555 as a switch (on off from rf controller)

Discussion in 'General Electronics Chat' started by Lineout, Nov 20, 2013.

  1. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Hi,

    I have a 12 volt 3 watt lamp that I would like to turn on and off with a 5 volt reciever module (as the controller) , so.... I've tried an npn and fried 0ne of two reciever modules that I have, (lol).

    Also a 6 volt relay would not operate via the 'module' though it works fine with a separate source..

    Question is, (since I have lots of 555's) , can I use one as a two source voltage simple on off switch ?

    If so , could you tell me which pins to use (1-8 of the 555) to allow this to work.

    I do have a solid state relay on the way to being delivered which I think might work , but would also like to do this with a 555 if possible.

    Thanks, for any help.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Here is a flip flop using a 555 timer, press the switch relay on, press the switch again relay turns off.
     
  3. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Thanks,

    I was actually looking to control the 12 volt load (switching function only) with a 5 volt source, I'll try the lower 'power consumption' relay when it arrives.

    Is there a way to do this with an npn ? (3-5 volt switching a higher voltage load)
    .
    Thanks again for the schematic.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A 555 will not source or sink enough current to power a 2 watt lamp. You need to use an N channel MOSFET with a logic level input. An IRLU8721 would do what you want. Connect the output from the receiver module to the gate through a 100 ohm resistor, tie the source to ground, and the drain to one side of your lamp with the other side to +12. You may also need a pull-down 100k resistor from the gate to ground. Should work.
     
  5. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Thanks,


    Where do I connect the module ground, does it need to also tie into the source ground for reference between the rf module, or just the 3 volt grnd?

    I'll have a couple more questions on this, but have to get back to work soon.

    I just drew out the schematic best I can, does the 100k pull down resistor just go to the 12v ground, is that a bleeder ?
    (I don't understand all of the terminology, still pretty new...)

    Thanks a lot for your help, I've been looking at the mosfet for this but having only 3 pins to handle the two sources is a bit confussing to me , still afraid I'll smoke more parts....lol...
     
  6. elec_mech

    Senior Member

    Nov 12, 2008
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    I agree with Tracecom. Below is a visual representation of his suggested circuit. The 100kΩ keeps the MOSFET (and lamp) off whenever there is no signal from the receiver.
     
  7. tracecom

    AAC Fanatic!

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    There will be only one ground, which will be common for the entire circuit. That's the beauty of a MOSFET switch.
     
  8. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Great schematic, very clearly laid out too. I'm trying to sort out some of the nomenclature and better understand why and how to determine certain values.

    The 100k keeps the gate 'low' and is sort of a bleeder like a capacitor bleeder only for a different reason (keeping the gate ,[-/low] )when you don't want the equip running accidentally from static stray currents ....?

    Since this is an N type mosfet in the schematic the meaning of the N is not the same as the meaning of the N in an npn transistor or is it ?
    I mean with the NPN , you need to take the base high to get it to close if I remember correctly, and the pnp base would need to go low, or do
    I have it backwards?

    Why the 100ohm resistor, ohms law tells me the schematic intends to limit the 'gate' to 50 mA , is that the idea , and why 110ohms was chosen?
    I need to check the current output of the rf module , cause I'm thinking it puts out next to nothing since it wouldn't close a magnetic relay.

    So with a mosfet as you mentioned, the beauty is the common ground, and actually it would probably not even need a ground to close from accidental or intentional stray or diff volt source, but of course that wouldn't be the way to go so we would need the 100k to ground as shown.

    The 100k is just a general value that works well for this kind of function (gate to grd)?
    100ohms would be way too little , and 1M ohm way too much ?


    Thanks....
     
  9. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Thanks, I inadvertantly addressed a question to this statement in my previous post.
    I'll ask here too..... would the switch close with no ground at all , could it also accidentaly close from stray static or other voltage source with no grd reference ?

    Thanks much...
     
  10. elec_mech

    Senior Member

    Nov 12, 2008
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    If nothing is connected to the gate when a signal is not present, then it floats. This means it doesn't know what state it is supposed to be in, so it may come on or go off randomly. The 100kΩ pulls the gate to the MOSFET low in lieu of a signal from the receiver so it is indeed off when no signal is present.

    If I understand your question, yes, an NPN and N-channel MOSFET both conduct when a high signal is placed on their base and gate, respectively. A bipolar transistor (the NPN) requires a small current on its base to allow a proportionally larger current to flow across the collector and emitter. A MOSFET on the other hand only requires a voltage on its gate to allow power to pass across the source and drain. Most MOSFETs require the gate voltage be the same as the voltage going across the source and drain, or at least 10V. A logic-level MOSFET allows a TTL voltage (5V) to be applied to its gate to allow a higher voltage across the source and drain.

