This is a digitally controllable electronic potentiometer. The heart of the circuit is a 16 channel analog multiplexer. Controlled by the BCD value at the inputs, the output pin of the multiplexer (pin 1) is connected to any one of the 16 outputs.
The output pins are connected with 1k resistors. The IC2 can therefore be considered as a linear potentiometer with 16 steps. The total resistance of the potentiometer is 15k. Other values or characteristics like positive logarithmic can also be achieved by using different resistor values.The setting of the potentiometer is controlled by the counter IC 4516.
The position of S1 determines whether the resistance increases or decreases every time S2 is pressed. u1 AND u2 function as anti-keybounce.A logic jump from 000 to 111 or vice-versa is not possible because once the maximum or minimum count is reached, the CO pin (pin 7) of IC2 blocks further pulse entry.
Pin CO is logic 0 when both the counter’s setting and pin U/D are logic 0. Once pin U/D is logic 1 and the counter reaches count 15, the output of pin CO reverses to logic 1, and blocks U3. This way, further pulse is prevented since the only way to go further is to count down. The current consumption is less than 1mA.