Goal: to have one box with one cable in (12V Battery) and one cable harness out (mount,DSLR,focuser,power). Simply to get rid of any cable tangle, thousands of power supplies. A touchscreen and a webcam for comfortable polar scope alignment without neck pain. Wireless control of the mount,dslr,focuser,guider via Indilib
Box material: thin (5mm) pressboard wood for easy editing.
The box should contain: The SyncScan motor controller, power supply for DSLR, relay to switch components on/off, RaspberryPi 2 with Touchscreen as Indiserver, a wlan usb stick, temp/humidity sensor inside the box, a cheap 7xUSB hub directly powered from 12V, a batterytester that periodically checks the voltage, a main fuse for the battery (~3A) and maybe some more for the power supply of the expensive parts (synscan,DSLR) and some switches.
Voltage converters: 3x cheap china step down converters 12V to 2x5V,7,3V. Relay will be not powered by 5V pin of the RPi2 because I observed heavy voltage drop, so it gets a dedicated buck converter.
Battery tester: I will check the lead-fleece battery voltage periodically with the RPi2. The voltage is checked with a MCP2003 AD and a voltage divider that makes the max. 13,7V below 3V that the AD can read it. If it drops below 10.7 volts the main power cable will be disconnected with the relay via a GPIO on the RPi2. I think it will not hurt the RPi2 to simply cut the power. I use a cheap 2 channel relay module from ebay that is 5V and active low. I use only one channel for now but I plan to use a EL-Foil for flat generation in future and I can use the second channel for that. I invert the active-low to active high with a BC337 transistor. After the relay cut the main power a push button needs to be closed to restart the box. Then the RPi2 boots up and immediately sets the GPIO to high and the relay closes and it will hold the main power on. Because I don’t want to increase the cable madness I decided to etch my own pcb for the batterytester.
the finished pcb:
Uncertainties: maybe humidity issue inside the box, wood may warp, shorts, explosion, black hole..
(measured between battery / step down converter)
|SynScan Controller||12V||idle: 0,3A||track sideral: 0,6A||slew 800x: 1A||max: 1A|
|Canon EOS 700D||7,3V||idle: 0,08A||take picture: 0,3A||max: 0,3A|
|5V||idle,touchscreen brightness 0:
|idle,touchscreen brightness 255: 0,43A||stress cpu,
touchscreen brightness 255: 0,52A
|Focuser Stepper||5V||max: 0,7A|
|Relay||5V||switched 1 channel||max: 0,02A|
Real power consumption:
The main fuse decision is 3A slow blow because of the inductive load from the mount steppers, relay and the focuser stepper. As I’ll add more electr. components (flatfield foil, automatic dust cover) I’ll have to increase my fuse rating.
While testing the assembly with my laboratory power supply I observed the current limiting led blinking while activating the synscan controller. That means there is a very short current spike greater that 5A. But it seems not to have any impact on the slow blow 3A main fuse.
Future battery decision: A 12V lead-fleece battery with ~20Ah. Using only 40% of its capacity should make it really long life and would give ~8Ah. It seems my box will not hit the 2A and seems to operate at an average current of ~1.6A while doing sideral track+guiding+taking images. That should give 8Ah/1.6A=5hours power for an astro session.
My first box was to small. I decided to make it new with slightly increased dimensions and reposition of cable in/outputs. Additionally a 12V powered 7x USB-Hub was dissasembled and fits perfectly into the box. I connected it directly to the main power battery cable but I consider to add a fuse inbetween.
So far I am satisfied with the box, but realworld outside test is pending and will show if it will survive.
Future plans are to add another cable for a servo and power cable for a EL-foil for a diy flatfield dustcover. This would take me a lot closer to my goal: a full automated remote telescope.