    You might get away without a resistor across the gate at all, but for reasons a bit outside my understanding of the subject, the MOSFET has some capacitance within it and will not respond well to fast switching without a gate resistor. Perhaps a smarter cookie will explain it better. In any event, a MOSFET is voltage-controlled, not current-controlled, so it does not require a current at its gate like a bipolar transistor requires at its base.

    With most DC circuits that require more than one voltage, the grounds are always tied together. If you truly needed to separate one voltage source from another, then you'd use an optoisolator, relay, etc.

    100kΩ and 10kΩ are good for pull-up/-down resistors. 100Ω might allow too much current to pass effectively over-riding the input signal when present and 1MΩ might not allow enough again allowing an input to float which you always want to avoid.

    Simply put, yes. It would be floating and could respond to and do anything.
     
  11. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Thanks, internet was down a couple days , just now back up.
    Lots of good info, I appreciate it.

    I received the solid state relay and it worked great, the magnetic coil was just drawing more current than the rf module was apparently rated for, (there is no data sheet available on the module).


    I'll try a mosfet as soon as I get one.

    Does 'logic level' mostly mean your going to get a 5v trigger or gate on a mosfet with that Logic Level tag , I think it was said you could get different gate voltatges, though 5 volts is probably all I'd usually need.

    Does the 5v gate voltage typically have a range of something like 3 to 6 volts or is it critical that you stick with 5v ?

    NPN Transistors:
    So far I only need npn transistors to be full on or full off, is there a typical voltage range that works for that, using current limiting resistors ?
    I've gotten them to work ok , but not really sure if they are full on off or somewhere in between.
     
  12. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Gate threshold voltage: , 2 to 4 volts, ?

    Is that saying that 2 to 4 volts is the safe range to use on the gate ?

    I found these fairly cheap, I don't need anything but about 1 amp of current , this one is good for 49 amps.
    The gate voltage could be anywhere from 1 volt to 12 volts.

    What is a good general source for getting 10 to 50 N Mosfets pretty cheap, and other parts in small lots ?

    Thanks.


    IRFZ44N




    Electrical Characteristics (TJ=25°C unless otherwise noted):
    Drain-to-Source Breakdown Voltage (VGS = 0V, ID = 250uA): 55V min.
    Breakdown Voltage Temp. Coefficient (Reference to 25°C, ID = 1mA): 0.058V/°C typ.
    Static Drain-to-Source On-Resistance (VGS = 10V, ID = 25A): 17.5mΩ max.
    Gate Threshold Voltage (VDS = VGS, ID = 250uA): 2.0V to 4.0V
    Forward Transconductance (VDS = 25V, ID = 25A): 19S min.
    Drain-to-Source Leakage Current (VDS = 55V, VGS = 0V): 25uA max.
    Input Capacitance (VGS = 0V, VDS = 25V, ƒ = 1.0MHz): 1470pF typ.
    Output Capacitance (VGS = 0V, VDS = 25V, ƒ = 1.0MHz): 360pF typ.
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You would be better off with the logic level version, which is an IRLZ44N; its gate voltage can be as low as 1 volt.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No. It's saying that the gate voltage at which the FET is just beginning to turn on is somewhere in the range 2V-4V. To be sure of turning the FET fully on you would need a gate voltage >>4V, typically ~10V.
     
  15. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Thanks, what does logic level mean in practical applications, 'low current/voltage' activated (arduino type controller interfacing) ?

    I actually don't need anything over 1 amp and though these are cheap enough at 49 amps 55 volts , they're super overkill.

    I only need 1 amp + - 12 to 18 volt. Is that range even available as a mosfet, or is the IRLZ44N about where they start ?

    Do you recomend any particular supplyer for transistors/timers/resistors/motors in piece to bulk quantities?

    Thanks.
     
  16. Lineout

    Thread Starter Member

    Nov 20, 2013
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    Thanks, where in data sheets is that full saturation gate voltage indicated?
    Is that the VGS ?

    Where you say 'typically ~ 10 volts' , I hope you don't mean 'AC' ?

    I'm using dc on my projects.
     
  17. Alec_t

    AAC Fanatic!

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    The squiggle is a maths notation meaning 'approximately'. It's DC volts.
     
  18. Lineout

    Thread Starter Member

    Nov 20, 2013
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    My bad , with my fail vision I thought it looked like a sine wave.
     
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