1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

False triggering of a 555 timer

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by jacbk6121, Mar 6, 2017.

  1. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,474
    Likes:
    191
    Location:
    Lancashire UK
    I would do it something like this.
    pump switch.jpg

    I would recomend runing this circuit straight from the battery and putting the regulator between this switching circuit and the motor. That way you will not have the cuurent taken by the regulator draining the battery all of the time. Almost any logic level N channel mosfet should work.

    Les.
     
  2. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Les, thanks for the sketch. Can you walk me through it step by step, what happens when the low level switch kicks in, and then when the high level kicks in. I'm a bit too new at this to catch the intricacies. Above the mosfet I see a coil and a diode, is that coil an inductance and what is its purpose. Also there is a NO contact shown, is that part of a relay and if so what energizes the relay?
     
  3. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,474
    Likes:
    191
    Location:
    Lancashire UK
    When the low level switch closes it will connect +6V to the top of the relay. As the mosfet will be switched on at this time by the +6V to the gate via the 100K resistor the relay will pull in and it's contacts will close. this will connect +6V via the diode to the top of the relay which will hold it in when the low level switch opens again. The relay contact also connects +6 volts to the motor. The diode is so that the reed switch does not try to directly power the motor when it first closes. When the top level switch closes it shorts the gate and source of the mosfet which switches it off. this causes the relay to drop out. During the time that the top level switch is closed the circuit will be drawing 60 uA. If the circuit was modified so the top of the 100K resistor was connected to the top of the realy coil instead of the +6 volts rail the circuit would draw no current except when the pump was running.

    Les.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0

    Ah, very ingenious. Thanks
     
  6. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Les, I like the idea of putting the regulator between the switching and the motor but I don't think I can get the manual switch to work in that configuration. I'm going to test this shortly.
     

    Attached Files:

  7. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,474
    Likes:
    191
    Location:
    Lancashire UK
    With your circuit in post #85 the manual switch will not work as yoy want it to. When pressed it will set the latching circuit on and it will remain on until the top level switch closes. You need to add a diode (1N400x series.) between the negative end of and the junction of the motor and manual botton. The negative end to D1. There is no reason why moving the regulator and it's support components to between the switching circuit and the motor should not work. You could also reduce the value of the 220 uF capacitor if you want to. Also I think from memory that the 0.1 uF capacitor should be larger. CHECK WITH THE DATA SHEET.

    Les.
     
  8. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    How's this?
     

    Attached Files:

  9. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,474
    Likes:
    191
    Location:
    Lancashire UK
    I think that should do what you want.

    Les.
     
  10. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Great! I'm giving it a whirl.
     
  11. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    I've described my understanding of the logic just to make sure I wasn't missing anything.
     

    Attached Files:

  12. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,474
    Likes:
    191
    Location:
    Lancashire UK
    You description fits the way I designed it. All you need to now is to build it to make sure that I have not overlooked anything.

    Les.
     
  13. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Les, the switching works fine, however the man. switch is affecting the mosfet starting the switching cycle; it's a recurring theme. I tried putting a diode/capacitor around the motor, no help. I tried a 220 microF cap. after the regulator, no help.
     

    Attached Files:

  14. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,321
    Likes:
    1,231
    Location:
    Cardiff, Wales
    Try adding a capacitor (say 100nF - 1uF) between the gate and source of the FET.
     
  15. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Sorry, I have some wiring issues around the relay, I'm checking it out.
     
  16. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Les, it works, lovely, switching is good, man. switch works as expected. I had the body of the relay in reverse, lot's of connections he he. I'm going to let it cycle for a few hours to see what the battery life is like.

    Les, thanks very much for your help and everyone else on the forum for you input.

    Cheers,

    Jacques
     
  17. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Les, instead of a relay would a mosfet do the job, might be more reliable and consume less power?
     
  18. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,474
    Likes:
    191
    Location:
    Lancashire UK
    I did think about an all electronic solution but rejected the idea due to your insistence on using a totally unsuitable power source (A PP3 battery) and what I suspect is your poor construction techniques. I have seen your failed attempts a trying to get the 555 timer circuit (Which worked first time when I built it.) to work here and on ACC with help from some very knowledgeable people. You can get 5 volt relays that only take 40 mA and the relay only consumes power when the motor is running. You can save about 60 uA drain when the high level reed switch is closed by moving the connection on the top of the 100K resistor as I suggested in a previous post. Your LED will probably be the most significant power drain (about 14 mA) which will be on all the time. You could try using a CD4013 driving a power mosfet but I think you would have problems with it. With the existing circuit you could use a cheaper mosfet such as a 2N7000.

    Les.
     
  19. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
  20. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Well! As my prototype advances I have realized the that 2 level switch system will not work.

    Two level switch system
    As the water evaporates there will be insufficient water in the bottom reservoir to fill the top reservoir to the point where the top-level switch turns the pump off. The pump will keep pumping until the battery is dead.

    Single switch (bottom) level switch system
    The single level switch system operates on a timer. When the bottom level switch is activated the pump starts and stops after a certain time, approximately 24 seconds. As the water evaporates the total amount of water in the system will decrease. The top reservoir will only be partially filled as at one point the pump starts sucking air. At which time more water has to be added.

    The single level switch system was working except that the manual switch was triggering the timer, a situation for which I could not find a solution until now, possibly.

    I have attached the schematic that was working with the timer but had the problem manual switch and another with my solution. The solution depends on whether a small multi-contact switch is available which is a big if I think. But maybe there is a better way...

    Jacques
     

    Attached Files:

  21. jacbk6121

    jacbk6121 Member

    Joined:
    Mar 6, 2017
    Messages:
    64
    Likes:
    0
    Woops, no this won't work, the MOSFET has to be activated. I think I just need to move the switch across the MOSFET
     

Share This Page