I've been tinkering with Arduino boards for few months now and I've found them quite useful for astronomy because they are so versatile. This is my first project and I hope someone finds it useful. Main purpose of this apparatus is to prevent roll-off roof from hitting a telescope thats not parked to desired position. My own telescope barely fits under the roof at any position but Erik's telescope doesn't so this creates a risk of collision because there is always a possibility of an user-error. The components are very cheap and the coding language isn't very hard to learn so it was a fun project to do. I haven't tested it in real world yet but it has worked great in tests I've done indoors.
List of components: -PCB board -Arduino Nano -HC-SR04 ultrasonic ranging module -Any two channel relay board that can be controlled with an Arduino. -Some 1mm wires and your preferred power connector, I used a JST XH 2-pin connector. -Indicator LED. Solder the components on the PCB, attach HC-SR04 pins to: Trig = D2, Echo = D3, VCC = 5V and GND to ground. For the relay module you can join 5V and GND pins to the same pins as in the HC-SR04. Solder relay 1 signal pin to D10 and relay 2 signal pin to D11 pin on the Arduino. After this you're almost ready to go, just solder negative lead of the LED to your power connector and positive to relay 1 NC port. Then solder a wire from the relay 1 COM port to the power connector to complete the circuit. You can do the same with the motor or any other device that you wish to control in relay 2. Now you should have a fully fuctional park sensor for your telescope, if the LED is on it means that your scope is parked and if not the power can pass though freely allowing power to your observatory roof motor. The sensor uses ultrasound so this is lightpollution free solution. I hope you find it useful! :) The code is available here, upload it to the Arduino and enjoy!