Coilgun Dev Diary: Charging Circuit

Today, I built the charging circuit’s switch. My sorting cabinet is still on its way. For now, I have to deal with this mess, not ideal when I’m working on high voltage. I’m constantly on my nerves for fear of anything shorts.

I decide not to use a mechanical button so fewer things that can go wrong. The Arduino script here reads commands from serial monitor.

/*----------------------------------------------------------------------------
 * Serial‑controlled PWM on pin 7
 * --------------------------------------------------
 * Baud rate : 115 200
 * Commands  :
 *   c   → set pin HIGH  (LED on)
 *   c0  → set pin LOW   (LED off)
 *
 * Any other input triggers a warning.
 *---------------------------------------------------------------------------*/

const byte PWM_PIN = 7;                 // digital‑7 (may be PWM‑capable)

// Keep an explicit list of recognised commands (helps with maintenance / help)
const char *KNOWN_COMMANDS[] = {"c", "c0"};
const byte  NUM_COMMANDS     = sizeof(KNOWN_COMMANDS) / sizeof(KNOWN_COMMANDS[0]);

void printHelp()
{
  Serial.println(F("Available commands:"));
  for (byte i = 0; i < NUM_COMMANDS; ++i) {
    Serial.println(F("  » " ));
    Serial.println(KNOWN_COMMANDS[i]);
  }
  Serial.println();
}

void setup()
{
  pinMode(PWM_PIN, OUTPUT);
  digitalWrite(PWM_PIN, LOW);           // start safe (OFF)

  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);

  Serial.begin(115200);
  while (!Serial) { /* wait for native USB boards */ }

  Serial.println(F("\nSerial‑controlled PWM ready."));
  printHelp();
}

void handleCommand(const String &cmd)
{
  if (cmd == "c") {
    digitalWrite(PWM_PIN, HIGH);
    digitalWrite(LED_BUILTIN, HIGH);
    Serial.println(F("PWM 7 → HIGH"));
  }
  else if (cmd == "c0") {
    digitalWrite(PWM_PIN, LOW);
    digitalWrite(LED_BUILTIN, LOW);
    Serial.println(F("PWM 7 → LOW"));
  }
  else {
    Serial.print(F("Unknown command: '"));
    Serial.print(cmd);
    Serial.println(F("'"));
  }
}

void loop()
{
  if (Serial.available()) {
    String cmd = Serial.readStringUntil('\n'); // read until newline (Enter)
    cmd.trim();                                // strip CR/LF and spaces
    if (cmd.length())                          // ignore empty lines
      handleCommand(cmd);
  }
}

Rough schematics of what I hacked together today, leaving out the capacitor on the right side of the boost converter. PSU is the power supply unit.

The red LED on the bottom of the breadboard is taking the place where the capacitor should be. It’s hooked up to the output of the boost converter. With a high value resistor sitting by it, it lights up to show both the switching and boost converter are working.

I was interested to see how fast can the charging gatekeeper IRLZ44N turn on. The scope give a 658 ns, which is pretty decent. 0.5–2 µs edges are typical when the GPIO can source/sink only 10–25 mA into a 5–15 nC gate. So, 0.658 µs is on the quicker side—indicates the device isn’t gate-charge-limited here.

Because I’ve only just started the project. It’s a good sanity check, but this good switch performance might all change when I put in the capacitor.

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