1st attempt: At the beginning, I thought the pipette dispenser aspirates/ejects the medium inside the pipette by changing the pump moving direction inside the pipette dispenser. If I added a PWM circuit into the pump control circuit to control the pump moving direction (for example, when the PWM signal is high, the pump rotates to one direction, and when the PWM signal is low, the pump rotates to another direction), it might solve the problem. Unfortunately, the pipette dispenser does not work that way. After I opened the pipette dispenser, I found that the pump inside the pipette dispenser always moves to one direction, and the aspirating/ejecting is done by mechanically pressing the valves to change the air flow direction inside the tube. I have to design something to press the up/down button mechanically.
2nd attempt: I decided to use the available parts inside my lab to design a PWM circuit to control a servo to press the up/down button. The position holding time of the servo (the duration of pressing the up/down button) should be adjustable, and the rotation position of the servo should be repeatable. The circuit I designed contains two main parts: a PWM circuit to control the servo position and another PWM circuit to control the holding time.