6 Simple transistor | police | ambulance Siren (audio alarm) circuits

The siren circuits (audio alarm circuits) are important in the various alarm systems. For example, emergency alerts, burglar alarm circuits, Fire alarm circuits, timers, sensor controls, etc. Today I would like to introduce the simple and economical siren circuits. It is suitable for beginners.

Simple transistor Police Ambulance Siren circuit

My son will gradually build this circuit onto the PCB layout. Look at below! 5 Circuits.

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Simple audio alarm circuit using transistors

I like to build a simple audio alarm circuit using transistors. Although it is an old circuit, it still useful. The circuit uses just two transistors and a few other components. We will try to build it to learn a simple oscillator circuit.

How it works

Simple audio alarm with transistor

simple audio alarm circuit using transistors

In the circuit diagram above. When we press the switch-S1. It will have the electric current flows through R1 to charge at the C1. While the voltage at the Base(B) of Q1 rises until makes Q1 work.

When Q1 is working, also the Q2 is working together. It causes a large current to flow through leads E into C of Q2, and R4 to drives the loudspeaker.

Recommended: Learn transistor works here

Next, the electric current begins to charge into C2, until it full. While the voltage at the B of Q1 will begins to go down until Q1 does not work.

Also, the Q2 stop work. It causes C2 to discharge to R4, SP1, and R3 to ground. Until the voltage at lead B of Q1 rises up. It will make Q1 begin working again, Q2 works too. This working generates the pops to SP1.

Which Q1 and Q2 will work together with high frequency. until merging many many pops into a continuous buzz or tone to the speaker.

When we release switch S1, C1 begins to discharge. It makes the voltage at B of Q1 go down. The output is a low frequency until the sound comes to silent.

Parts you will need

Q1 – 2SC1815
Q2 – 2SA1015
Resistors (All 0.25 watt,5% metal/carbon film)
R1 – 22K
R2 – 68K
R3 – 56K
R4 – 10 ohms
Capacitors and others
C2 – 0.068uF 50V, Ceramic
C1 – 100uF 25V, Electrolytic
SP1 – 8 ohms speaker
B1 – 9V battery

How to build it

This is a small circuit. You can assemble them on the universal PCB Board.

Keep reading: ‘Electronic bird sounds’ »

Electronic siren circuits using 2 transistors

The operation of the circuit is Q1 and Q2 will work with R3, R4 and C2 is a frequency generator circuit with output connected to LED1 and speakers SP1.

When I press the switch S1 to the C1 will begin to charge allows the voltage to pin B of Q1 to increase others.

The Q1 is working and Q2 is working with at the C1 charge full cycle will stop oscillator speaker is not.

The LED1 light and hold the release of the pressure switch S1, resulting in C1 will start discharger through R3, Q1 to ground.

The oscillator circuit sounds Sirens in the lower out.

If we press the switch S1 and then quickly leave many. The sound will be as continuous as C1 to charge and discharge alternately continuously.

Simple Transistor Electronic siren circuit

Parts will you need

  • Q1: 2N3904,C1815,BC548,BC547,BC549, 45V 100mA NPN Transistor
  • Q2: 2N3906,BC558,BC557,BC559, 45V 100mA PNP Transistor
  • C1: 100uF 25V, Electrolytic Capacitors
  • C2: 0.1uF 50V, Ceramic Capacitors
  • R1: 47 ohms, 1/4W Resistors tolerance: 5%
  • R2: 47K, 1/4W Resistors tolerance: 5%
  • R3: 33K, 1/4W Resistors tolerance: 5%
  • R5, R6: 68 ohms, 1/4W Resistors tolerance: 5%
  • D1: 1N4148, 75V 150mA Diodes
  • SP1: 8 ohms 0.5W Speaker
  • S1, S2: Normally open pushbutton
  • And others,

This is a simple circuit than upper.let’s build now! 🙂

Learn: How to use LM386 audio amplifier circuit

Simple Siren using 2N2907 Transistor

This is a simple siren circuit diagram that builds easily. Because of using the transistor 2 pcs only just.

We can use the 8-ohm size loudspeakers. Which, I use an old loudspeaker that not good for listening to music. But is so loudest for this.

The usability just you press the switch (close) S1 a loudspeaker will utter. Increase the frequency goes up (C1, loaded ). When liberating Switch(Open)S1.

The sound will decrease the frequency down (C1, disgorge load ) change the value R1. For a change, the time goes up-down.

Please see ะ้ำ circuit picture assembles.

Simple siren circuit diagram using 2N2907 Transistor

Parts will you need
Q1: 2N2222, 45V 100mA NPN Transistor
Q2: 2N29097, 45V 100mA NPN Transistor
C1: 22uF 25V, Electrolytic Capacitors
C2: 0.47uF 50V, Electrolytic Capacitors or ceramic Capacitors

0.25W Resistors tolerance: 5%
R1: 39K
R2: 47K
R3: 22K
SP1: 8 ohms 0.5W Speaker
SW1: Normally open pushbutton

And others.

