RCMixer
RCMixer - Microprocessor based radio control mixer for home constructors
The RCMixer is a microprocessor based radio control mixer designed for construction by technically competent amateur enthusiasts. The RCMixer consists of a small printed circuit board containing a microprocessor, a ceramic resonator, and a dual inline 6 way switch. Connection pads are provided on the board for connection to the two input channels and the two output channels. The RCMixer is designed to be mounted inline with the servo leads and encased in adhesive lined heat shrink tubing with only the 6 switches exposed. Current consumption is extremely low and could be considered insignificant compared to overall system usage. The RCMixer is extremely functional for such a simple circuit and is one of the few, if not the only mixer to implement a mathematically correct mixing algorithm.
The purpose of a mixer is to combine two channels of data from a radio control receiver in a linear fashion to produce two channels of information for delivery to the servos. This mixer can do this in one of two different ways.
Mixing "into" involves taking a fraction of one channel and adding it to the other channel. An example of "mixing into" would be manoeuvring flaps where an elevator input causes flaps to move but a flap input has no effect on the elevator.
Mixing "with" involves adding a fraction of one channel into the other and subtracting a fraction of the other channel from the first. An example of "mixing with" would be flaperons where an aileron input causes the flaperons to move differentially and a flap input causes the flaperons to move together. Other examples are V-tails (rudder and elevator) and Delta (elevator and aileron). A more esoteric use is to mix aileron into elevator on an aerobatic airplane to prevent torsional fatigue of the airframe.
RCMixer - Basic specifications
The following basic specifications are applicable to the RCMixer V1.0 currently shipping.
Note: Version 2 of the mixer is due for release before the middle of 1999. It will feature the option of selecting the mixing ratio by means of a third channel input.
RCMixer - Electrical description
The RCMixer consists of a high speed microprocessor (embedded controller), a ceramic resonator with built in capacitors to provide the processor clock and a 6 way DIL switch. The microprocessor is a Microchip PIC16LC61, selected to enable a wide operating voltage range. Two pins on port A are used to send out the pulse width modulated signals to the servos and the remaining port A pins are left unused. Two pins on port B receive the incoming data and the remaining pins are used to sense the switch positions. I just can't seem to get more than 4 lines of text out of 3 components !!
RCMixer - Software description
The RCMixer software is written entirely in Microchip assembler. The code is fully self documenting and describes the algorithm used to mix the two channels. This algorithm is unique in that it always allows full servo travel without over-ranging the servos regardless of the proportion of mixing taking place.
The remainder of the software description is contained within the source code. Of note is the method used to stop the clock to enable reading of a 16 bit timer value. This is an important technique. When the code was first written the servos would occasionally jump. It was a perplexing problem until I threw some heavy weight machinery at it and found that the output pulses were occasionally changing by 256us, just for one pulse. This was enough for the servo to jump. The 256us change was traced back to an uncertainty in reading the timer register. We maintain a 16 bit timer but only the lower 8 bits is managed by the hardware and the upper 8 bits is handled by the interrupt service routine. When the timer is read there is some uncertainty as to whether the timer low byte has rolled over, thus incrementing the high byte, between the reads of the low and the high byte. The fix up of course is to stop the timer, which can be done on PIC's if you read the fine print in the data manual.
RCMixer - Engineering information
Engineering information for the RCMixer is being made publicly available to enthusiastic home constructors so that others may learn the techniques involved in both the hardware and software design. To some extent this is a public service and I do not have to do this but one must always be mindful of ones roots. I have learned much from the internet thanks to other people unselfishly publishing the results of their hard work so it's about time I did the same. Just remember, this is copyrighted to Eagle Air so you can make as many mixers as you like for yourself and your friends but if you are a corporation you cannot make them for profit. I am not a hobbyist, I do this for a living, so if you "nick" this design for profit then you take food out of the mouths of my family and I take that very seriously.
Probably the best way to make the mixers is to buy the pc boards and programmed micros direct from Eagle Air. You are welcome to use the source code and program your own micros but making boards is expensive and I have already spent the necessaries on doing this. To buy the parts just email your order to me stating what you want and how you want it delivered. Prices quoted here are in USD FOB place of origin which is my home office in Australia. Don't forget, if you want code for neutral pulse widths different from 1500us then let me know when you order.
| Item | Price (USD) |
| Mixer PCB | $ 10.00 |
| Programmed microprocessor | $ 20.00 |
| Kit (assembled pcb and hs tube but no leads) | $ 45.00 |
| Completed mixer - (servo leads customer specified) | $ 50.00 |
| Freight | $ 5.00 |
| Place order using a secure order form. |
RCMixer - Construction
Construction of the RCMixer is very straightforward. You will need a fine soldering iron suitable for surface mount components and a magnifying glass such as a Mag Lamp. You will also need some practice at soldering surface mount components which can be obtained by securing old surface mount boards from junk equipment and heating them over an electric hot plate until the parts fall off. Then you just solder them back on again and keep doing it until you get a consistent quality.
When all of the parts are soldered on, all three of them, you then attach the servo leads to the end pads, making sure to place the heat shrink tubing over the servo leads BEFORE soldering them to the board. The heat shrink tubing should be the type which contains a layer of adhesive on the inside. Cut a small piece of stiff paper to the same size as the top of the switch and fix it on the switch with some easily removed adhesive such as a very small amount of contact glue. Centre the heat shrink tubing over the assembly and shrink it. After the glue has cooled then, with an Exacto knife, cut a rectangular window out of the heat shrink tubing to expose the switch. Remove the stiff paper protecting the switch and with a little heat, re-activate the adhesive around the edges of the window.
RCMixer - Installation
Installation of the RCMixer simply involves connecting servos to the output leads and plugging the input leads into the receiver. Set up the switches to suit your operation and go flying. One important point to note is this. If the two input channels are very close to each other in the pulse sequence then the mixer can sometimes have trouble measuring both pulses within one frame with the result that it takes two frames to acquire the two input pulses and the output pulses are then generated in a frame twice as long as the input frame. The effect of this is that the servo operation is jerky and unresponsive. The cure is simple - reverse the input connections then reset the switches to suit the new arrangement.
I hope you gain as much from this little project as I did - have fun !!