Simple Beeper Circuit

This post shows you how to build a simple beeper circuit.

When you press a button, it makes a sound using a special type of electronic circuit called an asymmetric multivibrator.

The sound comes out of a small speaker.

This is a fun project to learn about electronics and sound.

What is a Beeper Circuit:

The Beeper circuit is a simple yet advanced electronic gadget that generates a continuous beeping sound, suitable for alarms or indications in settings like hospitals, police stations or fire brigade stations.

Circuit Working:

Parts List:

The above beeper circuit operates as an asymmetric multivibrator, generating audible signals through the interaction of two transistors (NPN and PNP) in a feedback loop.

The circuit is initiated by a pushbutton and the audio frequency can be adjusted using a resistor.

Below are explanation of the circuits operation:

Initialization:

• Pressing the pushbutton triggers the circuit initiating the operation.

Initial State (Stable State):

• Before the pushbutton is pressed, the circuit is in a stable state with one transistor turned ON (conducting) and the other OFF (non-conducting).
• Let us assume Q1 (NPN transistor) is ON, and Q2 (PNP transistor) is OFF initially.

Pushbutton Pressed:

• When the push button is pressed, it causes a rapid change in the circuit’s state.

Transition to Unstable State:

• The pressing of the pushbutton turns OFF the initially conducting transistor Q1 in this case and turns ON the initially non conducting transistor Q2.
• This transition results in an unstable state where both transistors are temporarily conducting.

Capacitor Charging:

• As Q2 turns ON the capacitor connected to its collector begins to charge.
• The charging of the capacitor determines the duration of the unstable state.

Transition Back to Stable State:

• Once the capacitor is sufficiently charged Q2 turns OFF, and the circuit transitions back to a stable state.
• Now Q1 is turned ON again and Q2 is turned OFF.

Capacitor Discharging:

• With Q1 ON the capacitor connected to its collector starts to discharge preparing for the next cycle.

Repetition of the Cycle:

• The circuit continues to cycle between the stable and unstable states as long as the pushbutton is held down.
• This cycling generates a series of pulses producing an audible tone through the connected speaker or earphones.

Adjustable Audio Frequency:

• The resistor R1 in the circuit allows for the adjustment of the audio frequency.
• By changing the resistance of R1 the charging and discharging times of the capacitor are modified, influencing the frequency of the generated audio signal.

Optional Earphone Usage:

• If earphones with an impedance of around 500 ohm are used the circuit still operates similarly, providing audio output through the earphones.

Formulas and Calculations:

You may develop a basic astable multivibrator circuit to make a beeper circuit.

When the push button switch is hit, this circuit will create a square wave signal to drive the speaker and emit a beeping sound.

Calculating Frequency:

For a basic astable multivibrator the oscillations frequency f, can be roughly calculated as follows:

f = 1.44​ / (R1+2 * R2) * C

where,

• R1 and R2 are the resistors in ohms
• C is the capacitance in farads.

Use this formula to enter your inputs and get an estimate of the beeping sounds frequency.

In our diagram: R1 = 1.5k, R2 = 4.7k, C = 470nF

So the frequency will be approximately:

f = 1.44​ / (1.5k + 2 * 4.7k) * 470 ×10⁻⁹ = 1.44​ / (1.5 + 9.4) * 470 × 10⁻⁶ ≈

1.44​ / 11.4 * 470 × 10⁻⁶ = 1.44​ / 5.358 × 10⁻³ = 268 Hz

This gives an approximate value of the frequency, which may differ significantly based on the actual values and tolerances of the components.

How to Build:

Identify Components:

• Gather all required components ensuring compatibility with the specified circuit parameters.

Transistor Placement:

• Insert the chosen silicon NPN transistor Q1 into the circuit, connecting it as per the circuit diagram.
• Similarly, place the selected PNP transistor Q2 in the designated position.

Connect Speaker :

• Connect the loudspeaker with a coil impedance of 25 to 40 ohms to the circuit ensuring proper polarity alignment.

Resistor R1 Connection:

• Integrate resistor R1 into the circuit to enable audio frequency adjustment.
• Connect it according to the circuit diagram, considering its role in fine tuning the output.

Pushbutton Installation:

• Install the pushbutton for circuit initialization ensuring a reliable connection to facilitate user friendly operation.

Battery Connection:

• Based on the drain current specifications of Q2 select an appropriate battery and connect it to power the circuit.

Conclusion

Following these detailed construction, you will have successfully assembled a versatile beeper circuit with adjustable audio frequency range offering a unique and customizable audio output experience.

The circuit utilizes the principles of an asymmetric multivibrator to produce audible signals and the inclusion of a pushbutton and resistor R1 adds functionality and adjustability to the generated audio tone.

References

Buzzer or Beeper