The indicator described here is specifically designed for adjusting the dynamic limiter described elsewhere in this edition and checking whether the maximum level of the reference voltage (P1) needs to be modified. Here we use a 4-to-16 decoder IC (type 4514) to monitor the state of the four-bit up/down counter in the limiter circuit. This IC can be powered from the ±8 V supply voltages of the limiter. The limiter board has a 6-way connector (K5) that provides access to the four counter outputs and the supply voltages. Connector K1 of the indicator circuit can be connected to K5 on the limiter board.
One output of the 4514 goes high for each unique 4-bit combination on its inputs, while the other outputs remain logic Low. A separate current-limiting resistor is connected in series with each LED. It was not possible to use a common cathode resistor here because most LEDs have a maximum reverse blocking the voltage of only 5 V, while the supply voltage here (16 V) is a good deal higher.
The 16 LEDs arranged in a row provide a ‘fluid’ indication of the control process. You can enhance the display by using different colors for the first and last LEDs, such as red for D1 (maximum gain) and green for D16 (minimum gain), with yellow for the rest of the LEDs. While observing signals from various sources (TV set, DVD, media player, etc.), you can easily use the 16 LEDs to monitor the behavior of the limiter and adjust the setting of potentiometer P1 in the limiter circuit. It must be set such that D16 only lights up at the maximum signal level. If this is not possible and D16 remains lit a good deal of the time regardless of the position of P1, it will be necessary to increase the value of P1. Of course, it is also possible to adjust P1 so the strongest signal source extends slightly above the control range of the limiter.