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SCX Digital Jam Car Circuit

Updated on September 4, 2014

SCX Digital Jam Car Discontinued

The SCX Digital Safety Car had a "Safety Car" mode where the the car races on its own, at a constant speed, changing lanes randomly. This made the Safety Car a worthy adversary when practicing your racing skills by yourself. Unfortunately, the Safety Car has been discontinued and there are no indications on a replacement.

Jam Car History

Ideal came up with Total Control Racing (TCR) in the late 1970s. TCR used HO scale cars on a "slotless" track which allowed the cars to change lanes at the will of the driver. This slotless system gave rise to a new type of car called the "Jam Car" which was a slow moving drone that randomly changed lanes and which both racers had to avoid crashing into while trying for the pole position.

My Idea

Why not take apart an SCX Digital Hand Throttle and modify it so that you can preset the speed and have the car change lanes at intervals to simulate a "Jam Car" of yesteryear? After some thinking and fiddling around I came up with a circuit.

The Circuit

The circuit consists of two sections, one part of the circuit controls the speed of the SCX Digital Car, the other part of the circuit controls how often the lane change button on the SCX Digital Throttle is pressed.The speed of the SCX Digital Car is controlled by the six position switch SW3. I have taken the printed circuit board out of a SCX Digital Hand Throttle and soldered wires between the trigger contact points and the six position switch. The "SCX Digital Throttle common" connection gets soldered to the long semicircle contact on the printed circuit board. The "SCX Digital Throttle position (1-4)" get soldered to the different speed contacts on the printed circuit board in sequence. Caution: when soldering connections to the SCX Digital Hand Throttle printed circuit board, do not hold the solder iron on the connection for too long, otherwise you could cause the surface mount components on the other side of the printed circuit board to become de-soldered and fall off.The lane change part of the circuit consists of a 555 timer integrated circuit that actuates a relay. The relay's contacts are connected to the lane change switch on the SCX Digital Hand Throttle printed circuit board. You have to solder two wires between the normally open relay contacts and the lane change switch located on the SCX Digital Hand Throttle printed circuit board. P1 controls the how long the lane change button is pressed or "On Delay". P2 controls how long between lane change button presses or "Off Delay". D2 allows the change of the duty cycle, Delay On versus Delay Off, past 50%. Q1 energizes relay coil K1 causing the normally open relay contacts to close. D1 prevents counter-EMF voltage induced in the relay coil from reducing the life of Q1. LED1 is mounted on the project case to indicate the current state of the lane change switch. Push button SW2 is also mounted on project case to allow you to manually change lanes and to program your SCX Digital car. Caution, you need a basic knowledge of electricity and electronics before building this circuit. Build this circuit at your own risk! I am not responsible if this circuit breaks your SCX Digital slot car system or slot cars, causes damage to you or your possessions, or if it causes the moon to leave Earth's orbit!

Breadboard First!

When building a circuit for the first time. I like to build it on a solderless breadboard and test the circuit first before committing it to a more permanent medium like perfboard or a printed circuit board. This allows you to make any last minute changes as needed. The solderless breadboard in the picture can be purchased at Radio Shack for under $20.

Time to commit circuit to a Printed Circuit board.

After you thoroughly test the bread-boarded SCX Jam Car Circuit, It is time to commit it to a more permanent media like a printed circuit board. I like to purchase the pre-made circuit boards from Radio Shack so I don't have to fool around with any harsh chemicals when making my own circuit boards. This printed circuit board from Radio Shack with 417 holes can be purchased for under $3.

SCX Digital Stuff on Amazon

Packaging Counts!

Your finished SCX Jam Car Circuit should be packaged in an enclosure with no stray wires showing. Take pride in the device you built! I got this neat enclosure from Radio Shack for under $10. It is made of soft plastic and is easy to drill the holes for exposed switches and potentiometers and indicator lights.

Inside the packaging

Here is a picture of the "SCX Digital Jam Car Circuit" inside the enclosure. I used long screws to mount all of the electronics to the front panel so I would not have to run multiple wires to the case of the enclosure. The only thing mounted to the case of the enclosure is the 9v battery. The perf-board located toward the top of the front panel is the timer circuit. The printed circuit board towards the bottom of the front panel was taken out of the SCX Digital Hand Controller and is wired to a six position selector switch, mounted to the front panel, in order to control the speed of the Jam Car. The original cord attached to the SCX Digital Hand Controller is routed out of the front panel.

SCX Digital Jam Car Circuit Operation

Determine which car you want to be the "Jam Car". Unplug the SCX Digital Hand Controller for this car at the Control Unit. Plug the "SCX Digital Jam Car Circuit" into the Control Unit in place of the SCX Digital Controller. Set the "Delay On" knob, if turned full counter-clockwise the delay-on is about one second, it turned full clockwise the delay-on is three seconds. Set the "Delay Off" knob, if turned full counter-clockwise the delay-off is about one second, it turned full clockwise the delay-off is seventeen seconds. Slide the power switch to the ON position. You should notice the LED on the front panel turn on and off at regular intervals. This indicates the the delay circuit is working properly. Set the speed switch to the desired setting, your designated Jam Car should accelerate to a certain speed and change lanes at regular intervals based on your delay-on and delay-off settings.You can also manually cause the Jam Car to change lanes by pressing the Lane Change switch on the front panel of the "SCX Digital Jam Car Circuit" enclosure. Time to race! Pick up a SCX Digital Hand Controller and try to establish fast lap times while avoiding the dreaded Jam Car.


With a basic knowledge of electricity and electronics. You should be able to easily build my "SCX Digital Jam Car Circuit" in a couple of evenings.

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