Each IC has four CMOS bilateral switches for transmission or multiplexing of analogue or digital signals. It is pin-to-pin compatible with IC CD4016, but has much lower ‘on’ resistance, which is relatively constant over the input signal range.

Video and audio signals from SK1 (camera 1) are connected to S2a and S2b, respectively, and both enable pins (to switch on both the switches) are connected to port RC7 (pin 18) of the microcontroller. Similarly, audio and video signals from SK2 to SK6 are connected to IC2 through IC4 and then to port C of IC1 as shown in Table 1.

Enable pins of switch pairs S2a-S2b, S3a-S3b, S4a-S4b, S5a-S5b, S6a-S6b and S7a-S7b of IC2 through IC4 are shorted to each other and fed to the corresponding port of Port C as shown in Table I. The microcontroller provides high logic at Port-C pins to switch on a particular camera. The truth table for Port C is shown in Table II.

Immediately after switching on the mains, the standby LED (LED7) connected to Port-C pin RC3 glows and the LCD shows the welcome page (refer to the LCD screenshot).

On pressing the power key on the remote, the standby LED turns off and the LCD shows which camera is ‘on,’ in which mode it is now and default hold time for the corresponding camera in ‘Auto’ mode.

As described above, pins of Port C (RC0, RC1, RC4-RC7) are connected to IC2 through IC4 for selecting the camera to be switched on. LED1 through LED6 are used to check the logic status of Port C. Pressing keys ‘1’ through ‘6’ on the remote makes the corresponding LEDs (LED1 through LED6) glow,indicating ‘on’ condition of the cameras (CAM-1 through CAM-6).

Fig. 3: Screenshots of LCD

Here a 16-character, 2-line LCD is used in 4-bit mode. Data lines D4 through D7 of the LCD are connected to Port-B pins RB4 through RB7. Enable (EN) and reset (RS) pins are connected to RB2 and RB3 pins, respectively, and the read/write (R/W) pin is connected to ground as we have used the LCD in writing mode only. Preset VR1 (10-kilo-ohm) is used for setting the LCD character contrast.

Power supply

A simple circuit for regulated supply (shown in Fig.4) is used here. The +12V regulated output is used to power all the six cameras, IC2 through IC4 and IC5 and fed to another 7805 voltage regulator (IC7) to get the regulated +5V output for the microcontroller (IC1) and the LCD.

The current rating of the transformer should be around 2A, on the current consumption of the cameras. Use a good-quality heat-sink for both the regulators.

Fig. 4: Circuit for regulated power supply

Power amplifier

A simple power amplifier (see Fig.5) is used to amplify the sound signal from the cameras and hear it on a small speaker.

Fig.5: Circuit diagram of power amplifier

The audio power amplifier is built around IC UTC820 (IC5). UTC820 is a monolithic integrated circuit audio amplifier delivering an output of 1.2 watts at 9V on an 8-ohm speaker load with 10 per cent total harmonic distortion (THD) and good ripple rejection. It is designed for audio-frequency class-B amplifier with a wide operating supply voltage range of 3V to 14V and minimum external components.

The pin configuration of IC UTC820 is shown in Fig.6.2. Components at pin 2 of the IC5 decide closed-loop voltage gain (dB) of the amplifier. Here a 47-ohm resistor (R11) and a 22μF capacitor (C12) are used and, with these values, about 45dB closed-loop voltage gain is achieved. You can reduce the component values for more gain but it’s not required here as maximum gain of 75m dB is possible. Capacitor C10 (100μF) connected between pins 5 and 7 of IC5 is used for boot strap and improving the low-frequency signal. The amplifier output at pin 5 is connected to the speaker via C13. The audio signal from the cameras is fed to input pin 3 of the IC via volume control VR2 (10-kilo-ohm), capacitor C17 and resistor R13. Here the output power is limited to about 500 mW at 8-ohm load approximately, so use of a good-quality 6.4cm speaker is recommended.

Fig.6: Pin diagram of UTC820 audio power amplifier

Functions of remote control keys

The functions of various remote keys are shown in Fig.7.

Fig.7: Remote control of LG TV

Power on/off key. Press it once to switch on the switcher and press again to turn off or keep the switcher in standby mode and vice versa. (AC mains should be ‘on’ in both the cases.)



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