Digital potentiometer replacement for an analog potentiometer

Question

Is there a way to interface a digital potentiometer with this circuit so that the actual potentiometer could be replaced with a dual NO push button control?

I have thought about this and it would be much easier to use one of these circuits to control the duty cycle of a PWM fan. I many years of experience working with potentiometers with wipers becoming unable to function over time, and that is why I would like to replace a knob with push buttons.

If there is a way to do this and someone knows where to find a diagram for this, please let me know.

This is a modification of the circuit in the question here: Control duty time and frequency with different potentiometers using one 555 timer.

EDIT:
I do not want to use any form of NAND gate whether it be CMOS or TTL. I am asking how you would directly add a digital potentiometer to a 555 timer as a replacement for R2 without using any other form of chip in the circuit except for a potentiometer IC and a 555 timer IC.

Adding any further chips would make that circuit more complex than it needs to be.

I need to be able to interface the left circuit to the right one in the image below.

Image adapted from AD5116 datasheet, figure 1 & Electronics Tutorials, 555 Timer Tutorials

As for any circuit that uses an MCU processor because that is getting a little more complex that what I need for what I am trying to do. That is to control some DC powered PWM fans for use in a paint box for airbrushing. As for the use of a potentiometer, I have experience with those and over time they either deteriorate from use or get crud in them that will bind them. In a paint booth for airbrushing the paint from an airbrush is that crud that I need to avoid getting inside of any standard potentiometer.

Answer 1

An AD5220 can do it with a little logic. Something like this:-

^{simulate this circuit – Schematic created using CircuitLab}

The CD4093B contains four NAND gates with Schmitt trigger inputs. In this circuit we only use two of them. The top NAND gate is configured as a clock generator running at ~10 Hz (frequency determined by C1 and R1), controlled by the lower input which must be high for oscillation.

The bottom NAND gate output goes high if either push-button is pressed. If the 'down' button is pressed it also pulls the AD5220 U/D pin low to count down (otherwise it counts up).

One limitation of this design is that the AD5220 always starts in the center on power up. Non-volatile digital pots with EEPROM memory are available, but all the ones I looked at used an I²C or SPI interface. If the pot needs to retain its setting when powered off it might be better to use a small MCU to generate the PWM signal directly, eliminating the need for the digital pot and 555 timer as well as several other components.

Answer 2

Something like this should work, where R3 represents the AD5116 80kΩ version.

It's necessary to regulate the 12V down to 5 because the digital pot cannot handle more than about a 5V supply. If your fan is 5V it is not necessary.

A CMOS version of the 555 would probably be better.

M1 should be a logic level MOSFET with switching characteristics specified for 4.5V Vgs and adequate rating for the fan.

With the given components, the PWM frequency is about 20kHz. Adjust C2 for higher or lower frequency.

^{simulate this circuit – Schematic created using CircuitLab}