Day: July 14, 2014

Categories
Projects

New PCBs, New Problems

The new PCBs arrived and I soldered everything on, testing as I went. I used an Uno to bit-bang the Duemilanove bootloader to the Atmega328P. ICSP programmers are pretty cheap on eBay, so I ordered one for the future. The headers below the Atmega are for a USB FDTI adapter for uploading sketches. Everything worked great on the bench with no solenoids connected– you can see where this is going.

Once I installed the board It would cycle through one or two steps of switching on/off solenoids and reset. I pulled it out and gave the board a good looking over and found a possible problem. I left some points for the extra digital pins (which was down to 1 after I realized my mistake mentioned in my last post) in case I wanted them for something in the future. When I fixed the last problem I, for some reason, decided to also connect the 5V point, from a trace that passed the VCC supply pin on the Atmega to a trace supplying the relay coil power. I cut that extra trace and it now worked better, but only for a minute or two.

The only thing I could think of was some interference from the current in the 24v solenoid circuit. I now know that interference to be called di/dt (dee eye by dee tee). When the solenoids shut off the inductance of the 24V wire creates high voltage(radio wave) that jumps to the relay coil (antenna). That spike doesn’t do good things to microprocessors.

In my sketch I had 4 of the 8 relay pins randomly selected and turned off, then four randomly selected of the 8 again. So sometimes 4 relays could be shutting off at the same time. I changed the code to select one relay that was engaged and swap it with one that was off. It actually cycled through the entire program… sometimes.

The Darlington array (ULN2803) has internal diodes, but maybe they weren’t good enough? I soldered a 1N4803 Diodes between the coil pins on each relay. The polarity is backward of the current on the board, otherwise, it would short the DC– which would be bad. But it does still short the high voltage AC from the di/dt– which is good. Success! Working as planned. It’s been running for 2 days with no issue… so far. I’ll see how it works for a while before I bother creating a new PCB design including the new diodes. Even with a third (and let’s just assume I’ll need a 4th) round of PCBs, with the components for 3 boards (2 backups), I’ll be under $200. With a price tag of $2K on the replacement computer that I had no confidence in and would still be waiting to arrive, I feel pretty good about this. I’m not a programmer and the Arduino IDE is my only exposure so far to C++, but here is the sketch that I painfully got to work.

int stepLength = 6000;//6 sec steps
int steps = 50;//50 step
//6sec * 50 = 5min
int waitState = 1;


void setup(){
  Serial.begin(9600);
  pinMode(2, OUTPUT);
  pinMode(4, INPUT_PULLUP);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  
}

void loop() {
  digitalWrite( 2, LOW);//turn off motor
  digitalWrite( 4, HIGH);//HIGH is inactive for internal pullup
  int pin = 5;
  while(pin <= 13){//turn off all solenoids and run led
   digitalWrite( pin, LOW);
   delay(100);
   pin++;
  }
  waiting(); 
}

void waiting(){
 if(waitState == 1){ 
   Serial.println("Waiting for Input...");
   waitState = 0;
 }
 if(digitalRead(4) == LOW){//wait for input
   beginSeq();
 }
}

void beginSeq(){
 digitalWrite( 4, HIGH);//HIGH is inactive
 digitalWrite(13, HIGH);//turn on run led
 startFour();//open 4 solenoids
 digitalWrite(2, HIGH);//start motor
 delay(stepLength);
 int stepNum = 2;
 while(stepNum <= steps){
   Serial.print('\n');
   Serial.print("Step ");
   Serial.print(stepNum);
   Serial.print(": ");
   swapFeature();
   delay(stepLength);
   stepNum++;
 }
 Serial.println("");
 digitalWrite(2, LOW);//stop motor
 digitalWrite(4, HIGH);//deactiveate input pin
 delay(5000);//wait 5 seconds before all the solenoids slam shut
 waitState = 1;
}

void startFour(){
 Serial.print("Step 1: ");
 int pin = 5;
 while(pin <= 12){
   digitalWrite( pin, LOW);
   pin++;
 }
 int i = 0;
 while(i  ");
 Serial.print(turnOn);
 digitalWrite(turnOn , HIGH);
 digitalWrite(turnOff , LOW);
}

int turnOffPin(){
  while(true){
   int pin = random(5,12);
   if(digitalRead(pin) == HIGH){//check that pin is on
    return pin;
   }
  }
}

int turnOnPin(){
  while(true){
   int pin = random(5,12);
   if(digitalRead(pin) == LOW){//check that pin is off
    return pin;
   }
  }
}