Turn a Normal Pipette Dispenser to an Automatic Repeater

The challenge:

Recently, my coworker asked me to design a kit that can transfer a normal pipette dispenser to an automatic repeater. In cell culturing, he needs to use a pipette dispenser to create a water jet in the cell suspension to break the cells into individual cell after harvesting the cells from the flask. He needed to manually pressing the up/down button of the pipette dispenser for more than 15 minutes. His hand was super tired after doing that, and he wanted to me to make a kit for him to make his life easier.

Step 1: The Solution:

1^st 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.

2^nd 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.

Step 2: Circuit Description:

BOM:

Quantity	Description	RefDes
1	12V DC POWER_SOURCES
4	CAP_ELECTROLIT, 220uF	C6, C9, C1, C4
2	CAP_ELECTROLIT, 470uF	C3, C2
2	CAPACITOR, 100nF	C5, C7
1	CAP_ELECTROLIT, 100uF	C8
2	DIODES_1N4001	D1, D2
2	VCO IC	XR2209, XR2209_2
1	POTENTIOMETER, 100kΩ	R2
2	Trimmer, 10kΩ	R6, R15
1	Trimmer, 2kΩ	R7
1	RESISTOR, 100kΩ	R9
6	RESISTOR, 10kΩ	R5, R4, R3, R1, R16, R18
6	RESISTOR, 1kΩ	R13, R14, R11, R10, R17, R12
1	RESISTOR, 200Ω	R8
1	SG90 Servo	Servo
1	VOLTAGE_REGULATOR, LM317LM	U2
1	VOLTAGE_REGULATOR, LM7805CT	U3
1	OPAMP, LT1014CN	U1
1	MOS_3TEN, 2N7000	Q1
1	5V relay	X1

Others: a small project box, hookup wires, PCB, some standoffs and screws, a piece of plastic board, a small piece of ¾ aluminum angle.

Power supply:

I use D1, U2, C5, C6, R7, and R8 together to get 9V DC from 12V DC to power the circuit. For servo I use LM7805 to get 5V DC. C8, R18, Q1 and relay X1 together is a time delay circuit. The delayed time is about 3-4 seconds. The purpose of the delay circuit is to let the PWM circuit to settle down first before turning on the servo.

Servo position PWM circuit:

XR2209_2, C7, and R9 together is a triangle wave generator. F=1/C7R9 =100Hz. I use opamp LT1014 as comparator. By comparing the voltage on Pin 9 and Pin 10, Pin8 will generate a square wave. The frequency is the same as the triangle wave, and the duty cycle is set by the Pin9 voltage.

Servo holding time PWM circuit:

XR2209, C1, C2, C3, R1, R2, R3, R4, and R5 together is a square wave generator. The frequency is set by C2, C3, R1, and R2. U1B, R11, R12, R13, and R14 together is a summing amplifier. It sums the square wave signal from R6 and the reference voltage from R15.

Circuit Adjustment:

To adjust the neutral position of the servo, set the voltage on U1A Pin3 to zero by adjusting R6, and then adjust R15. When the U1A Pin3 voltage is zero, the output on Pin7 of U1B is a constant DC voltage, and the output of U1C is a 100Hz square wave with a fixed duty cycle set by R15. Slowly adjust R6 to adjust the servo position. The position holding time is set by R2.

Mechanical setup:

I use an BioRad cell counter counting slide packing box as the circuit enclosure, and it works perfectly. For the mounting board I use a scrap FR4 board. I use three standoffs to position the pipette dispenser and two standoffs to position the enclosure, and then I cut two small pieces of aluminum sheet to mount the pipette dispenser and enclosure. I also add a small piece of aluminum angle to the FR4 board so the whole kit can be clamped on a stand.