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Upgrade of a Radio Control from 4 to 7 Channels

The model of the Buessing 8000 was provided with a lot of functions which couldn´t 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 doesn´t 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.

By means of an internal generated reference voltage V_Ref = 5V and a voltage devider in the idle state of the control stick the input gets a voltage of 2,5V according to the neutral pulsewidth 1,5ms.
With operation of the control stick this voltage varies between 2V and 3V according to a pulsewidth between 1ms and 2ms.
The combination 10k/22pF probably works as HF-suppression to avoid influence to the NE5044.
The inputs of the non-activated channels 5, 6 and 7 (pins 5, 6 and 7 of the NE5044) are connected to the circuit reference (circuit ground) and because of that don´t appear externally.

The present input circuit
of the NE5044

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 can´t 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 doesn´t matter.

The input circuits of the channels 5 to 7

With the connection of the new control elements to the piggyback-board was to consider, that at the present control sticks because of the construction only a part of the potentiometer resistor range is used, but the additional elements exploit the full range.
Therefore into the input circuits of the new channels an additional 82k-resistor was to insert which makes sure, that the NE5044 gets the proper input voltage between 2V and 3V.

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 binary counter is clocked by the 50Hz clockpulse (pin 9 of the NE5044) and counts in the 3 least significant bits permanently from 0 to 7; the other bits are not relevant and therefore not connected.
The counter controls the multiplexer which in dependence of the binary number switches each of its inputs sequencial onto the output (pin 3).
Because the binary number changes all 20ms a complete sample of all 8 inputs takes 160ms. The delay between operating a switch at the transmitter and response in the receiver so can be max. 160ms.

The circuit of the
multiswitch encoder

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 modified transmitter...

...and the view into the case

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 shift register
for 7 channels

In the present receiver the dual 4 bit shift register 4015 is applied, from which only one half was used.
The upgrade included only the cascading of the both 4 bit shift registers to one 7 bit shift register and the insertion of the additional plugs.
For this the upper half of the case was to provide with additional holes and because of missing space some components had to be shifted. But the effort kept within a limit and therefore it isn´t necessary to discuss.

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.

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