555 Delay Power Off

Discussion in 'General Electronics Chat' started by coryfoss, Jun 27, 2011.

  1. coryfoss

    Thread Starter New Member

    Jun 17, 2011
    12
    0
    I found this schematic online and figured it would work perfect for my application. I am running a circuit with a 9V and after a set amount of time the circuit and current load on the battery will be disconnected. I breadboarded this circuit exactly as shown, but the load would never disconnect. It was on as soon as the 9V was connected without pressing S1. Did I do something wrong. According to what I read it should work as follows.

    S1 is closed; circuit powers up and supplies power to load.
    S1 is open: 555 starts timing based on RC combo
    At the end of the delay power to load and battery is removed.

    Any info would be appreciated.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Having the base and collectors swapped on either of the right two transistors would cause that to happen.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    I do not think the reset, p 4 , should be left floating; best return it to + temp. supply, p 8. Might add a 10k from diode anode to gnd. to eliminate possible leakage.
     
    Last edited: Jun 28, 2011
  4. coryfoss

    Thread Starter New Member

    Jun 17, 2011
    12
    0
    I did connect pin 4 (reset) high. I will check my connections and add the 10K resistor to see if that makes a difference.

    Thanks
     
  5. coryfoss

    Thread Starter New Member

    Jun 17, 2011
    12
    0
    It looks like I did have a connection issue, the load does now turn on/off with a press from S1, however the 555 doesn't start timing when it a low pulse is sent to the trigger input. It does output out of pin 3 when the low pulse is at pin 3, but once removed it shuts down the output rather than timing out. I have a 47uF and a 10M resistor so that should be approximately 10mins. Any ideas on that?

    Thanks
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    In the interests of sanity and keeping everyone on the same proverbial page, I have extracted the schematic from the PDF file, converted it to a .png image, added reference designators to the components, shown battery and load connections, and added the U1 pin 4 connection to +V.

    [​IMG]

    Now, where did you add the 10k resistor? From D1 anode to ground, or D2 anode to ground?

    What values did you use for R1 and C1? And, what is the voltage rating of C1?
     
    Last edited: Jun 28, 2011
  7. coryfoss

    Thread Starter New Member

    Jun 17, 2011
    12
    0
    I tore everything down are relayed it out on the breadboard. I must have had a bad 2N3906(Q3). Replaced and used 2N2222A in place of the 2N3904's and everything started working. I did place a 10K resistor on pin 3 to ground. I found that when the 555 times out there is still ~1.7V that is on the collector of the 2N3906(Q3)
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    Might be better to put 10k on base of Q3, rather than 555-3. Glad that it is working.
    10M makes me nervous, for 555, I like 2M max with low leakage cap.
     
    Last edited: Jun 28, 2011
  9. Dekster125

    New Member

    Apr 8, 2012
    1
    1
    pls post the working schematic. thanks
     
    ubaid1987 likes this.
  10. Chemdawg

    New Member

    Oct 29, 2012
    1
    0
    I'm just curious, but can someone show me a diagram of this using a 12 volt relay? This is exactly what I want to do.

    I built this circuit and it does not work....

    http://home.cogeco.ca/~rpaisley4/LM555PowerOffDelay1.GIF

    Is there something that is not right here?


    I am using this for a 12v motor I want the motor to run till the switch is open then shut off after a 1 second delay.


    Thanks,
    Pat
     
    Last edited: Oct 29, 2012
Loading...