555 timer output not producing time delay

Discussion in 'Digital Circuit Design' started by jacbk612, Feb 8, 2017.

  1. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    Hello everyone, I am having issues with the 555 timer output, I have included a schematic and a picture of the breadboard. Since there have been a lot of queries on this timer I hope I didn’t miss the solution.

    The pin 3 output will not maintain its high pulse for the duration of the pulse that is set by the RC combination on pin 6 & 7. The pump will run if the switch is kept on but there is no time delay.

    The purpose of the circuit is to start a 6V small water pump on low level of the tank and then run the pump for a time determined by the RC value on pin 6. Low level is determined by a magnetic switch which for the simulation I have replaced by a manual spring return switch. To avoid the problem of holding the switch to long and not having the output generate the pulse I have put some resistors and capacitor on the switch to generate a low pulse no matter how long the switch is held.

    The system works fine with a buzzer substituting for the pump. I have calculated that the pump will draw 240 mW (no info available from the manufacturer). Since the voltage to the pump is 6V that’s 40 mA. The current gain of the transistor is 100 so the current at the base will be 0.4 mA. The voltage at the base of the transistor is 4 (pin3) – 0.7; which means the resistor required at the base is 800 ohms. I suspected that there is not enough power to the pump being suppled by the transistor so I tried changing the resistor on the base to smaller values down to a 100 ohms but this did not work.

    Thanks,

    Jacques
     
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  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    If the LED on the '555 pin 3 lights appropriately then I think the problem is with the drive to the transistor base. Are you really sure about the current drawn by the pump motor as 40mA would be a very, very small motor?
     
  3. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    Hi, yes it is a small motor.It is 1" dia. by 1 1/2" long. I tried measuring the current but the ammeter affects the circuit, I'll see if I can measure it outside the circuit.
     
  4. AlbertHall

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    I suspect it will be more like 400mA than 40mA.
     
  5. LesJones

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    Jan 8, 2017
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    I think the starting current taken by the motor is pulsing the +6 volt rail to a lower level. I would suggest connecting a large electrolytic capacitor between 0V and +6V close to the 555. If this does not work take the pump supply directly from the output of the LM317. Also add a diode (1N400x) in series with the +6V feed to the 555 (- end to LM317.) with this if the motor does pulse the output of the LM317 low the capacitor should supply the 555 for the duration of the pulse. The diode will prevent the pump from pulling power from the added capacitor.

    Les.
     
  6. EM Fields

    Active Member

    Jun 8, 2016
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    In switching circuits like this, it's prudent to saturate the transistor by forcing its beta to 10, which means that if your collector current is 40mA, your base current should be 4mA.

    Vbe(sat) for a PN2222 will run about 0.7 volts, and your 555 is going to be putting out about 5 volts, so your base resistor should drop about 4.3 volts with 4mA through it, which is about 1075 ohms. 1000 ohms is a standard E24 value, and that little extra base current won't hurt anything.
     
    Last edited: Feb 8, 2017
  7. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    Just going to back up a few threads here. I can't measure the current while the pump is running, it seems the ammeter affects the circuit. My terminology is probably not right I'm a mechanical engineer and struggling with DIY tutorials, ect. However, I can measure the resistance which is about 10 ohms, which means that with 6V the current will be 600 mA. I'm going to try to lower the resistance to the base to see if I can boost the current to the motor.
     
  8. LesJones

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    Jan 8, 2017
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    I agree with EM Fields. I had not noticed that the base resistor was such a high value. I suspect that the motor takes more than 40 mA (Even though we have no data to support that.) I would be tempted to go even lower than 1K for the base resistor.

    Les.
     
    Last edited: Feb 8, 2017
  9. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    Just to modify my previous post. I have tried much lower resistors on the base, down to 100 ohms with no success. I will try Emfields suggestion.

    ...and Les Jones suggestion.
     
    Last edited by a moderator: Feb 16, 2017
  10. AlbertHall

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    Does the LED on the '555 output show that the output is triggered high for a period when the button is pressed?
     
  11. LesJones

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    Jan 8, 2017
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    One other thing to consider is the 9 volt battery. Is it large enough to supply the current taken by the motor ? (If it is a PP3 type then it is probably not large enough to supply the motor current without its voltage dropping well below 9 volts.

    Les.
     
  12. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    No luck with Les Jones suggestions but thanks anyways. And I have tested low resistance values on the base which covers EM Fields suggestions.
     
  13. AlbertHall

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    Yes.
    I hadn't spotted that. A PP3 style battery won't supply enough current for the pump.
     
  14. jacbk612

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    Feb 8, 2017
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    Well, I have a circuit with just the pump and the LM317 and it works fine with the 9V battery.
     
  15. EM Fields

    Active Member

    Jun 8, 2016
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    That 10 ohms is what the motor looks like when power is first connected, and that 600mA is called the "stall" or "locked rotor" current.

    The current will drop when the motor gets going, but 400mA is probably a reasonable number with the motor under load.

    That means that with 400mA into the collector and a forced beta of 10, you'll need about 40mA into the base, which comes out to about 100 ohms. But you tried that already and it didn't work. Hmmmm...

    With the motor connected, does the LED connected to the 555 time out properly when you press and release the trigger switch?
     
  16. Dodgydave

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    Jun 22, 2012
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    You've already proved the timer works in your first post, by substitution of the pump for the buzzer....

    So the problem lies with the psu not giving enough current smoothing, i would replace the transistor for a relay, and put a series diode on the 555 pins4,8 supply and 470uF cap, that will stop the re-triggering.
     
  17. eetech00

    Senior Member

    Jun 8, 2013
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    Hi

    There will be an approx. 1.7v drop at the output of the 555, so you'll need to take that into account when calculating the base resistor value and LED resistor value.

    For a quick timing test, disconnect the transistor, then trigger the timer while watching the LED. Does the LED light up for the correct length of time?
     
  18. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    No that's the main problem.

    I have had a relay as you describe previously with the same setup and it was worse, lots of chatter.
    Not sure if this will show up correctly as a reply.

    Yes, it works fine.
     
    Last edited by a moderator: Feb 16, 2017
  19. Dodgydave

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    Jun 22, 2012
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    The chatter will be caused by the psu, not being able to give the current to drive the pump, if the timer works on low current then its Definitely the psu!
     
  20. jacbk612

    Thread Starter Member

    Feb 8, 2017
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    Yes that works fine.
     
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