Simple Squelch Circuit

Enter the squelch circuit, your personal radio peacekeeper!

This clever little circuit automatically mutes the radio when there is no strong signal coming in.

So, instead of hearing annoying static you only hear what you want clear audio or video.

The best part is squelch circuits are super simple and work with tons of different radios, keeping them quiet when they should be and letting the fun stuff through.

Circuit Working:

Parts List:

The input signal, originating from the receivers RAA circuit is modulated by the R1-R2-P1 network.

The signal from potentiometer P1 is tapped at the input of operational amplifier A1 configured as a comparator.

A voltage reference of 200mV is applied to the non inverting input via the R9 and R10 voltage divider.

The output signal from A1 is fed into a schmitt trigger circuit, A2 through a low pass filter comprising capacitor C2 and resistor R2.

This filtering is crucial to ensure that weak signals such as noise or interference do not disrupt the proper functioning of the squelch circuit.

Capacitor C3 smooths the signal transition at the output of A2 enhancing the perceptual quality of the amplification control.

Subsequently, the output signal of A2 is directed to the base of transistor T1 via the R7-R8 voltage divider.

The open collector output of the squelch circuit can be utilized to suppress the audio frequency signal output of the receiver.

With a low current consumption of less than 10 mA, this circuit can be seamlessly integrated into nearly any receiver provided the power source can accommodate this level of consumption.

Formula:

The main method of squelch circuit employing an op amp is to compare the intensity of the incoming signal with a reference value.

Op Amp Comparator:

The difference between the incoming signal (at the inverting input) and a reference voltage (at the non inverting input) is amplified by an op amp set up as a comparator.

Formula for Op Amp Output (Comparator):

Vo = A * (V(+) – V(-))

here,

• Vo is the output voltage
• A is the op amp gain (very high for comparator configuration)
• V(+) is the voltage at non inverting input (reference voltage)
• V(-) is the voltage at inverting input (incoming signal)

Control Threshold:

A voltage divider network may be used to change the reference voltage, which gives you control over the signal intensity needed to activate the squelch.

Optional Schmitt Trigger:

To provide hysteresis, a Schmitt trigger circuit might be added after the comparator.

By doing this, you may prevent abrupt switching with loud signals by setting a threshold for turning on and off the squelch.

Stop Activation:

When the signal drops below the threshold, a switching device (such as a transistor or logic gate) controlled by the comparators output mutes the audio output.

How to Build:

To build a simple Squelch Circuit following are the steps:

Prepare the components:

• Gather all the necessary components and ensure they are in working condition.

Circuit layout:

• Design the layout of your circuit.
• Decide where each component will be placed and how they will be interconnected.
• You can refer to the provided schematic or create your own based on the description.

Connect the operational amplifiers:

• Place the operational amplifiers onto your breadboard or PCB.
• Connect their power supply pins V+ and V- to the appropriate voltage levels.
• Follow the schematic to connect the input and output pins as described.

Connect resistors, capacitors, and potentiometer:

• Connect the resistors, capacitors, and potentiometer according to the schematic.
• Pay close attention to their values and orientations.

Connect the transistor T1:

• Connect the transistor according to the schematic ensuring correct orientation and connections to the other components.

Power supply:

• Connect the power supply to the circuit, ensuring correct polarity and voltage levels.

Test the circuit:

• Before finalizing the assembly test the circuit to ensure it functions as expected.
• Check the behavior of the squelch circuit by applying input signals of varying strengths and observing the output.

Finalize the assembly:

• Once the circuit has been tested successfully finalize the assembly by securing components in place and tidying up any loose connections.

Optional:

• Depending on your application, you may choose to enclose the circuit in a protective case or integrate it into a larger project.

Note:

• Remember to double check all connections and component placements to avoid any errors or short circuits.
• If you encounter any issues during testing troubleshoot the circuit by verifying connections and component values.

Conclusion:

In conclusion, the squelch circuit described offers a practical solution for muting the audio output of a receiver in the absence of a strong input signal.

By leveraging operational amplifiers, resistors, capacitors and a transistor, this circuit can be constructed with relative simplicity.

With careful assembly and testing, it can be seamlessly integrated into various radio receivers enhancing their functionality and usability.

References:

Squelch

TLC272 Series Precision Amplifiers Datasheets