Often this is the case, where some bit or byte of data needs to be sent to a microcontroller without a direct cable connection. A simple way to achieve this is to use infrared receivers that are everywhere, such as Vishay's TSOP17xx or similar receivers, which are commonly used in infrared remote control devices such as televisions and video recorders. These devices are easy to implement because they do not require external components. These receivers typically use a 38 kHz pulse carrier with an amplifier, automatic gain control and demodulator.
The main problem with simple applications is to build a transmitter that requires a 38kHz start-stop oscillator, additional power, and modulation pulses in the millisecond and sub-millisecond range. It is difficult to control these elements with the PC operating system. On the other hand, the standard serial transfer rate of the PC serial port is 38400 bps, and a simple 38.4 kHz pulse data can be generated with a simple multiplier and two infrared LEDs (Figure 1). When transferring bytes in another O/I mode (HEX55), each HEX 55 byte generates a string of 18 pulses, plus the start and stop bits and consecutive bytes, which can generate longer pulse.
The receiver requires bursts ranging from 10 to 70 pulses with roughly equivalent pauses; these settings can easily be met. HEX0 bytes can be sent to generate a short interval, but each byte will transmit two pulses because of the relationship between the start bit and the stop bit. However, the receiver ignores these pulses. Stopping a transfer can generate a long pause. Occasionally, a longer interval must be inserted depending on the receiver used. Data transmission can be achieved with long and short bursts and appropriate protocols.
The circuit in Figure 1 together with the serial port output impedance and capacitance form a high-pass filter. A positive pulse drives an infrared LED; a negative pulse drives the other. Both should point to the receiver. The PC port typically provides a maximum current of 5mA to 20mA and a voltage of ±15V, so the output resistance is in the range of thousands of ohms. Current limiting resistors are generally not required. In most cases, the capacitance is 1nF~10nF. The receiver is lenient. If it is a non-PC port, you need to adjust the capacitance value, such as a microcontroller, because its impedance is low. In practical applications, if the LED is aimed at the receiver, the transmission distance of 2 meters to 4 meters can be achieved with the peak current of the LED as low as 5 mA. An example program for the PC can be provided.
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