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Circuit checking?

  1. Aug 19, 2008 #1
    Don't know if I can post this sort of thing here - apologies if it is not permitted/in the wrong place!

    But just for fun I'm trying to build a light-activated buzzer.

    Function:
    Light goes on, voltage at the divider of the first circuit passes threshold for Schmitt trigger (U1), causes trigger to go high and activate the transistor/relay (Q1), which closes the second circuit and causes the buzzer (R4) to go off - the buzzer should keep making noise until the system is reset.

    Questions:
    Will it work the way I expect it to?
    Will Q1 explode with a setup like this? (Or can I get a Schmitt Trigger thingy that regulates the current...?)
    Can I use one battery to power both circuits, or will I need to use 2?


    Attached is the circuit diagram - unfortunately I couldn't find the symbols I wanted, so here is a legend (also reproduced in the image)

    R1: Photoresistor
    R2: Variable resistor (to make adjustments for light)
    R3: Variable resistor (to make adjustments for the resistance of the buzzer)
    R4: Buzzer

    U1: Schmitt Trigger*
    Q1: Transistor/relay (activated/deactivated by the Schmitt Trigger)


    *Not sure how to get one of these... I found a 7419 chip that functions as a Schmitt Inverter or something - can I just use one of these?
     

    Attached Files:

  2. jcsd
  3. Aug 19, 2008 #2

    berkeman

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    Staff: Mentor

    It's certainly difficult to read the circuit drawn that way. First, use one DC power source -- no reason to have two in this circuit. Second, you need to show the power and ground connections to the comparator (and be explicit about where your ground connections are in the rest of the circuit). Third, you need to show the full comparator circuit, with the input bias circuit and the positive feedback hysteresis circuit.

    Can you fix it up with what I've mentioned, and post again?
     
  4. Aug 19, 2008 #3
    Thanks for the quick reply - I'll try my best :)

    1. Fixed (sort of... not exactly in standard format)
    2. Can I represent ground connections in the rest of the circuit by having wires go into the negative end of the power source/battery? Sorry, not certain of the standards for this :(
    3. The comparator, is sort of a stand-in symbol for 'anything with a Schmitt trigger', because I couldn't find the correct symbol anywhere in my circuit drawing software. I've done a pantomime in MSPaint

    Hopefully this is more clear!

    :)

    I'll also try to be more explicit with the questions:
    1. The voltage from the divider should be at the lower threshold when the photoresistor is on, so I'd have a high signal out of the Schmitt... Sorry to ask a basic question, but what type of transistor is 'on' when the input voltage is high? :(
    2. Uh... will it work, or will it explode on me? :)
    3. Can I replace the transistor with a relay? In my limited experience, transistors tend to explode on me...

    I think I'll try to prototype this tomorrow when the stores open again and I can pick up the components, so any help you can give would be greatly appreciated :) :)


    -V
     

    Attached Files:

  5. Aug 20, 2008 #4

    berkeman

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    Staff: Mentor

    The simplest way to do this is with an NPN transistor as a low-side switch to turn on the buzzer. There is no reason to use a FET in this simple circuit, unless you're bored with BJTs and want to experiment with FETs.

    I can post more tomorrow (it's late here). Pick up some 2N3904 and 2N3906 NPN jellybean transistors when you're at the store tomorrow, in addition to whatever else you are picking up. What comparator are you getting?
     
  6. Aug 20, 2008 #5
    Thanks for the tips - I'll give it a try and hope nothing explodes on me ;) I don't really have a proper power supply though so I'll have to make do with a 9V battery...

    For the schmitt trigger, I ended up grabbing a 7414 hex schmitt inverter.

    I may run out of alligator clips for this (my components don't all fit on a breadboard... darn) but I'll give it my best shot :)

    Thanks a lot Berkeman! :)
     
  7. Aug 20, 2008 #6

    berkeman

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    Staff: Mentor

    Do you have a 5V linear regulator to convert 9V to 5V for the 7414? Even a 74HC14 has Vcc max of 6V. a 74HC14 will trigger nominally at Vcc/2 at its input, but that's not how you normally use a 7414. To do the comparator function, you would usually use a real comparator like an LM139, and use positive feedback to set the hysteresis.

    Oh, if you do use the 7414, be sure to ground the 5 unused inputs.
     
  8. Aug 20, 2008 #7
    Thank you so much for your help - I kinda got it to work!

    Unfortunately, instead of switching between 'on' when there is light and 'off' when there's very little light, the buzzer kind of alternates between 'on' when there is light and 'quiet but still on' when there isn't.

    Looking at my circuit, I would expect an 'on-off' thing, right? Or does the transistor give me a whole spectrum, and I have to switch to using a relay?

    Thanks again! You guys (well, Berkeman, so far :P) are (is) awesome!!!

    -V
     
  9. Aug 20, 2008 #8

    berkeman

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    Staff: Mentor

    Are you using the FET or a BJT? Can you post your present circuit, showing the voltage regulator and all connections?
     
  10. Aug 21, 2008 #9
    It looks like this atm:

    I tested it by connecting the input of the 7414 straight to ground, and the buzzer correctly did not turn on at all. I think the problem is with my photoresistor, but not sure :( Will, alas, need to buy another photoresistor.


    -V

    PS: Thanks for your help :) I'll be sure to post another diagram at the end if it works... just in case somebody else reading this thread is having similar problems :)

    Edit: Currently using a 1.5V battery so it doesn't make that much noise when I test it - but I get more than enough noise with a 3V, so that's good :)

    Edit2: I actually started with a 9V battery, but my chip exploded. No real reason to mention this, but I hope it gives you a good laugh :P
     

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