This is a simple digital power supply circuit diagram to obtain variable DC voltage from 1.25V to 15.19V in reasonably small steps. The steps of up and down voltage can be down by press the up and down switch, and it use LED indicator to show the level of the output voltage. You can use DC input voltage between 20V and 35V. You must prepare a DC power supply before built this circuit.
The first section of the circuit comprises a digital up-down counter built around IC1— a quad 2-input NAND schmitt trigger (4093), followed by IC2— a binary up-down counter (4029). Two gates of IC 4093 are used to generate up-down logic using push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (4029). 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 sequential binary 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 realise a digitally variable resistor. This section consists of four N/O reed relays that need just about 5mA current for their operation.
More about this power supply circuit explanation and to show the diagram of voltage indicator with LED, please download this document: 1-15v 15-Step Digital Power Supply – powersupply33.com