Simple Beeper Circuit

How to build a simple beeper circuit is described in this article.

An asymmetric multivibrator is a type of electrical circuit that produces sound when a button is pressed.

A little speaker produces the sound.

Learning about music and electronics through this project is fun.

What is a Beeper Circuit:

The Beeper circuit is a simple yet advanced electrical device that constantly beeps making it perfect to use as an alert or indicator in places like hospitals, police stations or fire stations.

Circuit Working:

Parts List:

The beeper circuit mentioned above functions as an asymmetric multivibrator producing audible signals by means of a feedback loop using two transistors (NPN and PNP).

A pushbutton initiates the circuit and a resistor allows you to change the audio frequency.

Here is a description of how the circuits work:

Setting up:

• The process is started by pressing the pushbutton which activates the circuit.

Starting Point (Stable Point):

• With one transistor switched ON (conducting) and the other OFF (non-conducting) the circuit is stable before the pushbutton is pressed.
• Consider that Q1 (NPN transistor) is originally ON and Q2 (PNP transistor) is initially OFF.

Pressing the pushbutton:

• The condition of the circuit rapidly changes as the push button is pressed.

Change to an Unstable Situation:

• When the pushbutton is pressed the first conducting transistor Q1 in this case is turned off and the initially non conducting transistor Q2 is turned on.
• As a result of this change both transistors suddenly conduct in an unstable condition.

Capacitor Charging:

• As Q2 turns ON the capacitor connected to its collector begins to charge.
• The charging of the capacitor calculates 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.

The cycle is repeated:

• As long as the pushbutton remains pressed the circuit keeps rotating between the stable and unstable states.
• This cycle creates a sequence of pulses that are heard through the headphones or speaker that is included.

Audio frequency that can be adjusted:

• The audio frequency can be changed due to the circuits resistor R1.
• The frequency of the generated audio signal is modified by changing the charging and discharging periods of the capacitor by adjusting the resistance of R1.

Using Earphones Optionally:

• The circuit continues to function similarly when earbuds with an impedance of about 500 ohm are being used generating audio output through the earphones.

Formulas and Calculations:

Following are the formulas with calculations for Simple Beeper Circuit:

One can develop a basic astable multivibrator circuit to make a beeper circuit.

When the push button switch is pressed this circuit creates a square wave signal to drive the speaker and release a beeping sound.

Calculating Frequency:

For a simple astable multivibrator the oscillations frequency f is 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.

Enter your inputs and use this formula to calculate the frequency of the beeping noises.

In our diagram: R1 = 1.5k, R2 = 4.7k and 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:

To build a Simple Beeper Circuit follow the following steps for connection of the circuit:

Identify Components:

Assemble all necessary parts and confirm that they work with the circuits requirements.

Positioning of Transistors:

• As shown in the circuit diagram connect the selected silicon NPN transistor Q1 to the circuit.
• Also install the chosen PNP transistor Q2 in the allotted spot.

Connect Speaker :

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

Resistor R1 Connection:

• Add resistor R1 to the circuit so that audio frequency gets changed.
• As you connect it take note of its function in fine tuning the output and the circuit design.

Installing a Pushbutton:

• Make sure the connection is stable before placing the pushbutton for starting the circuits and for easy and clear control.

Connecting the battery:

• Using the given drain current from Q2 select the correct battery and connect it to power the circuit.

Conclusion:

By carefully assembling these components one can create a flexible beeper circuit that provides an interesting and adaptable audio output experience with an adjustable audio frequency range.

The circuit generates audible signals using the concepts of an asymmetric multivibrator and the addition of a pushbutton and resistor R1 provides the audio tone with more usefulness and flexibility.

References:

Buzzer or Beeper