This is a series regulator with variable output from the digital 1.25V to 15.19V The first section of the circuit comprises a digital up-down counter built around IC1, a quad 2-input NAND Schmitt trigger (IC CD4093), followed by IC2, a binary up-down counter (CD4029 IC). Two gates of IC CD4093 are used to generate the up-down logic using the push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (CD4029).
The frequency of oscillations can be varied by changing the value of capacitor C1 or preset VR1. IC2 receives clock pulses from the oscillator and produces a binary sequential output. As long as its pin 5 is low, the counter continues to count at the rising edge of each clock pulse, but stops counting as soon as its pin 5 is brought to logic 1. Logic 1 at pin 10 makes the counter to count upwards, while logic 0 makes it count downwards. Therefore the counter counts up by closing switch S1 and counts down by closing switch S2.
The output of counter IC2 is used to realize a Digitally variable resistor. This section consists of four N / O reed Relays that need just about 5mA current for their operation. (The original circuit containing quad bilateral switch IC 4066 has been replaced by reed Relays switches operated by transistorised because of unreliable operation of the former.)
The switching action is performed using BC548 transistors. External resistors are connected in parallel with the reed relay contacts. Particular if the relay contacts are opened by the control input at the base of a transistor, the correspond-ing resistor across the relay contacts gets connected to the circuit. The table shows the theoretical output for various combinations of digital inputs.
The measured output is nearly equal to the theoretically calculated IC3 outputs across the regulator (LM317).output of this regulator minimum (1.25V). As count-up switch S1 is pressed, the binary count of IC2 increases and the output starts increasing too. At the highest count output of 1111, the output voltage is 15.19V (assuming the in-circuit resistance of the preset VR2 as zero). Preset VR2 can be used for trimming the output voltage as desired. To decrease the output voltage within the range of 1.25V to 15.2V, count-down switch S2 is to be depressed.
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