Sidereal Technology Radio HandPad Saga

This page is mainly a (sad but true) story at this point, the nice thing is, even considering all the troubles I've had with this project, it's almost done!

I've always wanted to make a radio handpad for controlling my telescope, but I couldn't find the right radio chips, or boards, and never fully developed one.  Now, after completing the Servo Controller, I wanted to make this my next project.

I found a cool little radio reciever and transmitter at  These things cost about $20 for the pair.  They also sell encoder and decoder chips which I used on the first two boards.

Board One:
I designed my own transmitter and reciever board for the first prototype.  After carefully checking it, I ordered 4 boards.  The next day (it was too late to cancel the order) I thought of a much better way to route the final outputs to the Servo Controller.  The idea was to make the handpad input on the servo controller completely compatible with Mels old stepper system handpad when used in the autoguider mode.  This means all of the diode matrix's from the decoder chip outputs to the servo controller were completely wrong (also on the transmitter, as it can be used as a wired handpad).  After recieving the boards, I made a huge mod on them, and they seemed to work ok.

Board Two:
The first board was a huge mess due to all of the diodes sticking up in the air on the modified board, so I re-designed the board, and sent it off.  After getting the new prototype boards (hoping it to be the final design), I had our resident expert (Marion Beatty) solder the components and I tested it out.  This handpad worked fine, as long as the 9 volt battery wasn't in the battery compartment!  I found that the battery was right next to the transmitter daughter board, and it attenuated the signal enough to make it not work reliably.  Also, I found that the encoder/decoder chip had to send too much data too slowly and there was a small delay (about 1/6th of a second) between pressing the button and scope movement.  For slewing, this is not objectional, but for panning, it was a little annoying, creating overshoot.  I doubt if it would bother most folks, but I guess I'm a perfectionist.  Another annoyance was every time you were pushing one button, and then pushed another, the first axis would stop momentarily.

I'm currently using this prototype for my 28 inch telescope.  I have the 9 volt battery taped to the outside of the box so it won't attenuate the signal.  Even with its small annoyances, it's really cool to have a radio handpad.

Board Three:
At this point, I was thinking about the delay problem, and also the switching directions problem, and I figured if I put a micro controller on both the reciever and the transmitter instead of the encoder/decoder chips, I could solve both problems at once.  I selected a Cygnal C8051F310 cpu, rather expensive, but it had the features I needed.

I found that by using a micro processor, it opened up a whole new host of features!

One thing I like to use the radio handpad for (or any handpad for that matter) is to have people at public star parties view the moon at 350 power (if the seeing permits), and pan around the moon.  I did this at Riverside Telescope Makers Conference, in May, and people really enjoyed the view.  Most (even seasoned astronomers) had never seen the moon like this before!  Got a lot of ooh's and awe's!  The biggest problem was keeping people from initializing the dragNtrack mode by pressing the wrong button for more than 4 seconds!  They would also go into the slew mode, and the telescope would take off.

Using a CPU, the problem is solved!  Create a "KeyLock" mode, and no keys work except for the direction keys!  This feature is also going to even help me at 03:00 AM and I can't see straight anymore!

Another idea came to me while I was at RTMC.  I had my red flashlight hanging around my neck, and I was planning to make a necklace for my remote too (since I kept loosing it).  I thought, why not combine the two?  At almost no additional cost, I've got a flashlight combined with the remote control.  This is really cool!  It's even more efficient than most red LED flashlights, because I'm using PWM, and all 4 LED's will be in series, powered from 9 volts, so it will be very efficient.  No need to have two "necklaces"!  The flashlight will turn off automatically after about 15 minutes.  You can adjust the brightness using the keys.  I think this is the first micro-processor based flashlight I've seen, normally an overkill idea, but for just a few dollars more.....???  Why not?

Another problem with the very economical transmitters and recievers I've selected, is if there are 2 or more transmitters on the same telescope field in close proximity to each other, there may be interference.  With board one and board two, you could select an address, so one transmitter wouldn't control another's telescope with a different address, but if two people were transmitting at the same time, both may not work.

Again, the microprocessor solved the problem.  When you press a button, the transmitter transmits a short burst of address/data.  This only takes 30 mS.  While the button is held down, the transmitter transmits another burst every second.  If the button is let up, or another button is pushed, the transmitter will transmit another burst immediately.  This makes the handpad work instantly, on button push or letup, but it doesn't have to transmit all the time, in fact it will only be transmitting 1/25th of the time while a button is pushed.  The reciever will be programmed to expect the burst at one second intervals.  If it doesn't get another burst, it will stop the telescope from panning or slewing.  This should prevent the telescope from slewing too far if there is interferance from the "turn off" burst for one reason or another.

I eliminated the address dip switch on the transmitter and the reciever because of the microprocessor as well.  You will select your address using the push buttons on the hand pad, and it will store the address in the flash rom of the microprocessor.  To program the reciever module, simply put a jumper on, press a button on the transmitter a few times, then take the jumper back off.  As long as nobody with a different address transmits at the same time, the reciever will program it's address to match!

I gave close attention to the power system.  I finally found a very low power regulator that is "turn-offable", and so the microprocessor shuts itself off after about 1/2 an hour.

Using an alkaline 9 volt battery, the transmitter should last 160 hours of continuous operation.  I hope I run this battery down in one year, that would be considered a really good observing year!!

This board is now working, even with the battery in the battery compartment!  The response time is imperceptible!  When you press another button while holding the first button down, there is no slowdown of the first axis, it works perfectly!

Board Four:
I now have board 3 working with a few board mods, so I'm ready to finalize the design with board four.  I intend to finish this board this weekend, and test it out next weekend (early June, 2004).  During the waiting time for the boards, I'll finalize the firmware in both the reciever and the transmitter.  Hopefully I'll have a working board four in about 2 weeks, and I can retire my board 3.

Board Five:
Hopefully board 4 will work out perfectly.  I'm not crazy enough to order 100 of them though, just two!  If they work out, hopefully board 5 will be a production run of board 4's!

Why so many prototype boards?
In the case of the servo controller, the first board worked, but there were a few shortcomings.  I just sold some of the 2nd set of boards, they work great.  I have 100 of board 3, but there is only 1 very minor change for board 3.  This board is a lot more complicated than the radio handpad, so I either got very lucky with the servo controller, or I screwed up bad on the handpad!!!

I am pretty experienced with microcontrollers and power electronics, but I know practically nothing about radio, that fact, and the change in design mid stream two times, oh well, I guess that's the cost of R&D!

FCC Approval:
You may be able to help me.  I know nothing about FCC approval.  I've been trying to read up on the FCC reg's Part 15  but I'm still not sure how to handle this.  I'm pretty sure there is nothing in the design that won't be approved, but I don't really want to spend the 3-5K to get it approved at this point.  I'm pretty sure I'm going to have to sell the transmitter as a "kit", so the end user will have to do a bit of soldering to finish it.

If you have any ideas or expertise in this area, please let me know.  Thanks....

Dan Gray