RC Model Reviews

Here's the populated telemetry dashboard PCB, pretty simple really -- low parts count and through-hole construction for ease of assembly.

The LCD connects through the leftmost set of header-pins and there's a 6-pin ICSP connector plus connectors for four momentary-contact push-buttons that allow interactive programming of things like alarms (max/min) settings and mode-changes etc.

Hi Bruce, have you settled on any particular model PIC MCU for the RX (remote end) yet ?
I see the TX end has a 18F2520 in the photo.

Martin

I'm just using devices that I have laying around for the prototype work and have a bunch of 18F4520s in TQFP packages so I've used one of them for the airborne telemetry hub.

The good thing about the PIC series is that the programming code is very portable within members of the same family -- the only real difference is the amount of program memory, EEPROM, RAM and the number of hardware peripherals built-in.

Once I've fully tested this prototype system and got my hands on the "release" version of the FrSky 2-way system, I'll probably make some more pragmatic choices in respect to which processors to use. For example, if I go to a graphic LCD (rather than text-based) on the dashboard I may opt for some more processing horsepower at that end.

The main thing is to get something working ASAP using what I've got laying around.

Very cool project!

I have a question regarding how you are going to measure the voltages on the PIC. I have been working abit on similar problem but I've not come up with a good solution. For my project i would want the Hub to be able to work with batteries from 3 to 12 cells. And preferably autodetect the cell count. (i fly a TREX450 on 3 cell, and is currently building a TREX600 that will go on 6 cells in the start, but might get upped to 12 fairly soon).
I am planning on basing my Hub on a AVR rather than PIC though.

For voltage measurements, as I've understood it, one could either:

1. Use the balancing plug, and measure each cell on separate AD ports, however this would have some issues on large crafts like the TREX 600-700's running 10-12 cells, as very few micro controllers have that many AD's
2. Use the balancing plug as above, but somehow mux the lines to use only one AD port. (shift register controlled by a DO to swap cells maybe?)
3. Use a voltage divider on the full battery voltage, set up to your max cell count, i.e. 50V in gives the max on the AD port, would work, but require more logic in software, and would give low resolution for low cell count batteries, would also require soldering to the main power lines. (i really don't like doing that..)
4. Using a comparator or similar to put in a "battery ok" or "battery too low" into a Digital pin for each cell
5. Use the full battery voltage, but somehow create a circuit that would output only an average cell voltage, that could be measured on one pin
6. Use balancing plug, have a circuit that would somehow convert cell voltage to a "pulse train" where i could measure the period, or time between transitions on one DI for each cell

There are probably many more methods to do this, but i'm on deep water now..
I "think" there is probably a "best" way to do this (guessing nr 2...), and i am hoping that someone can point me in the right direction

i've had several electronic courses, but all of them where 15 years ago, and I have had very little practical application in this field...

Well i look forward to seeing your project progressing!

For my 2 cents worth I'd say use an something like a analogue multiplexer chip. I did a multiple thermocouple device some years ago using a DG509, and a AD595 plus a PIC, seemed to work well for me.

Looked up the DG509, and from the datasheet, the DG506A seems perfect for my purpose, would only need 4-5 Digital pins and 1 analog pin on the micro controller. Brilliant! Thanks!

Hi Bruce,

Hey I had a thought. What software do you do your schematics and PCB artwork with ?
If it's either Protel or Eagle I can help you with it. I used to use Protel, but lately have been cutting over to Eagle Lite (the free edition). I can help tidy up PCB artwork etc.
Great if you use Eagle, because anybody can then change or add components to the design for their own special customizing....

Cheers,
Martin

I use Eagle (the free edition) and I'll be publishing all the files so people can do whatever they want to modify things for their own purposes.

Had a busy week (damn) but since the weather's crap here I plan to spend most of Sat and Sun tidying up the loose ends on this project and perhaps making some video.

Bruce,
You suggested that you would be using a PIC18F4520 in the sensor hub, because you already have them. I happen to have some PIC18F452's in my parts bin. These are very pin compatible, but the newer part (18F4520) has more A/D pins. Could you try (if it's not too much trouble), to use the first eight A/D's on the 18F4520 in preference to the last five, as this will ensure pin-pin compatibility with the 18F452 (also the 18F4620 AFAIK). The more parts that are pin for pin compatible with your code the easier it will be to buy PIC's to suit and require less recoding.

Cheers,
Martin

PS. I need to buy some PIC's for my own FrSky vario project as the 18F1220's I was using just ran short on code space, so I'll buy some 18F2520's same as you've used in the display thus far...

Hi Bruce
hows your project coming along . I have ordered the FrSky system and am looking forward to seeing your finished product. Cant wait to give it a go.
David