Wednesday, 10 July 2013

Part Six. Setting up the Dish Controller.

First Look at the Arduino

My Radio Astronomy project is slowly taking shape. The LCD shield for the Arduino Uno controller arrived today. I paid an extra $1.60 for express post from Hong Kong and it was here in three days. Amazing! It did have $28 Hong Kong dollars worth of stamps on it!

Over the last couple of days I've been putting together some preliminary software for the Arduino to communicate with the LCD shield and the L298 motor driver shield. Mostly its 'sample' code that is available for testing shields and makes it quick to run through some ideas I've got for the actual Dish Controller software.

So far it reads the six pushbuttons on the LCD shield and shows on the display which button has been pressed. I'm really pleased with the look of the display backlighting.

The stack of three boards together looks pretty cool although the L298 motor driver shield is covered by the LCD shield which makes it difficult to access the terminals for the motor power wiring. I think it may be best to bring out the wiring from the motor drive shield to a separate terminal block to give easy access for wiring back to the Azimuth and Elevation motors.

Here's a picture of the three boards stacked together. I've obtained a suitable IP66 diecast metal box for the Dish Controller and power supply to be located in. I was a bit concerned about the height of the stack of three boards but it appears there's enough room to mount it with clearance.

Some thought has gone into the power supply for the Dish Controller. I'm not happy using a switchmode power module, tempting as they are. For most people they are an off-the-shelf regulated power box but I suspect the amount of high frequency noise that would be generated by a switchmode box would be unacceptable and no amount of filtering or shielding could minimise it enough.

I've settled for a toroidal power transformer with 2 x 15 volt windings capable of about 1.6 amps each. This fits nicely at one end of the control box and should give adequate current to power the motors. As the L298 motor driver is limited to 2 amps and the Arduino is using Pulse Width Modulation to limit the power to the motors I think this should be sufficient.

The dimensions of the box are Width 146mm, Length 222mm Height 55mm (including the lid).

One of the tasks this box will also be needed for is providing power and junction wiring for the Raspberry Pi and SDR receiver module mounted at the focal point of the dish. A CAT5 network cable will run from our main computer to the Raspberry Pi through this Dish Controller box and a USB cable will run from the Raspberry Pi back to the Arduino as Dish Position commands will be coming from the Raspberry Pi.

I suck at diagrams.

In my next post I should have the power supply built up and providing power. The Arduino should also be doing some basic functions with controlling the Azimuth and Elevation motors.


Rob Arrowsmith.