digital stopwatch trigerred by photogate or ldr

Discussion in 'The Projects Forum' started by rsrojas, Jan 31, 2008.

  1. rsrojas

    Thread Starter New Member

    Jan 31, 2008
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    hi to all! i am a new member of this community and this is my first inquiry. I've come across a circuit using ldr or a phototransistor to automatically switch a device via relay. I've tried to connect a digital stopwatch in the relay so that when the light falling into the ldr is cut off, the relay is activated and the stopwatch is triggered or "start", and when the light is cut off again, the stopwatch "stop". I wonder if there is a circuit configuration that would trigger the "start" and "stop" function of the stopwatch without using a relay.

    Also, if we are going to use this to a physics experiment about motion, to measure time interval between two positions of moving ball, we would need two sensors (ldr or phototransistor). Can I connect the two sensors in parallel?

    Using a digital stopwatch is a sort of an alternative to making an actual timer from a 555 IC.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A relay, being an electromechanical device, is very slow compared to electronic circuits. There are additional problems due to reverse EMF when the current to the coil is started/stopped, and "contact bounce".

    As far as connecting the two sensors in parallel - have you determined whether your stopwatch switch is a N.O. or N.C. connection? Does it cause a positive-edge or negative-edge trigger?
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,576
    231
    rsrojas,

    Are you willing to open up the stopwatch and solder wires to the START and STOP switch contacts? Could you post the LDR detector circuit you are using? I might have what you need.

    Ken

    Ken
     
  4. rsrojas

    Thread Starter New Member

    Jan 31, 2008
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    yap, i've also soldered wires to the start/stop switch of the digital stopwatch, and yes, i am experimenting on using transistor switch now, instead of the mechanical relay. as of now, i have tried only one configuration, the sensor is ldr, and i t connected the stopwatch switch to the emitter-collector of the n2222 transistor. the digital watch did not respond as expected. the display flickered!! Can you share where I should connect the stopwatch's start/stop button?? thanks in advance! Also, can you suggest a circuit where we can use phototransistor as sensor, instead of ldr
     
  5. rsrojas

    Thread Starter New Member

    Jan 31, 2008
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    hi ken! have already done that, soldered wires to the leads of the start/stop button of the stopwatch. somebody suggested to use transistor as switch, interface with the ldr sensor, instead of the relay. needs the circuit though. by the way, i came across the ldr circuit in this site
    http://www.reuk.co.uk/Light-Dependent-Resistor.htm

    reden
     
  6. rsrojas

    Thread Starter New Member

    Jan 31, 2008
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  7. rsrojas

    Thread Starter New Member

    Jan 31, 2008
    5
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    Hi john! I've seen your schematic. the stopwatch i used is really different, it's just an ordinary stand alone digital stopwatch, with its own power supply (a single 1.5V button cell) used in ordinary applications. I'll try to adapt your design. Thanks!

    reden
     
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