Upgrade of a Radio Control from 4 to 7 ChannelsThe model of the Buessing 8000 was provided with a lot of functions which couldnīt be supported by the present radio control. Thus either a new radio control with more channels was to buy - or an upgrade of the present system was to carry out.
The view into the inner side of the transmitter ATTACK-4 type FP-T4NBL from Robbe Futaba showed the programmable 7-channel-encoder NE5044. This chip is able to encode 7 channels, but in the present transmitter only 4 were used. From there the upgrade in principle was possible, and because in practise it worked excellent, the upgrade shall be discribed shortly.
Whether the time effort for the modification of such an old model is profitable may be questioned, but for an electronic engineer it is an interesting challange anyway.
For explanation of the principle shall be mentioned, that at this conventional radio controls the signal transmission occurs with pulses, whose length includes the information and can be changed by the operating elements (pulsewidth modulation).
In the idle state of the control sticks the pulsewidth is approx. 1,5ms (milliseconds), and the receiver-sided actuators this interpret as neutral position.
With operating the control sticks the pulsewidth can be changed between 1ms and 2ms with the consequence, that the actuators respond adequate: servos move right or left, speed controllers switch forward or backward etc.
The pulses of all channels are chained together without gap by the encoder chip, but are not transmitted in this constellation. Because the end of a pulse similarly is the begin of the next pulse, only the changeovers are marked by short approx. 0,3ms wide pulses (see also Pulse Diagram in Wikipedia). With this impulse sequence the HF-carrier is modulated, whereas the choice of the kind of modulation Frequency Modulation (FM) or Amplitude Modulation (AM) is irrelevant.
With 7 channels and a max. pulsewidth of 2ms the total length of the impulse sequence is max. 14ms, which is transmitted with a repetition of approx. 20ms (50Hz).
During the remaining 6ms no pulse transmission occurs; this gap is necessary to synchronize the receiver with the in the transmitter generated impulse sequence. If the gap is too small, the synchronization doesnīt work and the receiver-sided actuators get the wrong information. For this reason at this system the number of channels is limited to 7 or at most 8.
The Modification of the Transmitter
After sourcing the data sheet from the internet firstly the input circuit of the NE5044 was analyzed which is equal for all channels.
In the next step an adequate input circuit for all new channels was built.
Using SMD-resistors (Surface Mountable Device) and a piece of breadboard a small piggyback-board was formed, which could be mounted onto the present printed circuit board without problems.
Following the pins 5, 6 and 7 of the NE5044 were disconnected from the circuit ground by break of the printed tracks and connected to the appropriate points on the piggyback-board by means of fine wires. It is self-evident that this action only will be successful with the use of a suitable soldering iron and a little practice; otherwise the destruction of the NE5044 canīt be excluded.
Now only the additional control elements still were missing.
The input of channel 5 should get a 3-stage-switch for changing the gear of the model. Channel 6 should operate the brake by an additional potentiometer, and for channel 7 a special feature was intended:
Because as a result of the modification already a total of 6 proportional channels were available, the last channel should be upgraded to a switch-channel with further 6 switchable functions by means of a multiswitch module.
But firstly firstly the new control elements were mounted into the transmitter housing, and for that only very less space was available. In spite of using miniature components at some places in the interior of the housing a little plastic was to mill, but for the appearance and the stability of the housing this doesnīt matter.
The last step was the installation of the multiswitch module which consists of an encoder in the transmitter as well as a belonging decoder on the receiver side.
For the principle function shall be mentioned, that at this system the states of the connected switches cyclic are sampled and transmitted (time multiplex). The decoder reconstructs from this the equivalent states whereby a synchronization between encoder and decoder is also necessary.
The circuit of the encoder mainly consists of the 12 bit binary counter 4040 and the analog multiplexer 4053 (both in CMOS-technology).
The input states of the multiplexer are pretended by the position of the connected switches.
With an open switch the output voltage is 2,5V and the NE5044 generates the neutral pulsewidth 1,5ms, interpreted from the decoder as "Switch Off".
With a closed switch the output voltage is 5V corresponding to a pulsewidth of 2ms; this value for the decoder means "Switch On".
Pins 2 and 4 of the multiplexer are connected to circuit ground and generate an output voltage of 0V. The associated pulsewidth is 1ms, and this 2 one after another arriving short pulses act as synchronization between encoder and decoder.
The following pictures show the complete modified transmitter:
The Modification of the Receiver
The radio control receiver was also to be modified, but in relation to the preparation of the transmitter this was important simpler.
The receiver mainly consists of the HF-circuit, pulse regeneration and a shift register which allocates the control pulses to the belonging channels.
The reproduction of a smaller multiswich decoder was set aside, because large models usually have enough space.
Instead of this a multiswitch decoder Nr. 8884 from Robbe was bought as second-hand article which works very well with the modified radio control.