With the advent of digital television, it’s often necessary to use a separate receiver. If you have several television sets in your house, you have to buy a digital receiver (and accompanying subscription) for each set. The solution described here lets you watch television in two or more places in your home using a single digital receiver, while allowing the digital receiver to be controlled from both locations. The circuit needed for this is powered from one of the two television sets (see Figure 1). You’ll need a length of four-way shielded cable (such as Conrad Electronics # 606502) for the connection between the digital receiver and the second TV set. Two shielded conductors are used to transmit the audio signals (L and R) from the receiver to the second TV set, another one is used to transmit the video signal, and the last one is used to transmit the remote control signal from the remote control for the second TV set to the digital receiver located next to the first TV set. The infrared sensor of the second TV set receives the signal from the remote control unit for the digital receiver and sends it via a small circuit to an IR LED aimed at the infrared sensor of the digital receiver near the first TV set. With this arrangement, it’s convenient to buy a second (programmable) remote control unit so you don’t have to carry the original remote control unit of the digital receiver back and forth all the time. Most digital receivers have two SCART connectors for connecting a television set and a video recorder.
The second SCART connector can be used quite nicely for the signals to be sent to the second TV set (see the connection diagram in Figure 3). If this connector is already in use, you can always take the audio and video signals from the Cinch connectors (if present). The circuit necessary for converting the infrared signal received by the second TV set into a new signal for driving the infrared LED at the digital receiver location is shown in Figure 2). The infrared signal from the remote control unit consists of short pulse trains of modulated infrared light. The modulation frequency varies from one brand to the next and lies in the range of 30 to 56 kHz (B&O, different as always, uses 455 kHz). Frequencies in the 36–40 kHz range are most often used in practice. The modulation frequency of an infrared sensor is usually indicated in its type number. For example, the TSOP1736 responds to IR light modulated at 36 kHz, the TSOP1738 likes 38 kHz, and so on. Figure 4 shows a few IR receivers and their pinouts. Infrared sensors also have adequate sensitivity to other frequencies close to their design frequency. Consequently, we assume a modulation frequency of 38 kHz here, which covers the full range from 36 to 40 kHz. The IR receiver demodulates the infrared signal.
The demodulated signal forms the input to our circuit, which uses it to generate a new modulated signal for the IR LED located next to the digital receiver. The author opened up his second TV set (watch out for possible sources of high voltage inside the set!) in order to use the set’s built-in IR receiver and tap off power for the modulator circuit. However, you can also fit the circuit with its own IR receiver and use a separate power supply (AC power adapter).
The output signal of the IR receiver is used to trigger an astable multivibrator built around our old friend, a 555 timer IC. The data line of the IR sensor is High in the quiescent state and goes Low when it receives an modulated IR signal. As the Reset input of the 555 responds to an active-low signal, an inverter is built around T1, R2 and R3. The modulation frequency for IR LED D2 is set to approximately 38 kHz by P1, R1 and C1. Diode D1 allows the duty cycle of the output signal to be less than 50%, which cannot be achieved otherwise. The rise time of the oscillator signal on the Threshold input of the 555 is set by P1 and C1, while the fall time is set by R1 and C1. The ratio of P1 to R1 determines the duty cycle, which is approximately 30% in this case. With a 5-V supply voltage, P1 is set to 1 kΩ, but it must reduced to a lower value (around 500 Ω) with a lower supply voltage. If possible, use an oscilloscope to adjust the oscillator frequency to 38 kHz (period: 26.3 μs). To generate a test signal at the 555 output, temporarily connect the circuit input to ground. Place IR LED D2 in front of the digital receiver so it shines on the receiver’s IR sensor. Use the screen of the fourth shielded conductor of the cable between the receiver and TV2 for the negative lead of D2. Resistor R4 is dimensioned for a current of around 100 mA through the IR LED. If you use a 3.3-V supply voltage, R4 must be reduced to 3.3 Ω. You can also use this circuit for the remote control of audio or video equipment located inside a closed cabinet.