A little help please

Discussion in 'The Projects Forum' started by kris8583, Oct 20, 2010.

  1. kris8583

    Thread Starter New Member

    Oct 20, 2010
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    Hi all im new to the forum and would like a little bit of advice please.

    I would like to build a timer for a scalextric pit lane track and have found a digital stopwatch with 7 segment display that is perfect for what i need.

    Here is the schematic of the circuit:

    http://www.circuitstoday.com/wp-content/uploads/2008/02/stopwatch-1.JPG

    The problem I have is that i want to start and stop the timer between 2 points and use momentary switches at each end of the track, these would replace S2 (in parallel?) on the diagram. The idea is that when a car passes the switch and makes the contact it starts/stops the timer.

    So how would i change the single on/off switch for 2 momentary switches.

    The timer reset is already a momentary switch so that is not a problem.

    I would like to add a few LEDs to be on or off depending on wether a car has activated the timer, but need to sort the switch first. any help is really appreciated
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    I think an IR LED and sensor triggering a Flip Flop would do the trick. Place an LED and sensor at each end of pit lane. Or you could just use switches to trigger the circuit.
     
  3. kris8583

    Thread Starter New Member

    Oct 20, 2010
    6
    0
    Thanks for the reply.

    I would like to place a microswitch at each end of the pitlane to start and stop the timer. These momentary switches can be activated by the car guide blade pressing on the switch arm and making the contact as it passes over.

    I would use a third microswitch to control the reset function as I would like to keep the time displayed until another car makes entry to the pit lane.

    Because there is one switch that controls the timer on/off by being open and closed I need to keep the circuit "on" when a car presses switch one until it passes switch 2 and turns timer off. I presume the easiest way to do this is with flip flops?

    I was looking at flip flops on wiki, I have a little knowledge and can build a circuit from a schematic, but thats about all. From what I can see would an RS flip flop be what I rerquire, and would i need one for each switch?

    Could you please help a little more and help to show how it would be included in the schematic.

    Many thanks, kris
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Mechanical switches are slow, the contacts bounce, and they will affect the speed of your car.

    If you want it to be a decent design, you really need to use something like an opto-interruptor. These are much like optoisolators/optocouplers, except they have a slot between the IR transmitter and the IR receiver, so that an object passing between the two will change the state of the output.

    You could use one opto interruptor to trigger the SET input of a f/f, and another to trigger the RESET input of a f/f. Have the opto interruptor mounted under the track so that the guide pin passes through the slot.
     
  5. kris8583

    Thread Starter New Member

    Oct 20, 2010
    6
    0
    Thanks for the reply, I am open to options but would need for someone to show how this would be incorporated into a schematic. How would the switch stay in a closed posistion until the car passes sensor 2 and leaves the timed area, as the original schematic uses 1 switch to operate the timer.

    Thanks for your patience as I am very new to this.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ugh - a 555 timer based stopwatch. It won't be very accurate.

    If you want at least SOME degree of accuracy, you'd have to start with a crystal oscillator. For better accuracy, you could start with an oven-controlled crystal oscillator (OCXO).
    Roman Black posted a how-to on making your own OCXO:
    http://www.romanblack.com/xoven.htm
    Of course, it takes awhile for the ocxo to reach a stable temperature.

    A crystal is just the start. Then you need a divide-by circuit to get the frequency down to the range where you can count the fractions of seconds.

    A CMOS or TTL 4060 IC works as both a crystal driver and divide-by counter.
    http://www.fairchildsemi.com/ds/MM/MM74HC4060.pdf
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    We had questions related to sensing RC cars before, if this is a slot car or one with a large electric motor sometimes you can sense presence from the magnetic field - even in passing.
     
  8. kris8583

    Thread Starter New Member

    Oct 20, 2010
    6
    0
    Thanks again for the replies.

    The reason I have decided to use this schematic using the 555 is because I have no programming knowledge and would be even further out my depth that I already am.

    Are there any alternatives that dont involve going any more complex, that a total beginner could get to grips with. I looked online for a kit based stopwatch but could not find any with 7 segment display.
     
  9. kris8583

    Thread Starter New Member

    Oct 20, 2010
    6
    0
    OK so getting back to switches, what would be the simplest way of converting to a momentary switch?
     
  10. kris8583

    Thread Starter New Member

    Oct 20, 2010
    6
    0
    Could I use a latching relay to keep a momentary switch closed to replace S2 and put a second switch in parallell to operate it from the other end of the track to start and stop the clock? I know it wont be very accurate and i would like to keep it very simple.

    Heres the schematic of what i am starting with:

    http://www.circuitstoday.com/wp-content/uploads/2008/02/stopwatch-1.JPG
     
  11. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    On the more direct side a standard NO / NC / COM microswitch, with a lever and possibly a roller attached, could be triggered as you thought, by the car passing the guide blade....... sorry about the overkill otherwise :D
     
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