected. In contrast, with polarized capacitors, it does matter the direction of connection, can be easily identified by the arrow with minus signs (-) on it. The most common of polarized capacitors are electrolytic capacitors, which we are implementing in our police siren.

Step 4: Resistors

Resistors are passive components that impede the flow of electrical current in a circuit, usually, they hold a specific resistance value. This resistant value determines how much resistance is applied to the flow of current, the bigger the value bigger the resistance.

Resistors are usually used for different purposes such as current limiting, voltage division, and temperature sensing. As mentioned before in our circuit we are going to be using three different values of resistance, in this circuit they control timing, frequency, and current flow.

Step 5: How Does It Work

Our first 555 timer is on astable mode, and it produces a single tone. To make our circuit sound or imitate a police siren we need the second 555 timer, also in astable, making it go back and forward between two tones simulating the police siren.

The transition between these two tones is possible because of our second 555 timer. Which is connected to the output pin of our first 555 timer. Now depending on the output of our first 555 timer, we can hear two different from our second 555 timer, since it is acting as a modulator to the frequency of our first 555, giving us the two characteristic pitch.

With the help of our capacitor connected to our control voltage pin, in the first 555 timers, the voltage increased and decreased gradually, resulting in the similar sound of a police siren. Different tones are speeds can be achieved by changing the values of some of our components such as resistors and capacitors.