555 dual timer astable oscillator beeper.

Discussion in 'The Projects Forum' started by brian2037, Apr 3, 2011.

  1. brian2037

    Thread Starter New Member

    Apr 3, 2011
    3
    0
    I have completed a beeping circuit with a dual 555 timer, from the 555 out to speaker, I feed the base of a transistor NPN tip 31 and feed 12 volts directly into the speaker to the transistor then to ground. From the out of the off/on astable side of the 555 I feed a tip 31 to short the circuit on/off to the speaker to get the type of beep duration I want. As I turn up the speaker power with a 5k POT, the frequency of the beep speeds up as the speaker takes more power from the circuit. I have a 2000uf cap in the circuit. Not sure why the beep frequency is increasing and what component to use or where to place it to keep the beep frequency the stable no matter what speaker volum is:confused:.
     
  2. magnet18

    Senior Member

    Dec 22, 2010
    1,232
    124
    ...
    You might want to post a schematic, they really are worth a thousand words.
     
  3. brian2037

    Thread Starter New Member

    Apr 3, 2011
    3
    0
    Here is the schematic.
    Thanks for any help.
     
  4. magnet18

    Senior Member

    Dec 22, 2010
    1,232
    124
    From the perspective of this untrained eye, it appears that you forgot to wire your reset B (pin 10) line to Vcc, and for the astable circuit your reset A line (pin 4) should be run directly to Vcc without a capacitor, but one of the more experienced guys may correct me on that.

    You should also have a decoupling cap across pins 14 and 7, and your control pins (3&11) should have small caps on them to Vdd, to help prevent noise issues.

    556s (which is what this is actually called) usually need a cap in series when driving a speaker, but this is for driving it directly, so it may or may not be needed for your setup.

    I'll let bill or wookie handle the more technical troubleshooting part, I'll probably cause more confusion than help.

    [EDIT]
    Make sure your R1 and R3 values are at least a couple K, too low can damage the chip and mess things up.

    [EDIT_2]
    Just so you know, you drew your LED backwards and forgot the ground symbol on the cap attached to R4.
     
    brian2037 likes this.
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You should have a 1.2k Ohm resistor in series with R3, or you risk smoking the 556 timer.
    Likewise with R1; place a 1.2k Ohm resistor in series with it.

    If you accidentally set R1 or R3 to too low of a value, the discharge pins will become overloaded; they'll be trying to short Vcc to ground. You need a minimum of 100 Ohms per volt of Vcc to protect the discharge pin input.

    I don't know offhand why your timer is speeding up.
     
    brian2037 likes this.
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I'll be back with a redraw of that schematic, or at least what I'm thinking I'm seeing. Schematics are critical, and it is important to make them as readable as possible. It is the blue print of the circuit. I'll also make corrections for the stuff Wookie has pointed out.
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    This is your schematic with all the major and minor corrections pointed out. You scheme for beeping the speaker is not the best, especially since the reset pin (which you didn't connect, a showstopper) is perfect for the application.

    [​IMG]
     
  8. magnet18

    Senior Member

    Dec 22, 2010
    1,232
    124
    Bill, It might just be me, but I had a really hard time telling the difference between those colors...
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You should have seen the yellow.

    Here is a revised schematic. Your two transistors were pretty far off, I could not salvage the design.

    [​IMG]

    I really don't like the pots, fixed resistors are much better. As Wookie said R1/R6 and R3/R5 can not be 0Ω or the chip smokes. The reset pin is a digital input, it is usually tied to Vcc. The 555 is extremely predictable, the equations are very accurate. I tend to be one of the guys who know the 555 really, really well, and have been writing articles (some folks call them tutorials) on the 555 for the AAC book experiments.

    The phantom caps are recommended, but not required.

    Here are some resources I provide for the 555.

    Bill's Index

    The 555 Projects

    My Cookbook

    LEDs, 555s, Flashers, and Light Chasers

    http://forum.allaboutcircuits.com/album.php?albumid=41

    A couple of handy references for 555 design...

    http://forum.allaboutcircuits.com/picture.php?albumid=41&pictureid=308

    http://forum.allaboutcircuits.com/picture.php?albumid=41&pictureid=1233

    I used the first one to grab the 556 info to draw the schematic with.

    You will find this site is a really good source for help. If you have homework questions people insist you show you work first (which you did) before answering serious questions.
     
    Last edited: Apr 4, 2011
    brian2037 likes this.
  10. magnet18

    Senior Member

    Dec 22, 2010
    1,232
    124
    Hey, I said it too :p

    of course, I heard it myself from Wookie a few days ago, so I guess you could still say he said it.

    In any case, I found/find this site invaluable.
    http://www.kpsec.freeuk.com/555timer.htm
     
    brian2037 likes this.
  11. brian2037

    Thread Starter New Member

    Apr 3, 2011
    3
    0
    Great input from all. I will now include the resisters in series on all my 555's if I use a POT.
    I will test the schematic from Bill Marsden and see if it replicates the alarm sound I am looking for with the reset option. I have a very specific sound I wanted to replicate and the circuit I have replicates it to the T, except for the beep speed increase when more power is applied to the speaker. I tried some other variations but did not get the result I wanted, but the reset option schematic might do it. Thanks again. I will let everyone know how it works out.
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    There are some approaches to increase the volume, but if you can get it to work the way you want first we'll go from there.
     
Loading...