Help with circuit

Discussion in 'General Electronics Chat' started by lil_buckeroo, Jan 29, 2013.

  1. lil_buckeroo

    Thread Starter New Member

    Jan 29, 2013
    6
    0
    Hello, this is my first post. I am having a problem solving a circuit. The problem is that I’m not sure what is happening with the IC chip. I am 75% sure that it’s some sort of timer chip. Normally, the reed switch is closed and there is no voltage to the piezo siren. When the reed switch opens, the piezo siren emits an oscillating alarm. Oscillation is approx. 3-4 Hz. Any insights as to what is going on with the chip? I have included a drawing of the circuit that I am mostly sure is accurate. If there is some glaring deficiency, please point it out. Having said that, based on the yield of the circuit, does the drawing even look plausible? I did the best I could to make the drawing connections to the chip in the same order as they are on the board. Any ideas?


    Circuit diagram:


    http://www.flickr.com/photos/dryweave/8427498013/in/photostream/lightbox/
     
  2. SPQR

    Member

    Nov 4, 2011
    379
    48
    Hi, and welcome.

    Well, it's a small chip, 8 wires, some of them connected together, a resistor, but no capacitor.
    My first guess would have been a 555 timer, but without the capacitor, I wonder.

    Can you give us more information about the origin of the drawing?

    Can you give us the numbers of the pins?
     
  3. lil_buckeroo

    Thread Starter New Member

    Jan 29, 2013
    6
    0
    Thanks for the quick response!

    It is a small chip. The circuit came out of a little, battery operated $2 window/door alarm. When the window/door is closed, a magnet makes up the reed switch. When the window/door is opened, the magnet is removed, and the reed switch opens, sending voltage to the siren. I have a couple of these little units. One circuit does use a small capacitor connected to the transistor. However, this circuit does not use a cap yet the outcome is the same. The mystery is how it happens. I don't know the chip pin-out. I am hoping for some higher wisdom to help me solve the puzzle. Ultimately, I would like to discover the type of chip that makes this circuit work and the pin-out. If I have left out any obvious details, please let me know. I created the drawing by pulling apart the circuit. Thanks again.
     
  4. SPQR

    Member

    Nov 4, 2011
    379
    48
    By looking at the chip you can tell us the pinout

    One end of the chip will have a dent in it, or a little divot in it.

    The pin on the left or the pin nearest the divot is #1.

    1_____8
    2_____7
    3_____6
    4_____5

    Then all the pins are numbered as above.
     
  5. lil_buckeroo

    Thread Starter New Member

    Jan 29, 2013
    6
    0
    In all cases you are right. There is a small dot on the chips to indicate pin 1. My little problem is that the chip has a protective shell. I am sure you're familiar with the type. It is a shiny, black, dome shaped deposit of material over the chip. I haven't found a way to remove the material without catastrophically affecting the chip. I figured that I could draw out the chip, show where the supply voltage is pinned, show the ground pin, and the rest would fall into place. But, I can't see it with a standard 555 pin-out. The chip is very small, much smaller than the DIP 555. I would guess the dimensions might be 4-5 mm length.
     
  6. SPQR

    Member

    Nov 4, 2011
    379
    48
    Then it might be a custom IC specifically done for the manufacturer.
    And if it's a $2 alarm, with thousands of copies, they probably did a custom job.
     
  7. lil_buckeroo

    Thread Starter New Member

    Jan 29, 2013
    6
    0
    True, true. It might be impossible to make this circuit with shelf parts. Nevertheless, how is it done? The circuit seems very simple. However, I am having a hard time imagining a solution. I am hoping that someone smarter than me can see it.
     
  8. SPQR

    Member

    Nov 4, 2011
    379
    48
    While waiting for input from the experts, let's state what we know:

    1 - 5V supply - so probably TTL components
    2 - 470k resistor - might vary the tone or time to onset of the tone, or time to offset of the tone.
    3 - reed switch - controls flow of current to an internal transistor
    4 - NPN transistor probably driven by the out from an internal transistor
    5 - the transformer needs 2 more connections - are they hidden to ground?

    Is it still working?

    If yes, you could put a resistor in parallel with the 470 to see what happens (change in tone vs change in time to onset of the tone)
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I have a similar device that I bought at Harbor Freight for a couple of bucks. I don't think it would be hard to duplicate, but I am sure that it would cost more than buying more at HF.

    What do you want to do with it?
     
  10. lil_buckeroo

    Thread Starter New Member

    Jan 29, 2013
    6
    0
    SPQR:

    Your determinations are correct. Also, the circuit does still work. Here is some interesting news: I have one, older circuit with a four-post transformer (the circuit also uses a capacitor). The circuit that I have posted uses a transformer but it only has three posts. One post goes to the chip, the other two go to the siren. One of the posts that goes to the siren is also tied to the collector of the transistor. I did try another 470k resistor in parallel with the on board 470k resistor and the effect was a faster siren cycle (I am guessing the cycle doubled in frequency), but not a noticeable change in tone.

    tracecom:

    Agreed, it would be easier to buy more alarms if I was needing more alarms. But, I have found a convenient use for this circuit in another device. It would be great if I knew what was happening with the chip.

    Thanks both!
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I am curious about what's under the "blob," too. Frankly, I think there's no reason for it to be complicated. The functions that the device performs are very straightforward and don't require a high powered device. All that's really required is a driver for the piezo; the reed switch and the magnet provide all the "logic" needed.
     
  12. lil_buckeroo

    Thread Starter New Member

    Jan 29, 2013
    6
    0
    tracecom,

    Are you able to see a functional solution without an IC? Perhaps there is one. But, my eyes can't make it work. Please let me know if you can assemble a circuit with the submitted components that works (Reed switch closed, siren off. Reed switch open, siren on). It seems simple enough, right? It's killing me!
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I had incorrectly assumed that the reed switch was opened by the magnetic field which removed power to the circuit. However, I have confirmed that you are correct: the reed switch is closed when the siren is off. More thought is now required. :)
     
Loading...