Four-Channel Video and Audio Sequencer

The Four-Channel Video and Audio Sequencer is an electronic circuit designed to automatically cycle through four separate input feeds one after another. At the heart of the timing system is the NE555 timer integrated circuit (IC1), which operates as an astable multivibrator to produce a continuous stream of clock pulses. You can adjust the speed of these pulses using the potentiometer (VR1) and capacitor (C3). These clock pulses are then fed into the 4017B decade counter integrated circuit (IC2). As the 4017B receives each pulse, it sequentially activates its output pins one by one in a repeating loop. Each of the counter's first four active outputs connects to a BC547 NPN transistor (T1 through T4). Because the tiny current from the counter IC is not enough to drive mechanical relays directly, these transistors act as electronic switches that amplify the signal. When a transistor turns on, it energizes its corresponding DPDT relay (RL1 through RL4). Inside each relay, internal electromagnetic coils pull physical switches to toggle the connection of the video and audio lines. Diodes (D1) are included to protect the circuit from voltage spikes caused by the relay coils when they turn off, and LEDs provide a visual indicator showing which channel is currently active. The entire system is powered through a switch (S1) and assembled on a custom PCB.