Audio Circuit DiagramsIR devices

Infra-Red Transmitter Schematic Circuit Diagram

Wireless TV Sound Transmission with Infra-red Transmitter

Infra-red transmitter: Often, there is a desire to watch television without the sound causing disturbance to others in the room. In order to eliminate the need for a lengthy cable trailing across the floor, the current transmitter (along with its accompanying receiver) facilitates wireless listening to the TV sound.

Pulse-Duration Modulation in the Transmitter

The transmitter employs pulse-duration modulation to achieve its functionality. The modulated signal is generated by juxtaposing the audio signal against a high-frequency triangular signal in a comparator, following the traditional method. A suitable triangular signal generator is available in Chapter 6. If an alternative generator is utilized, it is imperative to ensure that its offset aligns with half the supply voltage (5 V) and that its peak value reaches 2.5 Vpp.

Infra-red transmitter Schematic diagram

LED Current Control through PDM for Enhanced Range

To achieve an optimal range, a substantial current must flow through the LEDs. However, to prevent LEDs from drawing continuous high currents, the solution lies in keeping the pulses short. This is precisely why Pulse-Duration Modulation (PDM) is employed. In PDM, the occurrence time of the first and last transitions (leading and trailing edges) is adjusted from their unmodulated positions. The XOR gate IC1 generates the pulses by comparing the original PDM signal with one delayed by R5-C3-IC2. This process results in short pulses. Each with a width equal to the time constant R5-C3, at every level change or transition.

LED Current Restriction and Pulse Adjustment

The output signal from IC1 controls T1, restricting the current drawn by LED1 (with a 10 V supply) to a peak value of 400 mA, regulated by R6. The circuit’s average current consumption is 90 mA. Presets P2 and P3 play a crucial role in equalizing the pulses resulting from leading and trailing edges. In the absence of an input signal, P3 is set, using an oscilloscope, to ensure all pulses in the output signal have the same width. Subsequently, P2 is adjusted to ensure equal pulse spacing. This process should yield a perfect square wave at the output of IC1.

Optimizing Receiver Performance and Additional Functionality

When used in conjunction with the transmitter, the receiver’s P1 is adjusted for the least interference on the received signal with maximum input. Lastly, it’s worth noting that IC2a and IC2b can function as the triangular wave generators.

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