Ambulance sirens circuit

This is a sirens circuit that has sounds like an ambulance siren. It is different from the general ambulance siren circuit. We use transistors instead of IC-555.

Which it saves and easy to find all parts.

Operation of the circuit

This cycle can be divided into three main parts is low-frequency production. Production of high frequency. The amplifier.

The ambulance sirens circuit

The schematic of Ambulance sirens

The manufacture low-frequency includes Q1, Q2, R2, R3, C2, C3.

Which will produce low-frequency out and to extend the Q3.

Before you sent with the high frequency of origin from Q4, Q5, R8, R9. , C5, C6 signal is included, then the output will come out to the leg E of the Q5.

And then be expanded with Q6(2N2222) to drive the speakers.

The C1, C4 is intended to stir filter collects them. The R11 will help prevent speaker damage.

Transistor Q1-Q5, you can use very many numbers that is NPN 100mA current collector. For example :
2SC1815 or 2N2222 or 2N3904 etc.

Parts will you need
Q1-Q5: 2N3904_45V 100mA NPN Transistors
Q6: 2N2222, 45V 100mA NPN Transistors
C1: 10uF 25V, Electrolytic Capacitors
C2,C3,C4: 1uF 25V, Electrolytic Capacitors
C5,C6: 0.04uF 50V, Ceramic Capacitors
0.25W Resistors tolerance: 5%
R1: 18K
R2,R3,R5: 1.2M
R6, R10: 10K
R7: 18K
R8: 12K
R9: 100K
R11: 100 ohms
SP1: 8 ohms 0.5W Speaker
9V battery

And others.

Look these:

American & Hong Kong Police siren circuit using transistors

Today I suggest police siren circuit. Another circuit is very interesting. It can mimic the sound of an American police car and the Hong Kong police. By which we use transistors mainly. For is so easy to buy equipment and budget.

How does it work?

In Figure 1 will see that include two sound generator set. The first sound generator consists of Q1, Q2, C2, C3, and R1-R4. Then a second sound generator will include of Q3, Q4, C5, C6, and R7-R10. The sound signal will Different as the level of parts in the circuit.

American police siren

Figure 1 The circuit diagram of a perfect American & Hong Kong police siren.

When an S2-switch is on “Hong Kong” position, the sound signal from a first series will out off from collector(C) of Q2 through R5 to control the working of the second sires.

Which causes can get the output signal at the collector of Q4 through R11 into Q5 will be transformer-T1 by it is used to match the output of such circuits to a speaker.

If slide sw1 to “191” position will have also some sound signal through C4. A capacitance-C4 will cause the feature of a sound signal is changed.

How to build it

actual-size of Single-sided Copper PCB layout

You can assemble parts on the perforated board or universal board. But if you want to make PCB in permanent version. You can see in Figure 2 is actual-size of Single-sided Copper PCB layout. And Figure 3 is a component’s layout and wiring.

Figure2 The Actual-size of Single-sided Copper PCB layout

components layout and wiring

Figure 3 The components layout and wiring

Application of this project

When assembling this project completely then to check for error, next apply the power supply 9-12volts, then test them by applying a 9-volt battery to the circuit. Next turn on power switch then press S2 we will hear a sound of siren from speaker anf LED1 flashes.

The applications can do many things. Such as be installed in the police car toy. Be installed on a motorcycle or a beep alarm for you.

The components list

Resistors ¼W +-5%
R1, R4, R7, R10: 1K
R2, R3, R8, R9: 56K
R5: 2.2K
R6: 560 ohms
R11: 4.7K
R12: 50 ohms

Q1-Q5: 2SC458 or similar (2SC1815 or 2SC828)

Polyester Capacitors 50V
C5, C6: 0.0047uF 50V
Electrolytic capacitors
C1: 220uF 16V
C2, C3: 3.3uF 16V
C4: 33uF 16V
T1: TO-601, Output audio transformer
S1, S3: SPDT- Single-pole, Double-throw switch
S2: Normally pushbutton
LED, speaker 8 ohms, PCB, Universal box.

T1 is an audio output transformer. It is small transformer. I found it on amazon.com It may work well.

AC 220V siren circuit using transistors

This AC 220V siren circuit is a simple siren that uses 220V AC as a power supply. It is small size and uses a piezo speaker cause use low power and saving.

How it works

This circuit is shown in Figure 1 which we use power source by reducing AC-voltage of 220 volts down by the resistors-R1, R2; the capacitor-C1 act as coupling AC-voltage.

Secondly, the current will rectifier into DC voltage by diode bridge D1-D4. And there is a C2 function to filter steady DC current.

220v ac siren circuit

Some voltage will flow to emitter lead of Q5 and another will is sent to R3-R6 to reduce voltage to ZD1 before, which will maintain the steady (DC) current regulated supply at about 9.1 volts.

The capacitor-C3 will smooth or filter the pulsating voltage from a power supply into direct current.

To begin with press switch-SW1, the 220V AC voltage will be reduced into 9.1 volts as the power source of this circuit.

Both transistors- Q1 and Q2 are connected to a low-frequency generator form, by output frequency depend on R7, R8, R9, C4, and C5. When begin to apply the power supply into the circuit.

