# DIY Drag Controller



## Weazle (Apr 26, 2021)

I had an old MRC blue 5 ohm controller from my 60's era slot car racing days, that I can use on my newly constructed track. By the way, pix of the track have been uploaded to the Hello From Newby Drag Racer thread.

For a second controller, I decided to have a go at building one myself, based on hints and information I gleaned from an extensive online searches. What I came up with can be seen in the pictures below.










The handle is from a cheep soldering gun which failed after just a few hours of use. It contains two micro-switches that I robbed from a defunct microwave oven. The rear thumb activated button is for rolling thru tire glue and/or staging at the starting line. The trigger launches the car on the green lite.










In the shot above, the components of the "Device Box" are from top right, a 10 amp self-resetting circuit breaker, bottom right a SPDT 30 amp automotive type relay, bottom left a portion of a variable resistor for the roll circuit, and top left a 30 amp stud diode.

The relay and diode work together to give me a two-step launch, to minimize tire spin and help with
consistent launches with hopefully no red lights! I would be happy to explain the circuit for anyone wanting more details.

Cheers, Weazle


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## LTAurora fanLI (May 2, 2021)

Weazle said:


> I had an old MRC blue 5 ohm controller from my 60's era slot car racing days, that I can use on my newly constructed track. By the way, pix of the track have been uploaded to the Hello From Newby Drag Racer thread.
> 
> For a second controller, I decided to have a go at building one myself, based on hints and information I gleaned from an extensive online searches. What I came up with can be seen in the pictures below.
> 
> ...


Cheers. I see you are in the great white north. Still a bit chilly here too. So I have a crude drag race HO set up here. I am interested in more info. I will have to look at your set up. Why a hand controller? I have foot pedals. I feel it’s more realistic for “foot eye “ coordination on this rather than hand eye. I have a tree with 2 foot pedals, timing, red light foul and win light etc. Do you have a need for more than an on off set up? What is the 5 ohm for? Do you have a third wire for brakes? Not nec here but I have seen the speed of some of the larger scale cars online. How long a run off-shut off lane? I will have to look at your pics.
Dan


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## Weazle (Apr 26, 2021)

Hi Dan, thanks for your reply. Hand controllers are the "standard" for lack of a better word, plus it's what I'm used to. You would be surprised at how many drag cars of the full size variety, launch with a push of a button on the steering wheel, rather than stomping a pedal.

As to the "on-off setup" I believe there is something to be gained in having the soft-launch that I get from the configuration. When the trigger is first pulled, the power to the car travels all the way to the hand controller, then back to the track connections (about 12 feet) via the 30 amp diode, which drops the voltage by 0.7 volt. Because the relay is a mechanical device, there is a pull-in delay of several milliseconds, before its contacts close, which applies the full power to the car through a much shorter path (about 2 feet), compared to through the handle, and it also bypasses the diode.

There is a third wire, but rather than for brakes, it just provides the return circuit for the relay's windings. The adjustable 0-15 ohm resistor sets the speed of the car for glue roll-out as well as staging of the car at the start line. It's set at about 10 ohms for my particular car.

My shutdown length is around 7 feet, with the polarity being reversed, to apply the brakes, as long as the trigger is still pulled. If that fails to completely stop the cars, there is a catch net at the very end, just like NHRA does it.

Cheers, Weazle


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## LTAurora fanLI (May 2, 2021)

Weazle said:


> Hi Dan, thanks for your reply. Hand controllers are the "standard" for lack of a better word, plus it's what I'm used to. You would be surprised at how many drag cars of the full size variety, launch with a push of a button on the steering wheel, rather than stomping a pedal.
> 
> As to the "on-off setup" I believe there is something to be gained in having the soft-launch that I get from the configuration. When the trigger is first pulled, the power to the car travels all the way to the hand controller, then back to the track connections (about 12 feet) via the 30 amp diode, which drops the voltage by 0.7 volt. Because the relay is a mechanical device, there is a pull-in delay of several milliseconds, before its contacts close, which applies the full power to the car through a much shorter path (about 2 feet), compared to through the handle, and it also bypasses the diode.
> 
> ...


I got you. Thirty amp draw and as hard as I have seen these larger cars accelerate and I get a two step (stage ) launch with relay control. Guess I should ask what powers each of your lanes. Just aurora txfr here. BTW , I have done some racing way back and am aware of transbrakes and the like-those are usually a release of a button and a mash the go pedal. Top fuel there is a release of a hand brake lever and a stomp the pedal. I have had cars of mine run the quarter mile in great meadows NJ and other tracks on the east coast so I am quite aware of foot brake , trans brake racing etc. maybe I can modify my set up for a button release stage and a pedal mash to tune up racers for the real thing. Good luck with your track.


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## Weazle (Apr 26, 2021)

Hi again, Dan.

I'm a radio amateur, callsign VE3EAR, and have an Astron power supply that was made redundant when I switched over to solar power some years ago. I brought it out of its
retirement to use for powering the track. It has adjustable voltage from 0 to 15, and
is rated at 50 amps, so power won't be a problem.

Each lane has its own dedicated power wire (#10 gauge) directly from the power supply
output terminals up to the circuit breakers and controller attach posts on the starting line backstop. The breakers are aircraft type, with a DC rating of 10 amps.

OK on your experience at drag racing the real things! Some friends had a souped up
'67 Chevy Nova with a 396 engine and four speed manual tranny. I drove it several
times, and believe me, it was a handful. The guys beside us in the pit had a neat
dragster that would be like today's super comp. I got to drive it too, which was a
real thrill for me!

I'll have some more pix of the track, once the surface is painted and the copper tape
put down. It has been bloody too cold here for the last week, and overcast too, to
add insult to injury : - )

Cheers, Weazle


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## Milton Fox Racing (May 27, 2014)

How is the build going?


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## Weazle (Apr 26, 2021)

Milton Fox Racing said:


> How is the build going?


The build is completed and we've been racing on it for most of the summer.
We went with the Dragon timing system and are really pleased with it.
The closest race so far has been 1/1000 of a second ET difference!
The Carrera's ETs of 2.4 seconds and scratch builds of 1.6 or so.

Weazle


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