555 timer and MOSFET

Discussion in 'The Projects Forum' started by chuyenle, Jun 5, 2013.

  1. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I've never used MOSFET in any application until recently. I don't completely understand how the Mosfet can be activated

    The application I've worked on is when button is push, it powers the 555 timer and turn on an LCD for a specific amount of time. After X time, it cuts down the power to the timer and the LCD to save battery. What I use are a push button, two N mosfet (from radio shack) and 555 CMOS timer.

    My set up:
    The push button (the type that you push, it's connected. You release, it's disconnected)
    Button connects to the Gate of 1st mosfet, Source to GND, Drain to Gate of 2nd N mosfet.
    Source of 2nd mosfet to GND, Drain of 2nd mosfet to Pin 8 and 4 of the timer and to (+) of LCD.

    Pin 3 of timer to Gate of 2nd mosfet

    The rest of the 555 timer is setting up as Astable mode. The 555 timer supposes to turn the LCD on for 40 sec and shut down all the power to save battery.


    My question is:

    1) Did I set up the mosfet correctly?

    2) When I try to measure the output current of the timer, I couldn't. The multimeter shows 0. What would be the correct way to measures output current of the timer?

    3) When I push the button, measure the Drain-GND voltage of 1st mosfet, there's nothing


    Any help would be greatly appreciated.

    Thank you


     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Post a schematic.

    Without one, I can only guess that the MOSFET you are using is not a logic level device and you are trying to gate it with 5V or less. It won't work.
     
    Last edited: Jun 5, 2013
  3. LDC3

    Active Member

    Apr 27, 2013
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    Are the MOSFETs enhancement mode or depletion mode?
    Are they PNP or NPN?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    To be precise, we don't classify FETs as PNP and NPN.
    PNP and NPN refer to bipolar junction transistors or BJT.

    FETs are classified as P-channel or N-channel type.
    You are correct on the modes. They can also be enhancement mode or depletion mode.
    Thus there are four possible combinations.
     
  5. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I designed 2 circuits for the same purpose. I use IRF510 Power Mosfet N-channel from Radio Shack in both designs.

    LED didn't turn on at all in Design 1 when adding N-mosfet. Without the mosfet, LED turned on and off for a specific amount of time; the time is way off for what I designed. The reason for using mosfet in this design is it can cut off the power draw by the LCD when it's not used.

    Design 2, I tried to measure the voltage across Drain and GND when button is pressed but I got nothing

    circuits.jpg
     
  6. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I don't think I follow the enhancement mode or depletion mode as well as the four combinations
     
  7. LDC3

    Active Member

    Apr 27, 2013
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    In design 2, you don't have any power going to the right side of the circuit (even if the switch is closed).
     
  8. LDC3

    Active Member

    Apr 27, 2013
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    Yes, I should have known that. I guess I'm just tired. :confused:
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The trigger input needs to be pulled low to trigger the 555.
    In circuit #1 connect the 10k resistor directly to the trigger pin, and connect the momentary switch between the trigger pin and ground.
     
  10. sheldons

    Active Member

    Oct 26, 2011
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    Heres the correct way to edge trigger a 555.....your schematic is totally wrong the way you are driving the mosfets.... Theres an example 555 timer and an output stage to drive a mosfet to switch a load be it leds ,relays etc
     
    chuyenle likes this.
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Let's get one thing at a time working, starting with the 555 one-shot. Here's a schematic that works; it uses your RC values of 680k for R1 and 100μF for C1 and gives a calculated on time of 74.8 seconds. After you get that working, we'll try to add the MOSFET. Note that my schematic shows an NE555, but your CMOS version will work just the same.

    If you want about 40 seconds on time, change R1 to 390k. (The calculated on time is 42.9 seconds.)
     
    Last edited: Jun 5, 2013
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Then you can add on R5, Q1, LED2, and R6.

    That will prove that your timer is working and the MOSFET is switching.

    If your LCD runs on 12V, you can put it in place of LED2 and R6. If it runs on another voltage, connect the positive input to the LCD voltage supply and connect the grounds of both supplies together.

    Then, if you don't want LED1 and R3, leave them out.

    The bad news is that the 11.1V out of the CMOS 555 isn't really enough to reliably turn on the IRF510. Even though it will probably work, you should change to a MOSFET with a lower gate voltage requirement such as the IRL510.

    What I don't understand is why you need a MOSFET at all. The TS555CN will source up to 100mA. How much current does your LCD draw?
     
    Last edited: Jun 6, 2013
  13. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    Thanks for all the help:). I'm totally confused with all this:confused:

    The button I use is not a actual switch. It doesn't stay close after you press the button. It connects when you press it and disconnect as soon as you stop pressing. I don't know the technical word for this type of button. But does that change the way the circuits should be?

    Anyway, I will try out some of suggestion here
     
  14. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I want the Mosfet cutoff the power that goes through the LCD. I was hoping with that, the LCD won't draw too much power from the battery while idling.

    My LCD draw 65-70mA when it's on
     
  15. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Your button is a switch; it's called a Normally Open Momentary Pushbutton and it is fine for the schematic I posted.
     
  16. tracecom

    AAC Fanatic!

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    What voltage does your LCD use?
     
  17. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    My LCD use 12VDC. It similar to a digital picture frame. I want to turn it on, play a slide show and turn off power completely (or minimize it as much as possible) to save battery.
     
  18. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    I adjusted my circuit according to your recommendation, it works well with LED. I tried to measure the current output to see if there's enough to run the LCD. Vgs = 11.48V - 12V when trigger and 0V when waiting for trigger. Vgd = 11.48V when trigger and 9.75V when waiting for trigger.

    Using a AC power meter, it shows that when my LCD runs, it takes 65mA-80mA. But when using DC power supply, it can play at up to 1.5A at 12VDC
     
  19. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Then use the schematic in post 12. Replace LED 2 and R6 with your LCD.
     
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  20. chuyenle

    Thread Starter New Member

    Jun 5, 2013
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    Thanks a lot. It's working. Is there a way to measure current that the 555 timer take while waiting for trigger? It doesn't seem right (0.1uA). I want to calculate the battery life.

    Can we discuss about design #2, using astable mode of 555 timer? My aim for this design is that, until the button is pressed, there's no battery loss to the circuit. Completely shut down circuit. That's why I connect the button to the gate of the mosfet, thinking that the button will trigger the first mosfet, then power the second mosfet and run the 555 timer. Like I said, I never use mosfet before, that's what I think it should work but obviously it's not and I have no idea why.
     
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