555 timer in monostable mode does not turn off

Discussion in 'General Electronics Chat' started by przachar, Feb 9, 2010.

  1. przachar

    Thread Starter New Member

    Feb 8, 2010
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    Hi,

    I am building a motion sensor based lamp that stays on for 10mins and then turns off. My problem is that the lamp turns on when motion is detected but does not seem to turn off.
    This is how my circuit is connected:

    Motion sensor output goes to a transistor which inverts the sensor output (555 monostable is triggered for input low). This triggers the 555 timer and the output of the timer goes high. This output is connected to a relay (through a diode and a protection diode) which turns the lamp on/off.

    I checked the voltage across the capacitor (the R and C of the timer) and it seems to have gone from 0 to Vcc and yet the output has not gone low as it should have for monostable mode.
    Can anyone tell me why this could be happening?

    Thanks,
    Mariam
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Have you seen this?

    Bill's Index

    555 Monostable

    Pin 2 must stay high. If it is low, the timer output will stay high as long as pin 2 is low, even though it has timed out.

    The way to solve this is to use an input conditioner circuit, shown in my 555 Monostable article.
     
  3. andreapg

    Active Member

    Feb 2, 2010
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  4. przachar

    Thread Starter New Member

    Feb 8, 2010
    16
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    Hi,

    I damaged my motion sensor :( so I was connecting Vcc (5v) as the input to the transistor to act as the motion sensor output. I did this just for the slightest moment and then removed the Vcc input to the transistor. So in this case the pin 2 of 555 gets a 0v for a slight moment and then it stays high as you have mentioned. But I still get the same results.
    Also, I am using an LED( and resistor) as my output instead of a relay just for now.

    Any ideas?

    Thanks,
    Preethi
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Show us your schematic. If wired correctly the timer works, but we are working in a vacuum.

    Did you read the link I gave about monostables?
     
  6. przachar

    Thread Starter New Member

    Feb 8, 2010
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    Attached is my schematic.
    I did go through the link and everything seems the same except for the part that ignores further triggers. The switch is closed only for a very short moment to replicate the motion sensor output. All voltage sources are 5V.
     
  7. andreapg

    Active Member

    Feb 2, 2010
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    I've never used a motion sensor, but if you connect as the input to a transistor to act as a motion sensor the schematic may be the following.

    When the start button switch in open, pin 2 is on the high level; pin 6 is on the low level, because discharge switch (pin 7) is closed to the ground.
    When I press the button, pin 2 goes to the low level, than the output (pin 3) will be energized. Discharge switch is now open, but pin 6 is closed to the ground by the capacitor, so it is on the low level.
    When the capacitor is full charged, pin 6 goes to the high level, than the output goes to the start condition.

    http://docs.google.com/fileview?id=...WMtYjk3OS00N2EzLTlkNzAtMDU3ZTA4YmRhNjFk&hl=it
     
    Last edited: Feb 9, 2010
  8. Wendy

    Moderator

    Mar 24, 2008
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    You may have blown the 555. While these devices can handle 200ma, it is a bad idea to push them too far.

    Try something like this with the 555...

    [​IMG]

    If pin 2, and pin 6 are high then the output will be low, this is core to the 555. Pin 7 shorts the capacitor when pin 2 is triggered, starting the timing cycle.

    Andreapg, you pointed to this drawing...

    [​IMG]

    Q1 should have a separate resistor going to Vcc through S2 in this schematic. This bias scheme for the transistor is usually meant for linear designs. You could also replace the transistor with S2, and it would work.
     
    Last edited: Feb 10, 2010
  9. andreapg

    Active Member

    Feb 2, 2010
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    Thanks for the info.
    I've a question about simulations and real circuits.
    When I simulated the circuit I used 12V as voltage source. But on the output I found 10 Volts only.
    If I set 5Volts as source (R4=150ohm) the led does not work.
    With real components is the same?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Look at the datasheet for the 555. With a 5V supply and a 100mA load its typical output high voltage is only 3.3V and its minimum output voltage is only 2.75V. Maybe your LED needs a higher voltage.
     
  11. przachar

    Thread Starter New Member

    Feb 8, 2010
    16
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    Thanks all of you, I will try out the suggestions.
     
  12. przachar

    Thread Starter New Member

    Feb 8, 2010
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    I have a new problem now. I built a very basic 555 monostable circuit without the sensors and the relays. Load is just an LED. There is a push button switch on pin 2. R = 1M and C = 10uF (delay ~= 10s). The problem now is the LED does not stay on for 10s. It turns on when the push button is pressed and turns off when the button is released. Why isn't the timer acting as a timer?
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    You do remember me saying I think you blew your chip?

    Pin 2 is high, pin 6 and 7 are held low (ground). When pin 2 is low, pin 6 and 7 should start charging. If you keep pin 2 low they should eventually go to Vcc, pin 3 is also high, when pin 2 is allowed to go high pins 6 and 7 go low again along with pin 3.

    Or to state it in sequence, measured with a DVM. Assumed, Pins 4,8 Vcc; Pin 1 Gnd; Pin 5 No Connection.

    Pin 2 Vcc
    Pin 6,7 Low
    Pin 3 Low

    Pin 2 Low
    Pins 6,7 Charging
    Pin 3 Depends on voltage at Pins 6,7

    Pin 2 kept Low.
    Pins 6,7 High
    Pin 3 High

    Pin 2 brought High.
    Pins 6,7 Low
    Pin 3 Low

    What is Vcc, the color of the LED, and the resistor driving the LED?

    If the power supply is 5V, and the LED is red (other colors may not work) Pin 3 will be 3.6-3.8VDC when high. You will need around 60Ω for 20ma, so use a 100Ω resistor for the LED resistor (it is a good idea to label this part as you have the others).
     
  14. przachar

    Thread Starter New Member

    Feb 8, 2010
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    I replaced the 555 IC as like you said I think I had blown the chip.

    Vcc = 5V, Red Led (could you tell me how the color matters? ) and resistor to the LED = 10k.

    I know all the things work because the LED does glow as long as the switch is closed but does not stay for 10s.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    10KΩ is way, way too much, you need 100Ω. A 555 does not come out with 5V, but 3.8V when high, it makes a difference.

    Color determines how much voltage the LED drops, for red this is 2.5VDC, give or take.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    It covers the basics on LEDs and 555s, and how 555s can drive LEDs.
     
  16. przachar

    Thread Starter New Member

    Feb 8, 2010
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    I will replace the resistor and the source and try again. Thanks.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    Why the source? 5V is 5V, it is a go/no go thing? Are you using a DVM?
     
  18. przachar

    Thread Starter New Member

    Feb 8, 2010
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    Yes I am using a DVM. what did you mean by "A 555 does not come out with 5V, but 3.8V when high, it makes a difference."
    I replaced the LED resistor to 100ohms. No difference except LED glows brighter.
     
  19. przachar

    Thread Starter New Member

    Feb 8, 2010
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    I think I understood what you meant. I did go through that link. It was helpful in understanding many things. But I still dont understand why my timer doesnt work. Everything seems fine.
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    A 1M resistor and a 10uf capacitor produce a delay of about 11 seconds in a 555 monostable.
    But a 1M/5% resistor could be 950k and a 10uf electrolytic capacitor could be 8uF or less so of course the timing is not correct.
     
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