Parts List 

Arduino Uno (or compatible board)
Servo, high-torque 25kg-cm
Power Supply (5V, 4A)
Jumper wires
Screw terminals (x2)
2 potentiometers (10K)
20mm PVC pipe, 1m (cut into x6 235mm sections)
M3 nuts and bolts ( lengths: 10mm, 15mm)
M4 nuts and bolts (lengths: 20mm)

Arduino Code (for quick copying/pasting) 

/* Ventilator Control
 *  Uses Sweep as starting code
 *  by BARRAGAN
 *  This example code is in the public domain.
 *  
 *  modified 8 Nov 2013
 *  by Scott Fitzgerald
 *  http://www.arduino.cc/en/Tutorial/Sweep
 *  
 *  modified 25 Apr 2020
 *  by Wayne Smythe
 *  
 *  modified 1 Jun 2020 
 *  by Brian Berletic
 *  servo pauses between delays in the resting position rather than compressing the bag
 *  
 *  Requirements: 2 adjusting knobs
 *  Air Volume Selection: 25%, 33%, 50%, 75%, 100% - determined by servo end position
 *  Rate of Compression: 6, 10, 14, 18, 24 cycles per minute (determined by delay)
 *  This requires 2 variable resistors connected to 5V and GND and to ANALOG IN pins 0 and 1 - I used 10K

 */
#include <Servo.h>
#define DEBUG 1
Servo myservo; // create servo object to control a servo

int pos = -70; // default -70: initialize the servo start position
int RateDelay = 2000; // default 2000: The delay used between strokes
int StopPosition = 150; //default 150: The stop position of the servo determining the volume of air 

int potpinVol = 0; // analog pin used to connect the 10K potentiometer for volume
int potpinRate = 1; // analog pin used to connect the 10K potentiometer for rate

int val0; //variable to read the value from the analog pin
int val1; // variable to read the value from the analog pin

void setup() {

  #ifdef DEBUG
    Serial.begin(9600); // BaudRate for debug
  #endif
  myservo.attach(9); // attaches the servo on pin 9 to the servo object
}

void loop(){

/* Number of Compressions per Minute
 *  rate of compression: 6, 10, 14, 18, 24, cycles per minute
 *  read the position of the Rate knob to determine the rate
*/
  
  val1 = analogRead(potpinRate); // reads the value of the potentiometer
  val1 = map(val1, 0, 1023, 0, 100);
    #ifdef DEBUG
    Serial.print("\t Delay map value = ");
    Serial.print(val1);
  #endif

  if (val1>=0 && val1<=20)
  {
    RateDelay = 10000; // default 10000
  }
  else if(20<val1 && val1<=40)
  {
    RateDelay = 6000; // default 6000
  }
  else if(40<val1 && val1<=60)
  {
    RateDelay = 4286; // default 4286
  }
  else if(60<val1 && val1<=80)
  {
    RateDelay = 1500; // default 3333
  }
  else
  {
    RateDelay = 1000; // default 2500
  }
  #ifdef DEBUG
    Serial.print("\n RateDelay = ");
    Serial.print(RateDelay);
  #endif
  
  val0 = analogRead(potpinVol);
  #ifdef DEBUG
    Serial.print("\t pot value = ");
    Serial.print(val0);
  #endif

  val0 = map(val0, 0, 1023, 0, 180);
  #ifdef DEBUG
    Serial.print("\t map value = ");
    Serial.print(val0);
  #endif

  if (val0>=0 && val0<=36)
  {
    StopPosition = 30;
  }
  else if(36<val0 && val0<=72)
  {
    StopPosition = 60;
  }
  else if(72<val0 && val0<=108)
  {
    StopPosition = 90;
  }
  else if(108<val0 && val0<=144)
  {
    StopPosition = 120;
  }
  else
  {
    StopPosition = 170;
  }
  #ifdef DEBUG
    Serial.print("\n Stop Position = ");
    Serial.print(StopPosition);
  #endif

  for (pos = -70; pos <= StopPosition; pos += 1) { // goes from 0 to 180 degrees
    myservo.write(pos);...