Introduction: Most Useless Lego Tower.

This useless machine uses a Digispark to control the servo.

Before wasting your time, have a look at how it works.
http://www.youtube.com/watch?v=sp4q9Gl0KuU&feature=c4-overview&list=UU53xZEgBce5oEcyNDEIld5w

If you are still interested, let me explain what I did. There are many implementations of a useless machine, some of them are pure mechanical machines which I love, most of them use two switches, some of them use a 555 timer ... and God know how many others.

The Digispark is a development board based on an Attiny85 microcontroller (http://digistump.com/products/1) it is an Arduino like hardware and software implementation. It is cheap and easy to embed in your projects. I decided to use this board instead of the more popular 2 switches, mainly because you have full control of the servo movement; you can decide the speed and the direction at will.

Parts needed.
- Digispark development board (any other Arduino will work as well using the regular servo library)
- DPDT switch, although I only use half of it
- 180 degrees servo, I’m using HiTech HS-422
- Lego structure, although any other will do
- 5v power supply

Step 1: Most Useless Lego Tower. Part 2.

Part 2. Wiring

I don't have the Digispark part for Fritzing, I'll just list the connections.

Arduino  Motor    Color
----------   -------    --------------
5V             5V        Red
Gnd         Gnd      Black
pin 3      Signal   Yellow

Arduino  Switch   Color   Connector
----------   ---------   --------   --------------
pin 0    Signal        red       mid
pin 2    SW Gnd     black   any end connector

The digispark has only one pin for ground, if you need more than one you'll have to use a regular pin (2 in this case) and set it to 0 (ground) in the setup section.

Step 2: Most Useless Lego Tower. Part 3

Part 3. Arduino sketch.

The digisparks uses a "special" version of the Arduino IDE based on 1.04. Most of the libraries are also special, basically due to the small memory on the attiny85 microcontroller. In this implementation i use the SoftRcPulseOut library which happens to be the attiny85 implementation of the servo library.

-------
#include <SoftRcPulseOut.h>

SoftRcPulseOut myservo;                           // create servo object to control a servo
#define SERVO_PIN         3
#define switchPin 0
#define gnd 2
#define REFRESH_PERIOD_MS 20

void setup() {
  pinMode(switchPin, INPUT);                      // set switch pin as an input pin
  digitalWrite(switchPin, HIGH);                  // turn on pullup resistor
  pinMode(gnd, OUTPUT);
  digitalWrite (gnd, LOW);                        // use this pin as a ground

  myservo.attach(SERVO_PIN);                      // attaches the servo on pin defined by SERVO_PIN to the servo object
  myservo.write(5);                               // tell servo to move to 5 degrees
  delay(REFRESH_PERIOD_MS);                       // waits 20ms for refresh period
  SoftRcPulseOut::refresh(1);                     // generates the servo pulse
}

void loop() {
  int val = digitalRead(switchPin);
  delayMicroseconds(20);
  if (val == LOW) {
    myservo.write(145);                           // tell servo to move to 145 degrees
    delay(REFRESH_PERIOD_MS);                     // waits 20ms for refresh period
    SoftRcPulseOut::refresh(1);                   // generates the servo pulse
  } else {
    myservo.write(5);                             // tell servo to move to 5 degrees
    delay(REFRESH_PERIOD_MS);                     // waits 20ms for refresh period
    SoftRcPulseOut::refresh(1);                   // generates the servo pulse
  }
}
-------

That’s all folks !!!

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