Building your own single transistor controller.


Why a single transistor?













The vast majority of slot racers will use an electronic controller of one sort or another. The main reason for this is that an electronic controller will allow you to adjust its’ characteristics to suit the car you are driving.

Most British built controllers use a variation on the circuit outlined by Chris Frost in his article on the BSCRA web site. These controllers usually have two transistors connected as a "Darlington Pair". This means that the transistor that handles the power to the car is effectively controlled by the other transistor. This combination works very well but has one major drawback; the difference between full power and just below full power is at least 1.2 volts. Consequently, a quick corner on a raceway style track may well be too tight to take flat out, but the next step down on the controller doesn’t give enough power.

To combat this problem of too large a bottom step, it is possible to use a single transistor. With a single transistor, the bottom step is reduced to a minimum of 0.6 volts. Controllers such as the Pro 2 use a single NPN transistor. The problem with this type of set up is that the power flow through the transistor is backwards compared to the PNP transistor used in British circuits and you have to have a separate set of brake and full power contacts to turn off the transistor when braking or applying full power. NPN transistors were used because it wasn’t always easy to get hold of PNP types that were capable of handling the large currents that may flow in the event of a short on the track. After a little searching on the Internet, I’ve found a suitable transistor, A PNP transistor with a type number of MJ14003. This transistor is capable of handling a constant 60 amps – more than enough if you fit a 30-amp fuse as circuit protection.

The circuit used is almost identical to the conventional British circuit, except that the Darlington pair is replaced with a single MJ14003. The values of the resistors in the resistor chain and the potentiometers that control sensitivity and choke are also changed to lower values than you would use in a standard British set up. The values are reduced to enable the controller to be more stable when the transistor heats up. I’ve given a list of parts plus prices and suppliers. You will see that the cost of parts for this controller is less than 100. However, the build time is several hours. So if you are willing to spend some time and care doing the build, you will have a top of the range controller for a very attractive price.



What you need


Also from SCD :-

3 pin 2amp plug  (MK) - 3.00

3 pin 2amp plug  (BG) - 2.00

Transistor MJ14003  at only 7.50

I’ve sourced all the electronic components from mainstream electronics suppliers and as a consequence, I may have paid over the odds for some of them. For items such as fans, if you have a local computer market, you will find that fans are much cheaper from there. At my local market, I can get a fan and grill for under 4. This is still good value if you have to pay an entrance fee to the market! Also, some of the items above (machine screws resistors etc) are sold in packs of 50 or 100. This will still leave you with stacks of them after you have built your controller. It’s always a good idea to carry a few spares with you. I used a Parma Turbo kit as the basis of my controller. To keep the cost down, you could use a Red Fox handle. This is slightly cheaper than the Parma, but maybe isn’t quite as well made. You would also need to ensure that the resistor was electrically isolated from the brake and full power contacts if you use this handle, otherwise you could blow the 10 amp fuse or operate the full power relay prematurely. Whichever handle you decide on, the parts cost under 100. You pays your money and you takes your choice!


Starting to build.

Assembling the resistor board

The resistance on the board of this controller is so low that you can actually do without a conventional resistor board. On my controller, I used a Parma 25 or 35 Ohm H.O. plus resistor instead. There are two main advantages to this set up. Firstly, it’s easier and cheaper than building a resistor board; secondly, the controller will have virtually no steps.

If you still want to use a conventional board, check the continuity with a meter once you have finished all the joints. The total resistance from one end of the board to the other should be roughly equal to the sum of the individual resistors present in the chain. There may be some slight variation because of the manufacturing tolerances in the individual resistors. If you have used a conventional board, it should look similar to the picture below.


Assembling the Diode Brake Selector.

First bend the selector switch contacts as shown in the picture and remove contacts 10, 11 and 12.

Using the supplied tagged washer set the switch so that you can select any of the contacts 1 to 9 but not 10, 11 and 12. Attach the red wire coming from the brake contact on the controller handle to position 1, and a black wire to the central common contact. The other end of the black wire will go through the handle into the transistor/relay box to the 10-amp brake circuit protection fuse. Between each pair of contacts 1 & 2, 2 & 3, 3 & 4, 4 & 5, 5 & 6, 6 & 7, 7 & 8 and 8 & 9, connect your diodes. Each diode should have the silver band at the left hand end (or anti clockwise end) when you view the switch from the contact side with the diode you are looking at in the 12 o’clock position.

If you need any further guidance about building this assembly, check out the AB Slot sport web site at

The finished brake unit should look similar to the picture below.


Assembling the Adjustment Unit

Next you need to mount the sensitivity pot, the choke pot and the diode brake kit onto the vertical side of a piece of aluminium angle (I got mine from B&Q). I’ve attached mine in the following order from left to right as you look at the control knobs, Choke, Sensitivity and Brake. This ensures that I can easily find the choke control during a race without having to take my eyes off the track. This angle needs to be bolted onto the top of the resistor guard on your Parma Turbo handle or onto two 90-degree brackets on the side of the Red Fox unit. You can make the brackets from the same aluminium angle. Make sure that you smooth any sharp corners off the angle. Attach any wires you need onto the pots and switches at this stage, and make sure that you know which wire goes where! The finished adjustment unit should look like the picture below.


Assembling the Transistor and Relay Box.

There are no setting controls on the box. The box houses the transistor and fan mounted on the outside and the fuses and relay inside. The transistor is mounted on the outside of the box with the base and emitter pins going through to the inside of the box. The connections to the transistor, fuses, fan and relay are all made inside the box. Next, take the fan protection grille and use it as a template to mark out where the fan mounting screws will be fitted. You should drill holes for the fan mounting screws, transistor terminals & screws and a small hole for the fan wires to pass through inside the box. You should also drill holes in the box endplates for the cables to the handle and the plug at this point.


Put some heat shrink cover over all wires where they pass through the holes to prevent them from being chafed. If you wish to fit LED’s to indicate track power and/or relay operation, do this now.

The fan should sit directly over the transistor as shown in the picture.

You will need to seat the fan on 4 small threaded spacers in order for the fan blades to clear the top of the transistor.


Final Assembly

Assemble the controller handle, resistor or board and adjustment unit. Wire all the parts together using the wiring diagram below as a guide. Note that there are 5 wires coming from the controller handle. 1 from the brake band, 1 from the choke pot, 1 from the sensitivity pot, 1 from the full power contact and 1 from the wiper. The wiper and brake cables need to be fairly heavy gauge (I used the ones supplied with the controller kit. The other wires can be lighter gauge. I used flexible Teflon coated lead wire. Use heat shrink sleeve to protect the wires where they go into the transistor box and controller handle, and use tie wraps to bind the wires together every 6 or 7 cm between the handle and box.

To make sure you have the transistor wired up correctly, take a look at the diagram. Notice that the connector pins are slightly below centre. With the transistor held with the pins facing towards you and below centre, the left hand pin is the emitter, the right hand pin is the base and either of the two fixing holes can be used as the collector.


Your controller is now ready for testing.

I’d like to pass on my thanks to Chris Frost, for filling in the numerous gaps in my knowledge of electronics. I really couldn’t have done it without him.

Paul Bucknell