USB Gamepad Slotcartimer - Ir-lotsensor

Introduction: USB Gamepad Slotcartimer - Ir-lotsensor

About: I studied Design and Context, Education in the Arts and Fine Arts (Department of Photography) at the Academy of fine arts Vienna. Have been teaching 10+ years since then. I love sailing, kayaking, canoeing ...

This is the second variation of my slotcar timer interface. Where the gamepad was triggered by a "deadstrip" the first time I am now using an IR-slotsensor to trigger the gamepad.


The third and refined version can be found here:


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Step 1: Partlist

  • TCST 2103 ir-slotsensor

  • Darlington transistor BD679 NPN

  • 5 V/DC Reed relay

  • 10kOhm and 100Ohm resistor

  • z-diode 5V LED

  • (optional but good for testing)

This will set you back some 4-6€, depending on where you source parts.

I am leaving cheap and going for a "perfect" solution.

Step 2:

Here is the circuit.
The LED is optional but you can test your circuit with it.

Step 3: Things to Improve...

I am not sure at this point how accurate the hardware will detect the cars.
It will probably work good with 1:32 cars but have a hard time detecting 1:43 scale cars.
When I pull a toothpick through the ir sensor they are not always detected. 1:43 slotguide pins will probably not get detected.

The reed switch closes very fast and works rather good.

Ideas how the stretch the switching would really be appreciated.

I also bought an opti coupler, this it the next part I would love to test.

Of course the slotsensors would have to be installed in the slot.

I am just learning electronics for some days now so please excuse any stupidity I may display writing about this vast topic.

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    2 Discussions


    4 years ago on Introduction


    If the pin is 2mm in diameter (I have no idea about slot car pins), it will block the light for the turn off time (8µs) or longer, at a velocity of up to 450km/h (125m/s).
    With a pin of 1.1mm the max. velocity will be 12.5m/s
    The above is determined by the 1mm aperture of the slotted optocoupler (SOC). Models TCST2202 (0.5mm) and TCST2300 (0.25mm) can be substituted for even higher velocities and/or smaller pin diameters.

    I think that your main issues stems from 2 things.
    With R1 being 100 Ohm, the emitter gets around (5V-1.25V)/100 Ohm = 37.5mA and as such, will likely swamp out high velocity narrow objects.
    I'd recommend going as high as 390 Ohm for R1, for a shade under 10mA into the emitter (The emitter will be happier as well then).
    R2, on the other hand, is way too large for swift switching of the transistor (probably why you went with a Darlington, which isn't really a good choice here).
    The current transfer rate (CTR) of TCST2103 is 20% (according to its datasheet) and this means that (within its limits), the internal transistor will allow a current of 20% of what you feed to the emitter.
    With your present 37.5mA, the transistor can sink ~9.4mA and R2 would be better changed to say 1kOhm. As is, the external transistor will see a base current of only 450µA (still no reason for a Darlington though).

    That said... Just forget what I said and try this for optimizing all parameters (make it faster, less power hungry and substantially cheaper), by loosing all of the circuitry to the right of the SOC and change R1 to 390 Ohm. It's set to 220 Ohm in the schematic, but go with 390 Ohm (allow me being to lazy to change it :)

    The output from the SOC connects directly to the game pad, emitter to common ground of the game pad and collector to the other part of the "fingers".
    Please verify that the trace I marked (on your own pics), is actually connected to common ground (the black wire usually), but I'll be surprised if this is not the case.


    Reply 4 years ago on Introduction

    Thanks a lot!
    I will definitely try that.
    My perception is that the problem is USB polling rate at 200HZ. The trigger might be to short to be recognised by USB. A better/more expensive gamepad could have built in some way to make sure every signal it senses gets relayed to USB...not in this cheap thing.
    I also wanted to be able to use the sensor circuitry with a wide range of sensors one being a "deadstrip" where I would lose galvanic isolation without the optocoupler.
    My new take on this can be found here: