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Keep relay actuated longer

Discussion in 'General Electronics Discussion' started by scott4957, Dec 30, 2010.

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  1. scott4957

    scott4957

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    Dec 30, 2010
    Here is a circuit that I built on a bread board, it works great. The only thing I need to change is to have the relay stay actuated for about a second.

    The photo transister is very sensitive, so cutting the light for only a few miliseconds will actuate the relay, but the circuit that the relay is setting off for whatever reason has a short delay, and needs that button "the relay" to be pressed for about a second.

    I tried putting a 100uf cap in parallel with R1, and then with Q1, neither worked. Where in the circuit would I place a capacitor to accomplish this goal?

    Thank you for any help, I am very green when it comes to building circuits, so your overly detailed instructions won't offend :D

    Scott
     

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  2. Laplace

    Laplace

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    Apr 4, 2010
    The first thing I would look at is to add another transistor in a Darlington configuration to drive the relay. Add the capacitor from ground to the transistors' emitter-base connection. That way the resistance of the relay coil will be multiplied by the gain of the added transistor when seen by the discharging capacitor. Just be mindful of the reverse breakdown voltage on the first transistor's base-emitter junction.
     
  3. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    I don't know if this is a school project, and you're stuck with existing components or not.
    A cheap solution would be at add an LM555 timer. Check a data sheet or the internet
    for some good circuits. National Semiconductor data sheets usually give you several good
    simple application circuits. You may not need it, but you can get your second, or longer
    with a LM555.
     
  4. scott4957

    scott4957

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    Dec 30, 2010
    No, I don't have to stick to any particular circuit or parts. I'm just starting out as a hobbyist and this was something I wanted to build. Another buddy of my suggested the 555 as well, I will take a look at it and see what that entails. I had hoped that simply placing a capacitor in the circuit would do the job.

    It's basically a laser trip wire, but when you move your hand through the beam quickly, or run very fast through it, it will still trigger the relay for a split second, but the secondary circuit that the relay sets off needs to be closed for a bit longer. Just walking through the beam or even a brisk jog will still set it off.

    Scott
     
  5. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    The circuit does not lend itself very well to adding cap's, being exceedingly simple and not of a very good design.
    Adding a cap across the relay coil is one thing to try. To get anywhere near 1 second delay you'd need 1sec/500ohm=2000uF.
    There is the question however that the circuit may not have the spare capacity to charge the capacitor quickly enough.
    If so I see no option but to add another transistor stage, like Laplace suggests.Then 100uF would be sufficient. It would be simpler than adding a 555 after the relay.
     
  6. scott4957

    scott4957

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    Dec 30, 2010
    I will look into both options, I have no problem changing the circuit. This is more about learning then anything else for me. I would prefer it to be efficient as I would like to be able to run the unit from battery as well as mains power.

    Thanks so much for the tips, if you have any ideas on how to improve the circuit please let me know, and maybe a brief explanation on why one way is better than the next.

    Thanks,
    Scott
     
  7. scott4957

    scott4957

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    Dec 30, 2010
    Another issue with the circuit, it draws way too much amperage. .5 ampers with light on the photo transistor. For the pot I have a fixed 15k resistor. The lower the resistance the lower the sensitivity to light on Q1. So to keep daylight from setting off the circuit I placed the 15k in. I am guessing that current consumption will go down if I raise the resistance on R1. This must not be the most efficent way to build this circuit. Any ideas?

    Thanks,
    Scott
     
  8. Laplace

    Laplace

    1,252
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    Apr 4, 2010
    Something does not make sense there. The more light shining on the photo-transistor, the less current should be drawn by the circuit. The photo-transistor shunts current away from the base of the transistor, turning off the transistor in bright light. If more than 1 ma current is drawn by the circuit in bright light, then that photo-transistor must not be very sensitive at all.
     
  9. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Half an Amp is hardly possible to draw from a 9V battery, and besides something would get very hot; 9V * 0.5A = 4.5W, so I guess you're measuring 8V / 15k = 0.53mA.
     
  10. neon

    neon

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    Oct 21, 2006
    you all seem to forget the other switch the relay that can feed back positive voltage from a cap to keep it alive no matter whatT. i would adda diode across the coil no matter where you put it emitter base collector where does the .5a goes the photodiode? not trought the 2n2222 for sure. except the base. and the photo as bias amplifier needs redisgn including concept.
     
  11. neon

    neon

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    Oct 21, 2006
    I looked up again the transistor is on because the photodiode is hi R . Te light turns R small turning off the transistor probably actually still conducting.It is real bad design . what you need is put the relay in the collector tie the photo to 9 v the pot to ground you will need about 5 ma base current to to turn on the 2n2222. that turn it on from the relay output tie a resistor to the NO TERMINAL put a resistor to 9v 100k and add a delay cap to that point that is your feedbach to get delay it would help a series resistor to ge tbetter on time on
     
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