The Q2 will work, at the same time, C6 will start to charge current. While transistor-Q2 is working, C5 will be charged until the voltage drop across C5 about 0.6 volts.

The transistor-Q1 will begin working cause Q2 stops. Then C4 starts charging. At the same time it will pull current from B of Q1, so Q1 stop.

Next Q2 will start to renew again, Will be this indefinitely. The voltage at E of Q2 will charge to C6 and stop charging according to the loop above.

While Q2 stopped, C6 is discharged by R10 automatically. The emitter lead ‘s voltage of Q2. In addition, will charge the C6 is also supplied to B of Q3 and Q4.

Which they are connected into an astable multivibrator circuit feature. The operation of this project will start by Assume that the function Q4 (ON), located in the state of saturation and Q3 inactive (OFF).

The C7 will be charged until the voltage across it is equal to the voltage of E of Q2.

While the Q2 performance, voltage at C of Q3 will increase, Until equal voltage from E of Q2. R13 serves to determine the size of the base current of Q4. To get the minimum, which makes Q4 can work in saturation.

Therefore it can be seen that when the capacitor-C7 capacitor fully. Q4 is working perfectly and is a state of saturation. And vice versa. When Q3 a state of saturation work. The capacitor-C8, it will keep fully charged, the voltage drop, equal to the voltage of E of Q2. While Q4 is in the works to make positive of capacitor C8 becomes negative.

All this work will result in Q6 and Q7 working alternately. When Q6 works, Q5 also works to follow with PZ1.

There was a shake-up until it was pitched up. But when Q5 and Q6 stop working, Q7 will work instead PZ1 so gradually stopped shaking and the sound stopped.

How to Assemble this circuit

Who created this project requires special precautions. It involves AC 220 V directly.

First is to start making PCB as Figure 2. which actual-size of Single-sided Copper PCB layout. Making PCB should check the accuracy of the PCB before.

Then Assemble parts when examining successfully. As Figure 3 Component layout for the PCB, Should be soldered from the low to the high, and if the device is a terminal. I need to see well before soldering.

actual-size of Copper PCB layout

Figure 2 The Copper PCB layout.

Component layout for the PCB

Figure 3 The component layout.


We should check for errors before. The external device as Figure 4. You should be very careful Because we are using the AC 220V directly.

When the circuit is correct Then apply AC 220V to the circuit and turns on SW1. You can hear the sirens coming piezo speaker.

the application-of-ac-220v-siren

Figure 4 the application.

Components list

Resistor size: 0.25 watt +-5%
R2: 220K
R7: 15K
R8: 470K
R9, R14: 1K
R10, R13: 10K
R11: 5K
R12: 33K

Resistor size: 0.5 watts +- 5%
R1: 50 ohms
R3-R6: 39K
R15: 20K R16: 100 ohms

C1: 1uF 250V AC/DC capacitor
C2: 2.2uF 350V, Electrolyte
C3, C6: 100uF 16V, Electrolyte
C4, C5: 2.2uF 50V, Electrolyte
C7: 0.01 uF 50V: Ceramic

Semiconductor and others
D1-D4, 1N4007 1A 1000V Diode
ZD1: 9.1V 0.5W, Zener Diode
Q1-Q4, Q6-Q7—BF422, Transistor or similar
Q5: BF423, Transistor or similar
Other parts
FUSE1-Fuse 300mA set
SW1-Switch AC PZ1- Piezo Speaker
FB-17 PCB, wires, and others

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24 thoughts on “6 Simple transistor | police | ambulance Siren (audio alarm) circuits”

    • please describe ” T1: TO-601, Output audio transformer ” in the ” American & Hong Kong Police siren circuit using transistors “. I search the world but not find it

    • please describe ” T1: TO-601, Output audio transformer ” in the ” American & Hong Kong Police siren circuit using transistors “. I search the world but not find it

    • please describe ” T1: TO-601, Output audio transformer ” in the ” American & Hong Kong Police siren circuit using transistors “. I search the world but not find it

  1. Where are the real schematics for electronics I can’t find any can you help me Please i don’t trust too many schematics now days they don’t seem that they work 210-309-2078

  2. please describe ” T1: TO-601, Output audio transformer ” in the ” American & Hong Kong Police siren circuit using transistors “. I search the world but not find it
    please help

  3. hello again and sorry from my infinite questions and your helps too. but I need more information about OT-601. Like the voltage and mA that the transformer can resist

    • Hi, Anthon black
      It’s okay. Do not be afraid. Sometimes I might answer (comment) too late, you might have to wait, I’m sorry.

      I measured the resistance of the OT-601 transformer. The output is 8 ohms and 2 ohms for the input.

  4. hello again. in the picture of hong kong and USA police siren, you say 2.2k resistor for R5 but in the component list, it is 22k. which one is true?

    • I thank you for visiting our website. But I am quite feeling bad that cannot tell you, the number of turns of copper wire that can be wrapped around a small transformer. However, I admire you for making it yourself. It is an interesting science learning activity. In the future, I might teach my daughter how to wind up a transformer herself like you.


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