Help combining 2 555 circuits

Discussion in 'The Projects Forum' started by webtracks, Jun 23, 2011.

  1. webtracks

    Thread Starter New Member

    Jun 14, 2011
    3
    0
    Hi all, I'm new here and I hope someone could help me out with my problem combining to simple 555 circuits. Sorry but I have very little knowledge on electronics and am just learning (already burned a couple of components trying to solve the problem :D).

    Anyway, I managed to get hold of a simple 555 alarm and a 2-tone siren (attached gif) from the web. Overall, the alarm (circuit A) should trigger the siren (circuit B). I was able to complete the circuit using a relay but have some questions.

    1. Is there an alternative to using the relay like making use of a transistor?
    2. Circuit A is triggered by a normally open switch (SW4), is it possible to modify the circuit and use normally closed switch as trigger?

    Appreciate any help you can give me and thanks in advance.
     
  2. kkazem

    Active Member

    Jul 23, 2009
    160
    26
    Although I probably wouldn't have designed the ckt the way you did, I'll ge glad to help you, but you've got to answer some Q's for me about the ckts or if you don't know, let me know that as well.
    1) CKT-A: what is it's purpose, I don't understand? Is it simply a timer to start and then shut-off the siren after some seconds or minutes go by or does it have another function?
    2) What's the specific purpose of SW3 in CKT-A. IS it simply sort of acting as an on-off switch or is it supposed to do something else?
    Answers for you so far:
    1) Since the output of a 555 is high when triggered and the RESET\ will inhibit it when held low, you can probably take the two RESET\ signals from CKT-B, tie them in parallel (they are basically digital signals), then just tie them to the CKT-A Output. You should need no relay. When CKT-A is triggered and for as long as it's programmed to be in the 'On' or 'high' state, it will enable the siren and when it times-out, the CKT-A Output pin goes low and takes both CKT-B RESET\ inputs with it, which should disable the siren when CKT-A is not triggered. Originally, I was thinking you needed to invert the CKT-A output signal by using a 2N7000 MOSFET or even a 2N2222 bipolar, but you don't need the inversion and since the RESET\ from CKT-B are high-impedance inputs, the CKT-A Output will easily control it.

    I await your reply and feel free to ask additional questions.

    Good luck,
    Kamran Kazem
     
  3. webtracks

    Thread Starter New Member

    Jun 14, 2011
    3
    0
    Thanks for the speedy reply. To answer your questions:

    Circuit A is a simple one-shot alarm. Output is initially low when powered up. Closing SW4 triggers the 555 set to high output and stay high even if SW4 is released.

    SW3 resets the 555 for the output to go low.

    Anyway, I tried your suggestion and it worked like a charm. Thanks.

    Hope I'm not pushing it though but I find the siren output too low so I added another circuit to amplify it (CKT-C attached). However, I can hear a hum through the speaker when I power up or when circuit B is not triggered. Anything I can do to remove the hum?

    Sorry if my circuit looks like I've just thrown bits and pieces in and connected them (which is what it is) but like I said, I'm just starting to explore.
     
  4. webtracks

    Thread Starter New Member

    Jun 14, 2011
    3
    0
    Wow I can't believe I found the solution. Placed a 100uF across pin 6 and 4 of the LM386 and hum is gone.

    Thanks again for the help.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I am quite familiar with the 555. The hum is an artifact of your power supply, what is it, a wall wart? Simple regulation should cure that.

    You mention a relay, but there is none shown on your schematic. I assume you are using a relay to drive a siren that is driven by line voltage. You need a swamping resistor if you are using a relay, something similar to this...

    [​IMG]
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I believe you will find the 555 much louder than a 386. It has much higher drive. Try connecting this to the ouput of the 555, pin 3.

    [​IMG]
     
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