This special electronic circuit makes a sound like a siren, but instead of a steady wail, it goes high low high low in a repeating pattern kind of like saying “Hee Haw.”
It is like a fun siren sound you might hear on a cartoon!
Circuit Working:
Parts List:
| Component Type | Description | Quantity |
|---|---|---|
| Resistors(All resistors are 1/4 watt unless specified) | 220Ω | 2 |
| 5.6k | 2 | |
| 100k | 2 | |
| 3.9k | 2 | |
| Capacitors | Ceramic 100nF | 2 |
| Electrolytic 100µF 25V | 3 | |
| Semiconductors | Transistor BC547 | 4 |
| Transistor BC557 | 1 | |
| LEDs | Any 5mm, 20mA | 2 |
| Speaker | 8Ω | 1 |
The circuit works by using two astable multivibrators also sometimes called oscillators.
These circuits can generate a continuous electronic signal without the need for an external input.
In this schematic, one multivibrator runs at a lower frequency than the other.
The lower frequency multivibrator acts as a control circuit, changing the timing of the higher frequency multivibrator which creates the “Hee Haw” sound.
Here is a working of the circuit:
It consists of two BC547 transistors Q1 and Q2, two 100uF capacitors C1 and C2, three resistors 3.9k, 5.6k, and 220Ω, and an LED.
The timing capacitor C1 controls the frequency of this oscillator.
When the voltage at the base of Q1 rises the transistor turns on and charges C1 through the 5.6k resistor.
As C1 charges, the voltage at the collector of Q1 rises, eventually turning it off and Q2 on.
C1 then discharges through the 220Ω resistor and the LED causing the voltage at the collector of Q1 to fall.
This turns Q1 back on and the cycle repeats causing the LED to flash and creating a low frequency oscillation.
It uses another two BC547 transistors Q3 and Q4, two 100nF capacitors C3 and C4, and two 220Ω resistors.
The timing capacitors C3 and C4 control the frequency of this oscillator which is designed to be much higher than the lower frequency oscillator.
The 220Ω resistors limit the current through the transistors.
The voltage controlled by the lower frequency multivibrator is applied to the base of Q3, which effects the output frequency.
The speaker is connected to the collector of Q4 through an 8Ω resistor.
The varying voltage from the collector of Q4 is what creates the “Hee Haw” sound.
By changing the values of the capacitors and resistors, you can change the sound of the siren.
While this is a relatively simple circuit it is important to be aware of potential safety hazards when working with electronics.
Make sure to use a power supply that is rated for the circuit and be aware of the risks of electrical shock.
If you are not comfortable working with electronics it is best to consult with someone who is.
Formulas:
The oscillation frequency and timing cycles of astable multivibrators are determined using formula.
The following are the main equations for a simple astable multivibrator:
Frequency (f):
f = 1 / T
where,
- T is the total period of the oscillation
Time constants T1 and T2:
T1 = 0.693 * (R1 + R2) * C
where,
- Time capacitor C charges through R1 and R2)
T2 = 0.693 * R2 * C
where,
- Time capacitor C discharges through R2
Total Period T:
T = T1 + T2
where,
- For a symmetrical output waveform
Duty Cycle D:
D = T1 / T
where,
- Ratio of high time T1 to total period
These calculations presuppose that the astable multivibrator is constructed using discrete parts (transistors, capacitors and resistors).
Remember that these equations offer a place to start.
The duty cycle and oscillation frequency can be influenced by the real component quantities and circuit layout.
How to Build:
To build a Simple “Hee Haw” Siren Circuit follow the below mentioned connections steps:
- Connect all the components as shown in diagram.
- Connect transistor Q1 collector to positive supply through LED1 and 220 ohm resistor, connect base of Q1 to output side 5.6k resistor and connect emitter of Q1to ground.
- Connect transistor Q2 collector to positive supply through LED2 and 220 ohm resistor, connect base of Q2 to input side resistor 5.6k, and connect emitter of Q2 to ground.
- Connect transistor Q3 collector to 3.9k resistor, connect base of Q3 to output side 100k resistor, and connect emitter of Q3 to ground.
- Connect transistor Q4 collector to outside side 3.9k transistor, connect base of Q4 To input side 100k resistor and connect emitter of Q4 to ground.
- Connect transistor Q5 collector to ground, connect base of Q5 between collector or Q4 and transistor 3.9k, and connect emitter to one end of speaker.
- Connect capacitor C1 100µF between 220 ohm resistor and 5.6k resistor.
- Connect capacitor C2 100µF between 5.6k resistor and 220 ohm and LED2 resistor.
- Connect capacitor C3 100nF between 3.9k resistor and 100k resistor
- Connect capacitor C4 100nF between 100k resistor and 3.9k resistor.
- Connect capacitor C5 100µF from positive supply to ground to negative supply.
Safety Reminder:
- Even though this is a low voltage circuit, it is still important to be careful when working with electronics.
- Double check your connections before powering on the circuit and use a power supply with the appropriate voltage rating.
Conclusion:
A “Hee Haw” siren circuit is a fun electronics project that creates a two toned sound effect mimicking a siren, but with a humorous twist.
It uses oscillators to generate the sound and control its variation.
While not used in real emergencies these circuits offer a creative way to learn about electronics and generate a classic sound effect.
Remember to prioritize safety when building electronic circuits.