Figure 2 shows the board and transmitter. There are modifications to be made to both sides. It is worth noting that there is a fair bit of room beneath the board inside the transmitter if you wish to add components or small boards of your own. There is however very little room above the solder side of the board so the components we need to fit will be on the underside.

The first thing is to decide how many channels you wish to transmit. For Tornado we use 6 even though we only transmit three signals. We have simply duplicated channels by connecting 1 to 5, 2 to 6 and 3 to 4. This means that we can sum these signals after decoding at the receiver and obtain some extra noise immunity over using just three channels. This is not the best utilisation of the extra channels but having realised that weight was not available in the robot we didn't fit the stuff the extra channels were going to be for….

Figure 3

Having decided the number of channels, we will need to take precautions against the significant RF field generated by the transmitter itself. This means adding capacitors (10nF is a suitable value) to the input pins we wish to use. Figure 3 shows these capacitors added to the IC input pins 3 to 7. Pins 1 and 2 already have surface mounted caps fitted on the solder side of the board. Adding these five caps covers all 7 channels. If you don't need them all you can miss out the channels not used. Ceramic caps are recommended (even though I didn't use ceramic ones!) since they have a lower impedance at RF than most other types. It didn't seem to matter though. Some people might argue that these caps are not necessary but since I prefer to be cautious I would fit them anyway.
All the caps should be commoned to ground (pin 8) as shown. Because we are soldering so close to the IC it is very important we apply the soldering iron for as little time as necessary to prevent overheating of the IC. Use a fine tipped soldering iron and tin both the capacitor leads and the IC leads before soldering them together. Then you can simply melt the solder already on the legs to form the connection.

Figure 4

It is necessary to remove the ground connection from the input pins we wish to use. Carefully cut the tracks to the required pins with a craft knife. Be careful to note that, as in the case of the Ranger 3S, the ground track connects to the unused pins and then goes on to other parts of the circuit. It is necessary to make sure that these other parts remain connected to ground otherwise your transmitter won't work. Referring to figure 4 you can see two wires from a ground point at the bottom of the board connecting the parts of the circuit previously connected by the tracks which have been cut. The other two wires connect channel 1 to 5 and 2 to 6 and there is a short link between 3 and 4, but these will not be required for your mods as they are specific to Tornado.

OK, as already stated the inputs take the form of DC voltages and they should only vary over a narrow range. The IC needs voltages that vary from 45 to 55% of the reference voltage available on pin 15 (5.04V) i.e. 2.27 to 2.77V to cover the full range of output span. The device you are adding (be it a potentiometer or whatever) should be fed by this reference voltage where possible. This will prevent drift of the output signal with temperature and battery voltage.

For an example we shall connect a potentiometer to a spare channel to demonstrate how its done. The potentiometer should be of around 10K ohm value and a linear type to provide smooth, proportional control. The ends of the potentiometer are connected to ground (0V) and pin 15 (5.04V reference) respectively. The centre (wiper) connection is then fed to a resistor divider that takes the voltage span down to that necessary for full range. The following arrangement is used (Figure 5):

Figure 5

The two 33k resistors (R3 & R4) provide an effective half reference bias (5.04/2 V) with an effective resistance of 33/2 = 16.5k ohms. Resistor R2 sets the attenuation ratio such that the voltage swing at the IC is 2.27 to 2.77V as required. To calculate the value of this resistor we draw a Thevenin equivalent circuit. We consider the potentiometer at one extreme (in this case 5.04V). The Thevenin equivalent circuit is simplified to allow easy calculation of the unknown resistor (see Figure 6).

Thus: R3

= R4 (V3 - Vout) / Vout = 16.5k (2.52 – 0.25) / 0.25 = 150k

Figure 6 - NOTE: These component numbers do not match those figure 5!

The same approach can be used for a switch input like this (Figure 7):

Figure 7

Modifying the Hi-Tec receiver for up to seven channels:

This is a relatively simple mod since the chip used in the receiver has all seven outputs on it already, there is simply no connections made to the unused channels.

Open up the receiver and locate the 4017 IC which is near the output sockets. This IC is a decoded counter with ten output pins. The first output is not used as this is active during the reset period when the transmitter waits for the next burst of PPM data. Output 2 is connected to servo output 1 and output 3 to servo 2 etc. The pins for each channel are listed, see also Figure 8:

Channel 1 2 3 4 5 6 7

Pin # 2 4 7 10 1 5 6

Figure 8

The servo connectors carry 0V, supply (receiver battery +) and the appropriate output channel as listed above. You can thus connect as many servo outputs (up to seven) as you wish by adding connectors from a model shop etc.

Good luck and happy roboteering!

Any questions/comments can be directed to me at dgamble@supanet.com

Addendum
Although all the information above is correct, the way we use the available channels has changed a lot since it was written back in Series 4! Since the Fifth Wars we have used all seven, transmitting motor drive information differentially (6 channels - 3 pairs of two, one pair for each motor) and the last channel is an ignition channel (used as an auxilliary kill switch). You don't need to spend hundreds on the most expensive radio gear to do well!

Where can I buy the Hi-Tec Ranger system?
We bought ours from a model shop in Kent called Model World (01622 735 855 - in Maidstone, at the Notcutts Garden Centre). However, your local model shop will be able to order you one if he is a Hi-Tec dealer, and that might save you postage...