Yesterday, I wrote briefly on a circuit which allows one to send or communicate audio using light as the communication channel. In that circuit only the NE555 Timer with the sine wave signal as input audio was used for purpose of testing. Here, I wanted to demonstrate the circuit with actual audio input from electret microphone.
Below is the transmitter circuit.
The circuit shown is a Pulse Frequency Modulation (PFM) Light Transmitter. It converts audio picked up by a microphone into high-frequency light pulses from an LED, which can be transmitted through the air to a receiver.
Following the routing logic, here is the breakdown of the three main functional stages:
1. Audio Pre-Amplifier Stage
This stage prepares the tiny signal from the microphone for the modulator.
Microphone Interface: The electret microphone (MIC1) receives DC bias through R6. The audio signal is then passed through C4, which acts as a coupling capacitor to block DC.
The TL072 Op-Amp: It is configured as an inverting amplifier. The non-inverting input (Pin 3) is connected to a "Virtual Ground" created by R3 and R4, which holds the input at 4.5V.
Gain Control: The gain is determined by the ratio of R5+RV2 to the input resistance. Adjusting RV2 allows you to control the sensitivity of the microphone.
2. Pulse Frequency Modulator (PFM)
The heart of the circuit is the NE555 timer operating in astable mode.
Carrier Generation: The components RV1, R1, and C1 determine the base frequency of the pulses. Based on your previous adjustments, this is set to approximately 25kHz—well above the human hearing range.
Modulation: The amplified audio signal from the TL072 enters Pin 5 (Control Voltage) via C3. This signal shifts the 555's internal voltage thresholds up and down. As the audio voltage changes, the time it takes for C1 to charge and discharge changes accordingly, effectively "wiggling" the output frequency to match the audio.
3. Optical Output Stage
LED Driver: The LED (D1) is connected to Pin 3 (Output). In this "sink" configuration, the LED turns on whenever the output goes low.
Current Protection: R2 (270$\Omega$) limits the current through the LED to prevent damage.
Power Stability: C2 (100$\mu$F) acts as a reservoir to smooth out power fluctuations caused by the rapid $25\text{kHz}$ switching.
The following shows the audio signal from microphone, the output of the audio pre-amplifier and the LED output signal waveforms.
As you can see the light signal from the LED is modulated by the audio signal.
Frequency of the carrier signal is calculated as follows:
Calculated Frequency
Using the standard NE555 astable frequency formula with the values provided in your schematic (C1 = 1nF, R1 = 1k$\Omega$, and RV1 $\approx$ 50k$\Omega$):
This calculator confirms that the calculated frequency of 555 timer is 27.5KHz.
The graph shows that modulation of the carrier signal by the audio signal is happening.
This diagram shows a sophisticated Multi-Stage Audio Receiver and Amplifier circuit, designed for a laser or light-based audio transmission system (Li-Fi). It progresses from light detection to high-power audio output.
Here is the step-by-step breakdown:
1. Optical Detection (Input Stage)
At the far left, D2 (Photodiode) acts as the sensor. It is reverse-biased through R7. When light (like a modulated laser) hits it, it generates a tiny current. C5 then couples this signal into the next stage while blocking the DC bias set by R7.
2. Transimpedance & Pre-Amplification (Op-Amp Stage)
The TL072 (U3:A) is configured as a high-gain inverting amplifier. With a feedback resistor (R9) of 200M$\Omega$, it converts the tiny photodiode current into a usable voltage. This is followed by U3:B, which appears to be a Sallen-Key Low Pass Filter (using R10, R11, C6, and C11) to remove high-frequency noise and "clean up" the audio signal.
3. Voltage Divider BJT Amplifier (Intermediate Stage)
The signal is coupled through C10 into a 2N3904 (Q1) transistor.
Biasing: It uses Voltage Divider Bias (R13 and R12) for thermal stability.
Gain: R15 and the bypass capacitor C13 ensure high AC voltage gain.
Function: This stage boosts the filtered signal to a level high enough to drive the final power stage.
4. Volume Control & Power Output (Final Stage)
Volume: The RV4 (10k$\Omega$ Potentiometer) acts as a variable voltage divider, allowing you to manually adjust the volume sent to the speaker.
Power Amp: The LM386 (U4) is the power "muscle." It takes the high-impedance signal from the BJT and provides the current gain necessary to drive the $8\Omega$ speaker (LS1).
Load: The Load Resistance seen by the BJT stage here is effectively the 10k$\Omega$ value of the potentiometer (RV4) in parallel with the LM386 input.
