I have talked a little about an electronic controller both here and in the 1/43 forum. Some have asked me to post schematics so they can try it out. I am posting it here. Please feel free to use these to make your own but I would ask you not to sell them. I have put a lot of time in in the last year to design this and some follow ons and plan to produce them. With that prenote now the fun stuff.
I started thinking about this after I got into the hobby about two years ago and started with 1/43 cars, then added HO Tjets and magnet cars. I then got an Artin 1/32 set. I enjoy racing all the cars. My personal favorites are the 1/32 and the Tjets. I really liked the 1/43 size but controllability wasn’t as good. Another problem was that I wanted to replace the stock controllers. I did get five Parma controllers 45-60 Ohm. This worked ok but for the Tjets a 125 Ohm controller works much better and the 1/43 Artins need a 7-15 ohm resistor. This meant to race all my cars and tracks I needed many controllers with different resistors. I thought of going with a diode controller but you still want to modify them depending whether it is an 18 volt HO setup or a six volt Artin setup. I also happen to be an Electrical Engineer with some experience in analog design so I started thinking of how to do a version with more control.
What I have come up with is a circuit that you hook up to any resistor controller (from 10-200 ohms) and can run any car (well the high powered ones need a really good heat sink) but at least any RTR car and a lot of others. It can be modified even for wing cars. The adjustability is outstanding. You can adjust the top speed and the starting speed. The schematics I’m posting today have these two adjustments. It feels a lot like a diode controller as it controls the voltage to the track.
I have working at home but not yet ready to post one with brakes. You get brakes even with a non-brake controller. With brakes there are two adjustments one for sensitivity and the other for active braking. Active braking means that the car is actively decelerated when you slow the car down on the controller. Some people like it, some don’t. I have a PWM version that is much cooler especially for the high powered cars but it is still in development. The last thing to add is the response curve. I would like to be able to make it linear or exponential (like an audio taper pot). The exponential gives more control at low speed for road courses. I then have a digital version in the early design stage that lets you save the settings for different cars and controllers.
For those who don’t like to build from scratch I will have a circuit board in a couple of months followed by a kit and then the assembled version.
So back to this design. Here is the schematic. The URL is to a larger schematic that is easier to read but bigger to download.
To make this controller just hook up the componenets on a breadboard or proto board. The pots can be mounted in a project box to give a finished look. I don’t have a very finished look yet so I don’t have any photos.
The basic controller uses three op-amps of a quad op-amp IC LM324 available at Radio Shack. It also needs a PMOS FET. This can be ordered from Mouser or DigiKey or JameCo. The 324 op amp is also a lot cheaper from the above sources than from Radio Shack. Get a good heat sink as well.
The first two op amps feed a current through the controller resistor and measure and amplify the voltage. The two potentiometers control the top speed of the car and the starting speed, or speed you get on the first band of the controller.
The third op-amp and the PFET make up a unity voltage gain buffer that provides lots of current to the car. That’s the simple description.
Here’s a little more in depth. The first op amp creates an appx 4 Volt reference using a regular diode (any will do). The 4 volts is applied across a voltage divider circuit consisting of the controller resistor and the 1K resistor. This creates a voltage that is at 4 Volts when the controller is pressed all the ways and is zero when released. The potentiometer adjusts the gain and adjusts for different controller resistances and also adjusts the starting speed. This voltage is then sent through the second pot to provide top speed adjust. The output feeds into the amplifier/driver section.
The 39K, 10K resistor and 10 uF cap on the output divide the output voltage and filter it so it is in the same range as the input. The third op amp compares the voltage from the input section (the desired voltage) and compares it with the output voltage. If they are not the same it adjusts the drive to the PFET to turn it on or off more. It is a nice simple circuit but the results will surprise you.
One not about the feedback. I use a diode based reference at the first rather than just a voltage divider because this makes the control voltage (the reference) mostly independent of the supply voltage. This means if the power supply voltage droops as an unregulated supply voltage will, the output you get to the car does not change. It also helps in sizing the resistor that is in series with the controller resistor. 4 volts allows us to use a standard ¼ W resistor instead of a bigger one. To match the output voltage we divide it by four. This circuit will work with 20 volt Ho supplies and six volt Artin 1/43 supplies. It also means that you can have one supply for a track that runs 1/43 and 1/32 and the max speed plays a double role as an adjustable voltage. You can set the max at the main supply and everybody can adjust their own.
Please feel free to e-mail me questions as slotfarm_at_spotten.org (replace _at_ with @) or post questions here.
Warning. This is a fun controller and you will be hooked and want to make them for all your cars. In about a month I will post the schematics for using a second quad 324 and an NMOS FET to add brakes.
You should make sure you put a fuse in-line because the PFET can conduct much more than 2A.
Hope you enjoy and tell me how it works. I’m going to work on refining it now.
Slot because we love slots and farm because we live on a one-acre mini-farm.