Prob. with discharge or reset ?

Discussion in 'The Projects Forum' started by brozizds, Feb 14, 2012.

  1. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    Hi guys,
    Im having a prob. with my old timed circuit. I've designed a PCB from Bill Marsden's sch. here http://forum.allaboutcircuits.com/showpost.php?p=281377&postcount=26 . It seams like cap. Ct is not discharging I think. The circuit runs its course (6 mins.)the green led is on at start then at the end the red led and piezo turns on, But when I try to run timer again the red led and piezo turn on again instead of the green led as it should. When I short across Ct it works again. Attached is pix of PCB & Components. HELP Please. Thanks JIM:)
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    You can see the cap discharge with a DVM. One of the neat things about slow circuits like this is a DVM can give real time measurements, which can be very useful.

    Did this circuit used to work?

    Just for the other users, the schematic is similar to this...

    [​IMG]

    Thinking about it, a backward diode on the power supply would discharge everything quicker.

    Anyhow, you should be able to see the capacitor slowly charge up, if it isn't then we need to look elsewhere for a problem.

    Pin 2 should always measure at Vcc. The power up pulse is so short you will not be able to see it. If you take Pin 2 to gnd it will start the timer from scratch.
     
  3. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    howdy Bill,
    Yep that is the basic schematic I used it has been working fine, I have added the 100uf cap across power supply and a .1uf across pins 1 & 8. The prob is it seems like Ct is not discharging because when I short it out the timer restarts fine. :confused:
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Have you tried replacing the timer? Pin 7 may be blown, people have mentioned the possibility, but I've never seen it before.

    If it is, and it turns out to be a repeating problem, we may need to add a resistor in the design. Or it could just be a faulty 555.
     
  5. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    yep change the 555 a couple times. Went over my traces and checked for shorts. the circuit works fine on a proto board though:(
     
  6. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    on my proto board I dont have the 2 extra caps as on the PCB
     
  7. Wendy

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    Mar 24, 2008
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    My next suspect is the trigger circuit. Shorting C1 should trigger it every time. The act of shorting C1 should discharge Ct through pin 7.

    Look close at R3 is totally redundant, it could be eliminated with no effect. So the circuit path for C1 or C1 may be messed up, as maybe R4.

    You definitely have a high (Vcc) on Pin 2? This is provided by R4.

    Then short C1 briefly, this should trigger the timer.

    Looking at R4, it looks messed up somehow, as in a broken lead maybe.
     
  8. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    yea Bill we did eliminate R-3 long ago. I did short C-1 and it does re-set and start again, but when I shut it off and turn it on it goes right to red LED and Piezo which should be at the end of the cycle any other ideas?:confused:
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Here are some things to check.

    When it is triggered capacitor Ct should go to ground. This is the start of the timing cycle.

    Just to confirm, are you seeing a stable Vcc on pin 2 of the 555? This is very important.

    When you short C1 you are shorting directly across C1, on the leads themselves?

    Another thought, discharge times. If the circuit sits for a while does this make a difference? If cold started after a day does it work then?

    R3 (when it was in the circuit) had one beneficial effect, it discharged all the caps in the circuit, albeit slowly. It was why I left it there.

    My current thought is C1 is being charged up, and there is no discharge path. C1 could also be bad.

    You could tack another capacitor across C1 to see if it makes a difference.

    How log did this circuit work before it died?

    I have some other ideas, but I want to give you a chance to do the experiments before I continue.

    The questions highlighted in blue are especially important.
     
    Last edited: Feb 15, 2012
  10. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    OK! yes if I let it sit approx. 10 to 15 secs. it will re start and run fine may be I just didn't notice this with my old hard wired timers lol :rolleyes: thanks for the comeback Bill
     
  11. Wendy

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    Mar 24, 2008
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    Lack of R3 is the cause then. It actually has a job. When the circuit is turned off C1 begins to discharge through R3 and R4, with 1 Time Constant (TC) = 2ms. We're taught in electronics class that 5 TC the cap is discharged, call it 10ms or 0.01 seconds.

    This is actually slowed a little by Ct discharging through the circuit, but even at worst it should only delay things 1 or 2 seconds, and I believe it will be a lot less.

    Let me know if I got it right.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think it's the charge on the 100uF cap across the power supply. Put it across the battery (on the other side of the switch). You might need to add 100nF from vcc to gnd on the IC side of the switch if the battery wires are long.
     
  13. Wendy

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    [​IMG]

    Thing is, the Rt is very large. Much larger that what is shown above. R3 is very small in comparison, so the C1 will only be around 1/10 or less of the Vcc voltage, which also happens to be the Vcc voltage. All of this comes from R3 being in the circuit (which it currently isn't). It is not obvious, but it really is doing something.
     
  14. Ron H

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    Bill, I see your point IF the 555 is CMOS. The voltage divider in the CMOS version won't discharge C1 below a certain voltage, because it is a stack of 6 diode-connected PMOS transistors (at least, that's what is in the TLC555). The resistance of the bipolar 555 divider is only 15k, so it would basically replace R3 as the discharge path.
     
  15. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    Thanks guys,
    Here is a pic of proto bd. versus PCB . The proto bd. works fine while the pcb takes time to discharge :confused: that is what I'm confused over neither have R3 but the proto works perfect and discharges with the red LED and piezo. And if I were to put R3 on the pcb where would it go? Thanks Jim Sorry I'll try to post pic diff.
     
  16. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    New pic . I did just break the wire on the piezo when setting for pic. lol
     
    Last edited: Feb 16, 2012
  17. Ron H

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    Are the 555s bipolar or CMOS? I can't read the full part number on the one on the PCB, and I can't see any part number on the one on the proto board.
    As I mentioned in post #14, if they are different technologies, it might explain what you are encountering.
     
  18. Wendy

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    Mar 24, 2008
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    Way back when we went with regular 555. It is off the shelf tech. I even sent the OP a few.

    I thought I saw a open lead on the PCB, was I mistaken? The interesting parts are behind the cap in the picture.

    In any case, the resistor can go across the power supply anywhere. I'm thinking it is there but with a broken lead or something. The cap is really hiding all that, including the pin 2 pull up resistor.

    It all goes back to R3 or R4, either way.

    *******************************

    OK, just realized there was a top view I had missed. Just add R3 across the power pins anywhere.
     
    Last edited: Feb 16, 2012
  19. brozizds

    Thread Starter Active Member

    Aug 15, 2010
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    yea guys I had to repair a trace in that location between R4 and C1 so it looked funny Bill since I used R4 to get past the small section of trace and replaced R3 and C1. Bill can you be a bit More specific on the location as I will drill and set it where it needs to be. I know you said across power but still newbie:rolleyes: lol. But wow are my PCB's comming out nice thanks to you! But why would my proto board work perfect and reset instantly but not the PCB? This is confusing me! :confused: same parts same layout. LOL Thanks Jim
     
  20. Wendy

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    Mar 24, 2008
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    Just solder a 10KΩ resistor between the + power supply lead and the - power supply lead on the PCB, that will be R3. The exact location is totally not important.
     
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