In the last tutorial designing Audio Induction Loop Communication System - Transmitter Design, I explained how to design transmitter circuit for audio communication using induction loop. In this part of the tutorial, I will explain how to design the receiver circuit for audio communication using induction loop.
Circuit Diagram
Below is the circuit diagram of the receiver circuit.
This circuit is a two-stage design: a high-gain NPN common-emitter transistor pre-amplifier feeding into an LM386 power amplifier.
Here is the exact, component-by-component explanation of how your circuit operates:
1. The Induction Input Stage
RX (Pick-up Coil with Ferrite Core): This is your sensor. The alternating magnetic field from the room's transmitter loop passes through this coil, inducing a small AC voltage across it via electromagnetic induction.
C1 (Input Coupling Capacitor): Passes the tiny AC audio signal from the coil straight to the base of $Q1$ while blocking any DC voltage so it doesn't disturb the transistor's bias.
2. The Transistor Pre-Amplifier Stage ($Q1$)
This stage handles the massive voltage boost required to lift the microvolt-level signal from the coil up to something the LM386 can comfortably work with.
Q1 (BC548 NPN Transistor): Configured as a Common-Emitter Amplifier, which provides both high voltage gain and current gain.
R7 (Collector-to-Base Feedback Resistor): This provides DC negative feedback to bias the transistor. It drops voltage from the collector down to the base, ensuring $Q1$ stays stable in its active linear region even if the temperature changes or the battery voltage drops.
R8 (Collector Load Resistor): Sets the collector current and converts the varying collector current into a scaled-up audio voltage signal.
R9 & RV2 (Emitter Resistors & Sensitivity Control): * R9 provides thermal stability for the emitter.
RV2 (Potentiometer) & C2 (Bypass Capacitor): This acts as an adjustable AC gain bypass network. By tweaking the variable resistor RV2, you change how much of the emitter signal is shorted to ground through C2 for AC audio. This serves as a sensitivity/gain control for the pre-amp stage.
3. High-Frequency Filtering
C9 (Collector-to-Ground Capacitor): This acts as a simple, effective low-pass filter right at the collector of $Q1$. It shunts ultra-high-frequency RF noise and harsh electromagnetic hiss straight to ground before it can be amplified further, cleaning up the audio.
4. Volume Control & Coupling
C10 (Interstage Coupling Capacitor): Blocks the DC voltage present at $Q1$'s collector from reaching the LM386 input, allowing only pure, filtered audio through.
RV3 (Volume Potentiometer): This acts as your main user volume control. The audio signal is applied across its full resistance, and the wiper arm taps off a adjustable portion of that signal to feed into Pin 3 of the power amplifier.
5. The Power Amplifier Stage ($U1$)
U1 (LM386 Audio Power Amplifier): Takes the filtered, pre-amplified signal from the volume pot and boosts its current to drive low-impedance headphones.
Pin Configuration:
Pin 2 (Inverting Input): Tied straight to ground.
Pin 3 (Non-Inverting Input): Receives the audio signal from the volume wiper.
Pin 6 (+9V) & Pin 4 (Gradients/Ground): Main power rails delivered from the $9\text{V}$ battery ($B1$) via the power switch ($S1$).
C5 (Pin 1 to Pin 8): This capacitor is placed across the internal gain setting pins. By bypassing the internal resistor with $C5$, the LM386's internal voltage gain is boosted from its default of 20 up to 200, providing maximum amplification for the headphones.
6. Output & Stability
C12 (Output Coupling Capacitor): Since the LM386 output pin (Pin 5) sits at half the supply voltage ($\approx 4.5\text{V}$ DC), this large electrolytic capacitor blocks that DC from damaging or heating up your headphones, passing only the final AC audio signal.
C13 & R10 (Zobel Network): Connected directly from the output pin to ground, this RC network stabilizes the LM386 against high-frequency oscillations that can occur due to the inductive characteristics of headphone wiring.
Headphone: The output port where you plug in standard low-impedance headphones to listen to the crystal-clear transmitted audio.
