Day: October 3, 2013

This may not look like much, but it’s the first test of the p-MOSFET power switch that completely kills power to the Arduino Pro Mini board and the Hall Effect LED Blinky Light:

The top trace is the base drive to the NPN transistor that holds the p-MOSFET on while the Arduino is running. When it’s time to shut off, the Arduino drops the base drive output, the MOSFET turns off, and the switched battery voltage in the bottom trace drops like a rock. The current is about 30 mA when the Arduino is running and immeasurably low when it’s off; the MOSFET spec says it’s less than 1 μA, which is fine with me.

I love it when reality matches the simulation.

That part of the schematic:

The PCB has those components clustered in the upper left corner, with the Arduino Pro Mini perched on header pins to the right:

The test code is a crudely hacked version of the canonical Blink sketch that waits 5 s after it starts running, then pulls the plug:

// Modified from Arduino Blink example
// Drives external p-MOSFET power switch
// Ed Nisley - KE4ZNU - Sep 2013

int led = 13;

// HIGH to enable power supply
int PowerOn = 4;

// HIGH to light Status LED
int Status = 10;

unsigned long MillisThen;

void setup() {

  pinMode(led, OUTPUT);

  pinMode(PowerOn,OUTPUT);
  digitalWrite(PowerOn,HIGH);
  pinMode(Status,OUTPUT);
  digitalWrite(Status,HIGH);

MillisThen = millis();
}

void loop() {
  digitalWrite(led, HIGH);
  delay(100);
  digitalWrite(led, LOW);
  delay(500);

  if (((millis() - MillisThen) > 5000ul)) {
      digitalWrite(Status,LOW);
      delay(50);
      digitalWrite(PowerOn,LOW);
      digitalWrite(Status,HIGH);
  }
}

It turns out that the Arduino runtime has a several-second delay after power comes up before the setup() routine starts running, so brief pulses from a vibration switch won’t last long enough to turn the thing on. That’s not a fatal flaw for now and, in fact, having to hold the power button in for a few seconds isn’t entirely a Bad Thing.

However, once the power turns on, a vibration switch could trigger an Arduino interrupt pin to reset a power-off timer. I’d be tempted to put the vibration switch in parallel with the button, with a pair of steering diodes that isolate the raw battery from the input pin.

This is, of course, a pure electronic implementation of a Useless Machine…