SA- NE555 timer anomoly

Discussion in 'The Projects Forum' started by lost_bro, Aug 11, 2014.

  1. lost_bro

    Thread Starter New Member

    Mar 17, 2009
    Good day all!

    I have recently completed an automatic water pump control circuit for a solar powered water pump.
    It will sense water levels (with remote sensor, copper wire) and run the pump according to the water /sensor levels.
    Also it has a -run dry- protection feature that will shut off the pump if the -run dry- sensor loses contact with the water.

    OK, I designed and built the circuit using some -old- SA555 T.I. chips that I had on my bench from maybe 5 or 7 years ago. And the circuit runs as expected.

    Now, I ordered from China some 'T.I.' NE555 chips and to my surprise the timer delay function for the -run dry- feature no longer works as it does with my 'old' T.I. chips from many years ago!!!

    Basically, the observed problem involves Q1 and D1 circled in RED. Q1 should not be conducting when the start up timers is active and counting.... (when the start up delay timers expires then Q1 begins to conduct.).
    With the new Chinese NE555 chips , Q1 conducts ALWAYS except for when I remove the Ground to D1. Then the circuits runs normally as with the 'old' T.I. SA chips. So, the path to ground to bias Q1 is thru D1 with the Chinese chips. I don't understand because if that is the case then why does Q1 NOT bias thru the D1 ground when using the old T.I. SA chips?

    So, I put the old 'T.I.' SA555 chip back in the circuit and all functions are a GO.

    So, my question is: Can there really be that big of a difference in these common 555 chips to make my circuit STOP functioning? :confused:

    Sorry about the hand drawn circuit. Didn't have time to LTspice it.

    I would appreciate any and all commentaries, probably I am overlooking something very obvious here.....;)

    Thanks in advance, just let me know if anyone requires more info.
    Last edited: Aug 11, 2014
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    You either forgot to 'burp' your Tupperware or the chips are bad/counterfeit/designed differently than your old chips.
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Your schematic shows pin 5 disconnected which is a big no-no. :eek:

    Try putting the recommended noise stopping capacitor from pin 5 to ground. The value is not ciritical, just a small greencap will do fine.
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Sounds like pin 3 isn't going to full supply V+, i would put two extra 1N4001 diodes in series with Q1 base, or put a 1K resistor across its B/E terminals to help shut off..
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    I agree with Dodgydave. A resistor across the b/e pins should help.
    It won't on any 555 (unless it's a CMOS type).
  6. lost_bro

    Thread Starter New Member

    Mar 17, 2009

    Good Day Alec & Dodgydave

    Hit the nail on the Head!!!!
    Scoped pin#3: almost 1 volt difference (less) with Chinese NE555 than with old T.I. SA555.

    Not all chips created Equal!!!!!

    So, placed 1k between B & E on Q1 and now BOTH chips work fine!!!!

    Thanks for the assistance, you guys rock......

    Last edited: Aug 12, 2014
    Alec_t and Dodgydave like this.
  7. lost_bro

    Thread Starter New Member

    Mar 17, 2009

    Hello The_RB

    Yes, you are correct, forgot to show that cap on the diagram.....
  8. lost_bro

    Thread Starter New Member

    Mar 17, 2009
    Good day GopherT

    Yes, after burping the tupperware _and_ adding a 1k between B&E on Q1, all works fine...... never forget to burp the tupperware;)

  9. takao21203

    AAC Fanatic!

    Apr 28, 2012
    I'd say the old chip is out of spec.

    And you also used it out of spec.

    quite complicated circuit.