RADIO CONTROL IS a virtual necessity for serious outdoor model railroading. It eliminates the need for complicated wiring and, in some cases, for cleaning track. Three basic approaches to outdoor radio control exist.

The most common is where the transmitter talks directly to a receiver on the train. Onboard batteries and/or electricity from the track may power the train.

Another approach is "command control" where a transmitter talks to a receiver distant from the train and the receiver in turn talks to a command system controlling and powering the track and thus the train. My Cranis Garden Railway uses that method. Both systems require modifications to the locomotive.

The third approach is where the transmitter talks to a receiver distant from the train and the receiver in turn talks to a power pack; the power pack controls the train through the track. Such a system requires no modification to the locomotive but it limits operation to one train (unless the railroad has separately controlled sections or blocks). I recently put together a system using such a method. It offers radio control at minimum cost.

THE TRANSMITTER

The radio portion of the system uses a pair of 49 MHz Radio Shack TRC-505 (catalog #21-405) FM walkie-talkies. They cost $24.95 each. We must modify the transceiver so it is always in the transmit mode but actually only transmits when we send new control information. Fabricate a 2 1/2 by 4 1/2 by 1 3/8 inch housing for the controls from sheet styrene. Secure it to the front of the transmitting walkie-talkie. I used black electrical tape.

The main throttle control is one pole of a DPDT momentary flip switch (Radio Shack #276-709). I located the switch an inch from the bottom of the control housing. The down position is "GO" (accelerate) and the up position is "STOP" (brake). The SPDT momentary center-off reversing switch is 2 1/2 inches above the bottom of the housing. I used a rocker type switch from my spare parts box but another Radio Shack momentary flip switch would work. Orient it to operate from side to side rather than up and down.

Finally, 3/4-inch below the top of the housing is a Radio Shack (#275-1547) SPST momentary push button. It serves as a PANIC button to stop the train without momentum.

The system uses Dual Tone Multiple Frequency (DTMF), or "touch-tone", signals to transmit control information. Such signaling is virtually immune to interference and "glitching" effects. My own radio controlled command control system has transmitters with actual telephone key pads. The keys function as follows: 1=accelerate; 2=forward; 4=brake; 5=reverse; 8=panic.

I retained the same DTMF signal assignments for the radio controlled throttle system we are building. A TP5089 tone dialer chip (ECG #1690) generates the DTMF signals. Mount it, along with the other necessary components, on a piece of circuit board in the control housing. Disconnect the microphone in the transmitting walkie-talkie. Then feed the output of the tone dialer chip (pin #16) into the walkie-talkie circuit board, where the microphone had connected, by means of a 10K resistor and a 10K trim pot (to set the tone modulation level).

Disconnect the black battery lead from the circuit board and connect it to the control switches. That way, the walkie-talkie will be on only when you operate a control switch. Connect a jumper wire across the push-to-talk switch so the walkie-talkie always will be in the transmit mode. Adjust the modulation level by listening on the companion walkie-talkie with its volume control set to a moderate level. Advance the modulation trim pot while actuating any of the control switches until you hear the tones just starting to distort; then back off the trim pot just a little.

THE RECEIVER

Modifications to the receiving walkie-talkie are much more simple. Since it will usually be in a fixed location and away from the elements, we may power it with a 9 volt battery eliminator (Radio Shack #273-1552). A 3/32-inch closed circuit jack (Radio Shack #274-292) mounts on the side of the receiver, near the bottom. Wire it so, when you insert a plug, the speaker turns off and the receiver output feeds to the jack. Make a patch cord to connect the receiver output to the throttle unit from a pair of 3/32-inch phone plugs.

HOW THE THROTTLE WORKS

The throttle unit has a DTMF receiver chip and a 74LS138 decoder chip to convert the DTMF signals to logic signals the throttle can understand. The DTMF receiver chip may be hard to find. I used a United Microelectronics Corporation UM9203 but UMC has now gone out of business. Some UM9203s have shown up at electronic surplus outlets. Crystal Semiconductor Corporation CS202 and CS203 chips are similar. Connect an LED to pin #14 of the DTMF receiver through one section of a 74LS00 quad nand gate (as an inverter) to indicate when the unit has received a valid command.

The throttle is a pulse width momentum type. It uses half of an LM393 dual comparator chip as a triangular wave generator. The wave's frequency determines the throttle pulse frequency. You may adjust that frequency with a 100K trim pot. The generator's output feeds into the negative input of the other half of the comparator. The comparator's positive input receives the d.c. voltage developing across the 1000 µfd capacitor. The voltage builds up when the capacitor receives an accelerate signal. It falls when the capacitor receives a brake or panic signal.

The accelerate signal (DTMF #1) appears on pin #14 of the decoder chip. The accelerate signals feed to another section of the 74LS00 also serving as an inverter. The brake signal (DTMF #4) appears on pin #11 of the decoder and the panic signal (DTMF #8) appears on pin #15. The 25K trim pots set the acceleration, braking, and panic rates.

The comparator literally compares the triangular wave with the d.c. voltage. When the d.c. voltage is low (slow speed), the comparator looks at the top of the triangle and, since the triangle is narrow, it puts out narrow pulses. When the d.c. voltage is high (fast speed), the comparator looks at the bottom of the triangle and so puts out wide pulses.

The 1K MIN trim pot applies a d.c. voltage to the capacitor just above the top of the triangle wave and the 4.7K MAX trim pot limits the voltage to the capacitor to a degree just allowing the comparator to put out a continuous d.c. pulse. The comparator output is just a series of pulses, always of the same voltage, but with the pulse width (on time) proportional to the d.c. voltage on the capacitor. The pulses from the comparator control a power transistor switch, the TIP120. It delivers pulses of sufficient power, through the reversing relay, to the track and, thus, to the train. The setting of the 4.7K TOP SPEED trim pot controls the output of the LM317T adjustable voltage regulator and the trim pot's setting determines the amplitude or voltage of the pulses. That, in turn, sets the top speed of the train at full throttle. The pulse frequency trim pot setting allows good low speed control without excessive buzz from the locomotive motor.

The forward signal (DTMF #2) appears on pin #13 of the decoder and reverse signal (DTMF #5) on pin #10. Those signals go to the two remaining sections of the 74LS00; those sections work as an R-S latch. The latch output operates the reversing relay by means of a 2N2222 driver transistor. An external a.c. or d.c. 18 to 24 volt supply powers the throttle. Current capacity should be at least 2 amps.

A full wave bridge rectifier in the throttle facilitates using a.c. or d.c. of either polarity. A 7805 regulator supplies the 5 volts the DTMF receiver, decoder, comparator, and NAND gates need. A 7812 regulator supplies 12 volts for the reversing relay and transistor driver. I had a 12 volt DPDT relay. If a 5 volt DPDT relay were available, the 7812 would be unnecessary.

FINAL COMMENT

I assembled the throttle unit on a circuit board in a small plastic experimenter enclosure. Terminals on the side connect the throttle to the power supply, track, and to the jack for the radio receiver signal. I had many components in my junk box, but I estimate the cost of buying everything new would be slightly more than $100. In my opinion, that is a modest price to pay for the ability to move freely with your train.