Simple signal delay circuit - For homemade IR laptimer

Discussion in 'Electronics & Electrics' started by Azrael, Sep 18, 2006.

  1. Azrael

    Azrael Member

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    Ok, so instead of forking out big bucks ($600 or so) for one of these: http://www.aimsports.com/products/tg/index.html I have gone the cheapo route and decided to build my own.

    Sofar im using a cheap DSE stopwatch with lap function:
    http://dseau.resultspage.com/search...13bed82743c0a87f9c0723&method=and&isort=score
    And an Infrared detector from Jaycar:
    http://www.jaycar.com.au/productVie...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

    And ill connect the two together with a relay so that the IR detector can trip the lap "button" on the stopwatch.

    However, my issue isnt so much with the triggering but the fact that on any given track day there are normally 5-10 people out with laptimers and each will have their own IR beacon down the pit straight. The way it will be connected now the IR detector will trip the lap button multiple times when i drive down pit straight, which obviously isnt what i want.

    What i want to do is create a small delay circuit for the signal from the IR detector to the relay. I had first thought of using a 555 timer in a monostable trigger, but thatll mean fabbing up a breadboard etc to set it up.

    So can anyone think of a simpler way of doing this?
     
  2. LethalCorpse

    LethalCorpse Member

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    EDIT: Never mind - irrelevant.

    I need to go to bed now.
     
    Last edited: Sep 19, 2006
  3. other ed

    other ed Member

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    Hi,
    my 2 cents,

    1) I had a 555 in my old car once, the spikes from the ignition sent it haywire and made it unreliable.
    2) What if your beacon is not the first one in the pit straight? You will be getting triggered by the other beacons before yours. (assuming the delay you want is to mask the later beacons??)
    3) I'd be looking more into setting up a matched pair, so the beacon is flashing at frequency X and the reciever does not send the trigger to the stopwatch unless it sees a beacon flashing at frequency X.
    4) You might be able to get the (3) effect from an old vcr. Remote with stop button stuck down for example and find the wires on the vcr board that come on at the right time. Although this would need the right voltages, and this would involve heaps of unused circuitry.

    Good Luck, I'd be intrested to know what you come up with.

    Cheers,
    Ed.
    :)
     
  4. OP
    OP
    Azrael

    Azrael Member

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    This is exactly what the delay is for. Basically a 2s delay will be enough to mask all the timers down pit straight and therefore give me a reasonably accurate laptime.

    Pulse modulation is a pain because with a limited viewing angle its common to miss the pulses for various reasons and get either no or spurious triggering.
     
  5. other ed

    other ed Member

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    Can you use a capacitor/resistor to turn on a transistor? I remember one in a DSE project years ago. The resistor slows the cap charging for the time delay. Then the right voltage is on the transistor base it turns on iirc. prolly need 2 transistors for enough grunt to switch a relay?

    Cheers,
    Ed.
    :)
     
  6. OP
    OP
    Azrael

    Azrael Member

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    You mean like this:
    [​IMG]

    Sure, i could do that, but the capacitor wil only discharge when its at capacity, and im not sure if the brief pulse from the IR detector will be enough to trip the circuit.
     
  7. blakeyboysmith

    blakeyboysmith Member

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    doesn't really help you with the electronics but i am pretty sure you can pick those up for less than $600..

    and there are some decent second hand units floating about that may be resonably priced... most likely with more functions than simple times.
     
  8. Dr feelgood

    Dr feelgood Member

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    does it have to be IR, can you use sonics or another light spectrum?
     
  9. OP
    OP
    Azrael

    Azrael Member

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    Why not IR. I already have an IR detection circuit, and the timer. Just need a delay circuit.
     
  10. dakiller

    dakiller (Oscillating & Impeding)

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    I've tried drawing up a schematic of something that will do it but it is getting a little more complex than it should, using a handful of resistors, capacitors and diodes.

    Use the 555 circuit.
     
  11. OP
    OP
    Azrael

    Azrael Member

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    I was thinking of using a pair of relays.... ill keep thinking about it some more, but the 555 does look like the way to go.
     
  12. switchblade327

    switchblade327 Member

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    Guys, be wary of using relays...they can generate enough back EMF to blow your watch to Kingdom come :)

    What u need is a simple switch debounce. See which uses simple components http://www.all-electric.com/schematic/debounce.htm

    This might work but would be dependant on what your watch needs to trigger a lap event i.e. does the pushbutton force it low or high etc

    Alternatively, Jaycar do a Flexi Timer Kit which you could use to do the initial trigger then 'blanking'. Needs 12-15V but I reckon by swapping the relay for something a less brutish for your application, you may be able to get away with a 9 VDC battery. Don't forget to 'clamp' the output to rid yourself of EMF problems.
     
  13. OP
    OP
    Azrael

    Azrael Member

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    Hmm, interesting, ill be looking into those.
    The laptimer just needs two pins to be connected, as the switch itself is merely a reed contact type.
     
  14. OP
    OP
    Azrael

    Azrael Member

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    This kit seems to do what i want it to do. The 555 is triggered by a voltage input of some sort (havnt checked it yet) and then triggers a buzzer at the end. Ill just use the voltage output from the buzzer to trigger the laptimer via a relay.

    Should be pretty easy :D
     
  15. Caffeine

    Caffeine Member

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    Having designed and built lap timers over a few years, it's a very common problem you face.

    It's completly bizarre how 5 or 10 people will set up identical beacons in a line down the pit straight. Everyone triggers off the first beacon and the rest are redundant.

    I toyed with using 56khz IR receivers instead of 38khz, but then youd have to build your own beacon.

    Bear in mind also that most beacons (AIM etc) aren't straight 38khz IR, there is some modulation on that IR so that the in car units can distinguish between the beacon and other sources of IR (sinlight, fluorescent lamps etc)

    From memory the AIM beacon modulates the 38khz IR at 5ms intervals or thereabouts.

    The Jaycar units you're showing there doesn't have any sunlight filtering. You'll be getting multiple false triggers every time the IR receiver is exposed to sunlight.

    I achieved what I wanted with a vishay TSOP36238 receiver, an Atmel ATTiny13 microcontroller with some filtering code. It's pretty reliable now.
     
  16. dakiller

    dakiller (Oscillating & Impeding)

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    I thought that it was modulated at 35-40khz to stop it getting triggered by other light sources, why would you need to modulate that further again?
     
  17. Caffeine

    Caffeine Member

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    It is, but all IR receivers (including the excellent vishay ones with sunlight filters) will still false trigger when exposed to sunlight (exacerbated by movement)

    I've had laptimers setup on cars at wakefield park in the early afternoon, and they would trigger off the beacon, and then again as they passed from the shadow of the main control tower into sunlight on the main straight (the beacon was setup mid track so the dead time had expired)

    The modulated 38khz carrier comes out of most IR receivers as a square wave, which can then be measured by the micro to make sure it matches the expected modulation.
     
  18. OP
    OP
    Azrael

    Azrael Member

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    Yeah, i think ill have that modulation problem myself, but for a combined total of $40 sofar im not going to complain too much. Normally im timed using the Dorian anyway.
     
  19. dakiller

    dakiller (Oscillating & Impeding)

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    Hmm, interesting.
    I've been playing with a IR receiver, the one Jaycar sells, using it to interface a remote control to a micro and I haven't seen this phenomenon yet, I should try it out a bit more
     

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