ENHANCED LOCO CONTROL ON A BUDGET by Mike Jeffries

Mike Jeffries provides some improvements to the LED dimmer control systems discussed in our March issue.

In Garden Rail No 320, Ray Waters described using an LED dimmer to provide radio control (RC) of a locomotive but noted the range was limited to around 20 feet. As he says, the system offers ‘cruise control’: the transmitter is only active when you’re pressing a button to change your loco’s speed, and so this is the only time the loco needs to be within range of the transmitter; the rest of the time it will carry on doing whatever it was last told to do. This being the case, we don’t need a huge range, but even so, any improvement we can achieve seems desirable.

The Controller was designed to drive LEDs whereas we are using it to drive a motor – a much (electrically) noisier device. This noise finds its way into the Controller’s electronics, reducing the range, but don’t worry – simply adding a couple of decoupling capacitors should reduce the noise.

Figure 1 shows the insides of the Controller and Figure 2 shows the two capacitors soldered in place. One is connected across the battery terminals on the printed circuit board and hence should have a voltage rating at least as high as your battery voltage, while the other is connected across the voltage regulator circuit’s 5V output and hence can have a rating of 6.3V to save space. A steady hand will be required to solder the positive end of this latter capacitor to the regulator transistor.

If you don’t feel up to adding the capacitors yourself, Model Engineers Laser (www.modelengineerslaser.co.uk) is planning to offer controllers with them already fitted, subject to demand, and so I suggest you give the new owner, Edward Parrott, a call.

The Controller includes a built-in antenna as shown in Figure 3, but it’s a simple matter to add an external antenna for maximum range. This should be a quarter of a wavelength long – 173 mm at the unit’s operating frequency of 433MHz (not 2.4GHz as Ray said).

As well as the capacitors, Figure 2 also shows the point to which the antenna is soldered - the end of the surface-mount capacitor at the top right-hand corner of the board. To disconnect the internal antenna, cut the track on the bottom of the board at the end nearest the tiny hole.

When mounting the system in your loco, try to keep the antenna as straight and vertical as possible.

Combining both the capacitors and the external antenna, I managed a range of between 20 and 35m, depending on the particular loco I was using, which should be enough, after all, how often do you want to change the speed of a loco when you are further away from it than this?

3V locos

One of the nice features of the system is its tiny size, which makes it eminently suitable for small, low-cost 3V locos. However, the voltage range quoted for the Controller (5-24V) makes its use on such locos look unpromising.

Fitting six 1/3AA (or AAA) NiMH cells into a standard two-cell battery holder solves this; the resulting 7.2V is high enough for the Controller to work properly and won’t blow up your 3V motor because the (mean) voltage that appears at the motor is under your control – and it takes up the same amount of space as your original batteries. It also provides your loco with a bit more grunt for curves, gradients, etc.

The heading photo shows the system mounted on a 3V chassis from Phil Sharples. Just for fun, I ran this chassis on my test track for a few circuits at the full 7.2V. This gave a scale speed approaching 40mph, and at the end of it, the motor was only slightly warm to the touch.

By the way, if you model at 16mm/foot (i.e. SM32 or SM45) to estimate the scale speed of a loco, count how many seconds it takes to cover a yard length of track and divide the result into 40: for example, if it takes four seconds, it’s doing a scale 10mph (with a calculation error of less than 3%). Handy, eh?!

Figure 4 shows how the Controller is connected into a loco, i.e. between its existing on/off and forwards/reverse switches. If you only have a combined forwards/off/reverse switch, you’ll need to add a separate on/off switch; the receiver part of the Controller still draws about 10mA from your loco’s battery even when you’ve switched off the electronic speed control (ESC) part by pressing the red button on the remote (Incidentally, because it turns the ESC on and off, this button provides an ‘emergency stop’ function).

Variations

The design of these little units seems to change fairly often: in the five years since I wrote the articles in 16mm Today, the hardware has changed at least twice and the firmware inside it a number of times. If your unit doesn’t look like Figure 1, drop me an e-mail (via the Editor) and I’ll try to help you out.

One recent change seems to be the minimum operating voltage the Controller will work at (despite the label still saying 5-24V). Ray said he has operated down to 4.8V, but one of my correspondents has had trouble even at 6V. If you do too, I suggest you try the 7.2V approach described above.

Another change is the ‘pairing’ of each unit’s transmitter with its Controller. With the earlier systems, each Controller would only respond to the transmitter supplied with it, but later systems seem to have lost this feature, despite the claims of the eBay listings.

In a future issue, Mike will be showing us how to build a circuit board that adds directional control and consistent lighting to the cheap RC unit.

Figure 1: The view inside the Controller after removing the white plastic sleeve and unsoldering the battery connections; those on the left go to the motor. The regulator transistor is the three-terminal device immediately to the right of the big silver thing (the crystal).

Figure 3: The underside of the board. The internal antenna is the track going from bottom right across the top of the board to the top left (and not connected to anything when it gets there); the cut to disconnect it is at bottom right.

Figure 2: The 470uF capacitors soldered onto the board. Make sure you get them the right way round – the end of the capacitors marked ‘–’ or with a thick white line goes to the battery negative connecting pad. The positive end of the larger 470uF capacitor (10V in my case) goes to the battery positive connection and that of the smaller (6.3V) to the bottom righthand termination of the regulator transistor. The 173mm long external antenna is the yellow wire soldered to the end of the capacitor in the corner of the board at top right.

Figure 4: Connecting up the controller. Note the suppressor capacitor soldered across the motor; if yours hasn’t got one, add 0.1uF.