Simple Ultrasonic Security Alarm Circuit

An Ultrasonic Security Alarm Circuit acts like an invisible force field for your home.

It uses high frequency sound waves, like super squeaks that humans cannot hear to detect movement.

Imagine the circuit sending out these sound waves and then listening for them to bounce back.

If something walks through the area and disrupts the waves the circuit knows someone might be there and triggers an alarm.

This type of circuit uses a transmitter to send out the sound waves and a receiver to listen for their return.

Ultrasonic alarms are a great way to add an extra layer of security to your space.

Circuit Working of Ultrasonic Transmitter:

Parts List:

This circuit creates inaudible ultrasonic to create a barrier at the entrance.

When the circuit detects a breach in the sound barrier it triggers a loud alarm indicating someone has entered.

This setup can secure entry points and detect unauthorized access to protected areas.

The circuit consists of two main sections: an ultrasonic transmitter and a receiver.

Both sections use a 40kHz ultrasonic transmitter TX and receiver RX pair for transmitting and receiving ultrasonic signals.

The transmitter section utilizes the ultrasonic transmitter TX with an emitter coupled oscillator consisting of T1 and T2 both BC547 transistors.

The values of resistors and capacitors determine the oscillation frequency at 40kHz, matching that of the transmitter.

Formula:

Positive feedback is usually used in a two transistor feedback oscillator circuit from the above diagram to maintain oscillations.

The following formula may be used to approximate the frequency of oscillation f in this simplified configuration:

f = ​1​ / 2π√ C * R

where,

• C is the capacitance in the feedback network.
• R is the resistance in the feedback network.

Note:

Above formula offers a simple method for figuring out how often an oscillator circuit with two transistors in capacitive feedback oscillates.

Within its design range, the oscillator may be tuned to function at different frequencies by varying the values of the capacitor and resistor.

How to Build:

To build a simple ultrasonic Transmitter circuit following are the steps:

Transmitter Section:

• Connect the ultrasonic transmitter TX to the emitter coupled oscillator circuit T1 and T2.
• Use resistors and capacitors to set the oscillation frequency to 40 kHz.

Receiver Section:

• Connect the ultrasonic receiver RX to a circuit that can detect interruptions in the received signal.
• This can be a simple comparator circuit.

Alarm Section:

• Connect an alarm device e.g. buzzer to the receiver circuit so that it triggers when the received signal is interrupted.

Power Supply:

• Provide power to the circuit using an appropriate power source e.g. batteries or a DC power supply.

Note:

• Additionally, ensure you have the necessary skills and safety precautions in place when working with electronics.

Circuit Working of Ultrasonic Receiver:

Parts List:

The receiver section features a 40kHz ultrasonic receiver RX connected to the base of T1.

It operates as a common emitter amplifier boosting the signals received by RX.

The amplified signal, available at the collector of T1 is then passed through C2 to the signal detection stage consisting of T2, D1 and C3.

When the RX continuously receives ultrasonic from the transmitter TX, T3 remains saturated and conducts.

This condition maintains the conduction of T4, as its base is held at ground potential.

However, when the ultrasonic beam is interrupted, T3 turns off, causing T4 to conduct.

This action triggers the piezo buzzer connected to the collector of T4 sounding the alarm.

A red LED indicates the standby mode showing that the alarm system is active.

Range of the Circuit The effective range of the circuit is a few meters.

Proper alignment of the TX and RX on opposite sides of the entry is crucial.

This setup ensures that the alarm remains inactive when the RX receives ultrasonic signals from the TX.

Formula:

Transistors are often used in an ultrasonic sensor transistor amplifier circuit to increase the signal received from an ultrasonic sensor (such as an ultrasonic transmitter and receiver pair).

Below is a general formula for ultrasonic sensor transistor amplifier.

The following formula can be used to approximate the gain (𝐴𝑣) of the transistor amplifier circuit that amplifies the ultrasonic signal:

A_v​ = R_C​​ / r_e​

where,

• RC is the collector resistor.
• re​ is the intrinsic emitter resistance of the transistor.

An emitters inherent resistance, re may be found using the following formula:

r_e​ = V_T​​ / I_E​

where,

• VT​ is the thermal voltage (approximately 26 mV at room temperature).
• IE is the emitter current.

Note:

The foundation for creating an ultrasonic sensor transistor amplifier circuit is provided by this general formula and related considerations.

The precise specifications of your application, such as the required levels of signal amplification and operating frequency, will determine the values and modifications of each component.

How to Build:

To build a ultrasonic Receiver Circuit following the below mentioned steps:

Connect the RX to T1:

• Connect the 40kHz ultrasonic receiver RX to the base of transistor T1.

T1 as Amplifier:

• Configure T1 as a common emitter amplifier to amplify the signals received by RX.
• Use appropriate resistors in the base biasing circuit of T1.

Signal Detection Stage:

• Connect the amplified signal from the collector of T1 to the signal detection stage consisting of T2, D1 and C3.
• Ensure proper biasing and connections for T2, D1 and C3.

Alarm Triggering:

• Use transistor T3 to control the alarm triggering mechanism.
• When T3 is saturated due to continuous reception of ultrasonic, it keeps T4 conducting which keeps the alarm off.
• When the ultrasonic beam is interrupted, T3 turns off allowing T4 to conduct and trigger the piezo buzzer.

Indicator LED:

• Connect the red LED to indicate the standby mode showing that the alarm system is active.
• Use an appropriate resistor in series with the LED for current limiting.

Power Supply:

• Provide power to the circuit using an appropriate power source ensuring it meets the voltage requirements of the components.

Note:

• Ensure proper grounding and isolation between components to prevent interference and false alarms.
• Adjust component values and circuit layout as needed for optimal performance.

Conclusion:

An Ultrasonic Security Alarm Circuit is an effective and reliable way to detect movement and trigger an alarm in a monitored area.

By using ultrasonic waves, this type of alarm system can detect intruders or unauthorized access making it ideal for home and industrial security applications.

References:

Security System Based on Ultrasonic Sensor Technology