No worries mate, I'm glad that someone is willing to think with me on this!

The details:

We're building a project where a computer could choose a number as strength. This number will determine how hard the spring will be pushed in. Upon reaching the chosen strength (or pressing a button, doesn't really matter) the mechanism should fly forward (like a slingshot) to hit something which in return will start its course (in this case, a small rollercoaster vehicle).

I already thought about creating an automatic linear slide with a mechanism (servo, solenoid, ...) that can move itsself out of the way to release a rubber but I was wondering if there was a motor that can release itsself.

The object that needs to be pushed isn't that heavy: think about a small toy car (matchbox-alike).

I am looking for a system that could pull something in and then suddenly release it (like a slingshot).
Normally I would use a system where the gear has one side that is cut off, but I need different "power levels", meaning:

The idea is that you hold a key for a certain amount of time, the longer you hold it, the tighter the slingshot gets. When you release it gets released immediately.

Anyone have a clue?

No, I am afraid not: I hooked it up exactly like that, also implemented the suggested changes but the out of sync error still remains.

I have the following sketch (see below) on my PC which compiles perfectly. When I upload it to the board however I keep on receiving the "out of sync" error when the wires are hooked. If I upload it and then make the wire config I do not get an error, but also no readings.

Can it be my URM is fried?

Code:

/* Ultrasound Sensor
*------------------
* URM V3.2 ultrasonic sensor TTL connection with Arduino
* Reads values (0-300) from an ultrasound sensor (3m sensor)
* and writes the values to the serialport.
* Pin12 (Arduino)-> Pin 1 VCC (URM V3.2)
* GND (Arduino) -> Pin 2 GND (URM V3.2)
* Pin11 (Arduino) -> Pin 7 (URM V3.2)
* Pin 0 (Arduino) -> Pin 9 (URM V3.2)
* Pin 1 (Arduino) -> Pin 8 (URM V3.2)
* www.yerobot.com
* Last modified 20/04/2009
*/

int UREnable = 11; // Ultrasound enable pin
int URPower = 12; // Ultrasound power pin
int val = 0;
int USValue = 0;
int timecount = 0; // Echo counter
int ledPin = 13; // LED connected to digital pin 13
boolean flag=true;
uint8_t DMcmd[4] = { 0x22, 0x00, 0x00, 0x22}; //distance measure command

void setup() {
  Serial.begin(9600);                  // Sets the baud rate to 9600
  pinMode(ledPin, OUTPUT);            // Sets the digital pin as output
   pinMode(UREnable, OUTPUT);
   digitalWrite(UREnable, HIGH); // Set to High
    pinMode(URPower, OUTPUT);
   digitalWrite(URPower, HIGH); // Set to High

delay(200); //Give sensor some time to start up --Added By crystal  from Singapo, Thanks Crystal.
}

void loop()
{
 
  flag=true;
  //Sending distance measure command :  0x22, 0x00, 0x00, 0x22 ;

  for(int i=0;i<4;i++)
  {
    Serial.print(DMcmd[i]);
  }
 
  delay(75); //delay for 75 ms
 
while(flag)
{
    if(Serial.available()>0)
    {
        int header=Serial.read(); //0x22
        int highbyte=Serial.read();
        int lowbyte=Serial.read();
        int sum=Serial.read();//sum
       
        if(highbyte==255)
        {
          USValue=65525;  //if highbyte =255 , the reading is invalid.
        }
        else
        {
          USValue = highbyte*255+lowbyte;
        }
   
       Serial.print("Distance=");
       Serial.print(USValue);
       
       flag=false;
     }
}
  delay(50);
}

Can you post a usage example?

Did you get the temperature from the URM37? If so: how?

Can't seem to get this working. I used the sketch in the post by Ioby. What I don't understand is why the 5V is not used.
It says Pin 12 to VCC (urm37): how does it get power?