Generate particular pulse using Arduino
Take two 10K or so resistors:

    +5V
     |
    10K
     | 
Pin2-+---&gt; output
     |
    10K
     |
    GND


The following program does the trick:

const uint8_t kPin = 2;
const uint32_t kDelay = 10;  // 100Hz

void setup() {
}

void loop() {
  pinMode(kPin, OUTPUT);
  digitalWrite(kPin, HIGH);
  delay(kDelay);

  pinMode(kPin, INPUT);
  delay(kDelay);

  pinMode(kPin, OUTPUT);
  digitalWrite(kPin, LOW);
  delay(kDelay);

  pinMode(kPin, INPUT);
  delay(kDelay);
}


When Pin2 (or any other pin for that matter) is set as INPUT, it is in high-impedance state (theoretically), so effectively you have a voltage divider giving you 1/2 Vcc. When it is set as OUTPUT, and now assuming that its output impedance is &lt;&lt; 10KΩ, it drives the output to either +5V or 0V.