RCU Forums - Aerobird Challenger Detailed Electronic Analysis

Aerobird Challenger Electronic Analysis

(Thanks to all who have made similar posts!!! You have given me a start to take the thread further)

I plan on building my own RX board while also simply replacing the MCU only on the RX current board while using the same transmitter. Here we go......

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MAIN CHIPSETS
MC3371 or MC3361 Low bandwidth FM IF
Function – takes in the RF signals, demodulates and supplies the digital signals to the MCU. (Note – this part has been discontinued by Motorola, but a replacement (TK83361M-G-ND) can be found at www.digikey.com)

Hynix HMS87C1204A (16 PIN sop PACKAGE)
MICROCONTROLLER –Function – Main Controller

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RA4 |-1 16|- RA3
RA5 |-2 15|- RA2
RA6 |-3 14|- RA1
RA7 |-4 13|- RA0
VDD |-5 12|- VSS
RB0 |-6 11|- /RST
RB2 |-7 10|- Xout
RB4 |-8 9|- Xin
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This analysis may be intended toward engineers and techs so sorry of some things seem high level. Some things have been taken from other posts to prevent typing it twice, most is from what I tested.

1 - Servo potentiometer wiper (L)
2 - Servo potentiometer wiper (R)
3 - 35% of Battery Voltage (voltage divider with small filter)
4 - ACT input (voltage divider between upper / lower sensor)
5 - VCC
6 - CN4 jumper (10K to ground - jumper to VCC)
7 - (also pin 3 of xport) . An Interrupt line that receives demodulated digital signals
8 – 1K Ohm to pin 2 of Xport / Motor FET (combat module gnds line to disable motor)
9 – Clock in 4MHz resonator
10 – Clock Out 4MHz resonator
11 - Power on reset RC
12 - GND
13 - Forward servo FET drive (L) – to H bridge of /NPN transistors
14 - Reverse servo FET drive (L) – to H bridge of /NPN transistors
15 - Forward servo FET drive (R) – to H bridge of /NPN transistors
16 - Reverse servo FET drive (R) – to H bridge of /NPN transistors

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GENERAL FLOW OF OPERATION
(RF Data in)
The RF signal comes into the antenna through a series of balancing transformers (the square silver things with wax on the top). This matches the LRC of the antenna and to the 50 ohm input of the FM modulator. It also aids in tuning the input to the proper frequency. The MC3361 (also could be an MC3371) take the input on pin 16. The RX board has an oscillator that is 455KHz less than the one that is in the transmitter. If the Transmitter is sending on 27.5MHz, the Receiver clock is 27.045MHz. The mixer inside the chip uses this offset to strip out the digital data that is riding on the carrier wave. Similar to how you car radio works.

(Data to the micro-controller)
The demodulated data goes to the MCUs pin 7. This is the MCUs interrupt line and uses it to detect rising edges. More than likely, it uses a timer to detect between the pulses and counts the pulses. There are 4 pulses that come in every 20ms. These pulses represent three things: Throttle, left, right. I will explain this in greater detail below with JPG pictures.

(Servo signals)
The servos are connected to 4 NPN transistors in a classic H-bridge configuration. The servos have 5 pins. The 3 pins that come on the ribbon cable are nothing more than a POT. VCC, out, GND. The voltage at the servo’s POT (center wire) is centered at 2 volts and varies between 1 and 3 volts. This is what connects to back to the MCU and the MCU uses its A2D (anlog to digital) convertor to determine the position of the servo.

The RED and BLACK wires is what is connected in the H Bridge. Pulse one sie high while the other side is low and the servo moves one way. Pulse the other side while keeping one side of the bridge grounded, and the servo moves the other way. The pulse width and how many pulses in a row determine how fast the servo moves. Narrow pulses in repetition will cause the servo to move more smoothly in one direction. Wider pulses in repetition will cause the servo to move much faster.

The MCU then will send a series of pulses to the H-Bridge to move the servo until it is within its programmed range. Therefore, it acts like a closed loop system. The pulses come in bursts of 8 and are spaced apart around 2.3ms to 3.8ms. The pulse widths can vary widely from 523us to 1700us or greater.

The idea is that the MCU will vary the width of the pulse to move the servo faster or slower depending on what it reads on the POT. This prevents servo “jerks” and gives it a ramp or acceleration.

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RF to MCU INPUT SIGNAL EXPLANATIONS (MCU pin 7)

I did screenshot so take a look at them in reference to the descriptions below:

Pulse 1 = start sync signal
Pulse 2 = throttle
Pulse 3 = up/down stick
Pulse 4 = left/right stick

Pulse widths always seem to remain at 280us in width

Measurements were take in reference to ground

DEFAULT IMAGE
There are 4 pulse trains that come in 20ms apart. In the default setting with everything centered and the throttle off, each rising pulse is 280us in width. The distance between pulse 1 and pulse 2 is 925us.The space between pulse 2 and pulse 3 is 1480us. The space between pulse 3 and pulse 4 is 1480us.

STICK LEFT
With the stick move far left, the last pulse (pulse 4) moves inward. The distance from pulse 3 to pulse 4 is now 970us. All others remain unchanged.

STICK RIGHT
With the stick move far right, the last pulse (pulse 4) moves outward. The distance from pulse 3 to pulse 4 is now 1940us. All others remain unchanged.

STICK FORWARD (UP)
With the stick moved up, the 3rd pulse (pulse 3) moves outward and pulse 4 tracks pulse 3. The distance from pulse 2 to pulse 3 is now 1940us. Distance from pulse 3 to 4 is still

STICK BACKWARD (DOWN)
With the stick moved down, the 3rd pulse (pulse 3) moves inward and pulse 4 tracks pulse 3. The distance from pulse 2 to pulse 3 is now 940us. The Pulse width remains the same.

STICK UPWARD & RIGHT
The 4th pulse moves out to a distance of 1940us from pulse 3. The 3th pulse moves out to a distance of 1940us from pulse 2.

STICK UPWARD & LEFT
The 4th pulse moves in to a distance of 940us from pulse 3. The 3th pulse moves out to a distance of 1940us from pulse 2.

STICK BACKWARD & RIGHT
The 4th pulse moves out to a distance of 1920us from pulse 3. The 3th pulse moves in to a distance of 940us from pulse 2.

STICK BACKWARD & Left
The 4th pulse moves in to a distance of 940us from pulse 3. The 3th pulse moves in to a distance of 940us from pulse 2.

FULL THROTTLE
The 2rd pulse moves outward from the sync pulse (pulse 1) as the throttle is increased. The distance from pulse 1 to pulse 2 is now 2160us. The distance from to pulse 2 to pulse 3 to pulse 4 is remains the same at 1460us. This is the approximate center between 1960us and 960us.

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PLANS

I plan on doing two things. First, I’m going to create a PCB that is an interposer card. This way I can use a more standard MCU (Microchip, Atmel), and use its port pins and A2D for the RX board. Code should not be a problem. I’ll keep it al in C. This way, if anyone wants to replace the MCU on the board, I can sell one at cost ( a few bucks) and also give the code away.

Next step is to spin the PCB so that the receive can still use the transmitter, but rplace the crappy servos with a standard 3 pin interface.

Now I’ve got two choices.

If this is of interest to anyone, let me know. I’ll be updating this link very soon and often.

JC