Reed Switch Slot Car Lap Counter

Hey folks
So I went old school…4511, 4029, 556 and a couple of reed switches under the track for slot car lap counter. Everything worked perfectly in testing phase until I actually powered up the track and raced cars with my boy…suddenly the display was flicking through numbers totally randomly.
I assume this is from em field generated by dc motor noise??? (can someone confirm or offer other hypothesis). I tried decoupling both sides of the reed switches but that didn’t help??
Anyway this is only for interest as I’ve gone and replaced (test run this afternoon) the reed switches with infrared triggers but I’d still like to know why it didn’t work.


Hi Roger,

How were you testing it? My guess is that the track itself is creating the magnetic fields that are triggering the sensor, the current to the cars will have to travel to them through the track this field will also be fluctuating due to the emf feedback from the motors.

That’s exactly what I thought Clinton. I tested on bench top with a single section of track and manually pushed the cars back and forth over the reed switch. I tried decoupling the tracks as well but no good.

Yeah, I would say it is the current in the track causing the switching, even DC electric current will make a magnetic field so decoupling won’t stop it triggering. The field is proportional to the current in the track which would be proportional to how hard the motors are working so every time you speed up or slow down it would trigger.

As an interesting side note the larger the capacitor the more of the changes it removes, so in theory you could add a capacitor to make it field constant enough but it would result in the car taking a really long time to change speed.

Well now you have me thinking! That might be a cool effect. Slap a full farad cap on there and you’d have gradual acceleration!!

With regard to the EMF … I have seen other people use the reed switch for this…I wonder how they overcome it. btw I do have a solid (full second or so) debounce happening through the 556 monostable. Anyway I’ve decided to go IR…that should be less clunky!

Could be down to the reed switch or position, I imagine the field from the motors would be a lot bigger than the one from the current so moving the switched further from the conductor may make it work.

Just be careful with large capacitors, not that I know but they are a nightmare to clean up if you pop them… the dog would never stay in the same room with that robot again.

This link to the British Slot Car Racing Association may give you some different ideas. They have been counting laps for years.

Thanks Robin. So I’ve gone and replaced the reed switch with ir leds above the track and photo diodes and npn transistors to trigger the 556… Once again all works perfectly when the track is not powered up. When I actually race the cars it is still (pulls much hair out) false triggering.
Nowhere near as bad. I have noticed it is much worse with some cars than others and mostly happens when they go over a ‘crossover’ section (in which the contacts are momentarily separated from the track).
There are NO sparks and I even setup the counter circuit to run off battery so that it was 100% isolated. I really can’t see how the emf from the car track can be triggering IR receivers!
Any bright ideas as to what I have got wrong would be so well received (particularly by my boy!)


My guess is that is probably RF radiation causing the problem. The motors in the cars are probably brushed motors, and very very noisy, more than the brushes to the track. A few things to try:

A small ceramic cap (.01 - .1 uf) to shunt spikes on the power supply.

Shielded cables on the connections to the IR LEDs and transistor, with the shield grounded.

Put the electronics in a metal box, and ground the box. (Aluminium foil can be used as a starting point if you ensure nothing shorts out)

Ferrite beads on the connections to the IR and transistor.

Hope this helps.

Thanks Robin…I was thinking shielding might be needed. I don’t have much experience with it but it is next on my list. I just didn’t think the RF would trigger so much, particularly with an isolated circuit. It definitely is to do with the motors as some of the cars, the slower ‘crunchier’ ones are spiking it much more often than one or two nice smooth running ones.
I might wrap the board in insulator than grounded metal box and decouple the power supply and see how I go.

You can always see if you can source new motors for older cars. The brushes are probably worn, and the rotor coated in carbon. The hardest part of this is getting a pinion gear to fit the shaft and gears on the car.

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And of course, complete brain fade on my part.

Arc suppression caps fitted to the motors themselves. Can be hard to do with the sometimes extremely limited space inside the car chassis.

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I thought about that and may try it…some caps across the pickups on the cars to keep those spikes down!


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Good recommendations, thank you.

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