1 momentary switch and a thyristor - on/off

Discussion in 'General Electronics Chat' started by WonHungLo, May 9, 2011.

  1. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    i have worked up this simple circuit to use a thyristor to turn a circuit on and off but it involves 2 momentary switches. i am trying to get it to use 1 momentary switch instead. i understand if i need additional parts not included in the schematic but im trying to keep the design simple and parts down (as always K.I.S.S.)

    any advice would be greatly appreciated

    [​IMG]
     
    Last edited: May 9, 2011
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    #1. You really need to put a current limiting resistor in series with the LED...~470Ω.
    #2. I've never seen any way to do a push-on/push-off with a single switch and an SCR.

    Ken
     
  3. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    ok sorry the schematic on paper had a 330 ohm resistor for the led.
    i may be going about this all wrong.

    the goal is to have a momentary switch that latches on / off with a push of the button. i was just thinking a thyristor bc it latches current once a signal is applied to the gate. is there a better way to do this than a thyristor????
     
  4. KMoffett

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    Is this just an experiment or do you have an real world project? Are there limitations you need to meet?

    ken
     
  5. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    its a smaller circuit inside of a larger circuit. i am building a gps data logger and i didnt want a toggle switch sticking out of the enclosure and i didnt want a slide switch bc they are harder to water proof. i can mount the momentary switch behind a tail cap cover like is used on flashlights and seal it from the inside. i know i can also use the push style on-off switches but they are usually larger and space is a factor in this project. plus i have a ton of the tactile momentary switches left over from other projects. plus i would just like to know if it can be done. ( i have seen other commercial products that uses a momentary switch to power on and off the device )
    so there are some limitations to the project but i can flex them a lil bit.
     
  6. KMoffett

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    So, may I assume the the data-logger is a microcontroller that you are programming?

    Ken
     
  7. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    no it is a usb gps data logger and i am building the power supply for it. what about a way to use a 555 timer to latch a momentary switch for on and off operation. im just throwing ideas out here. i think it can be done i just don't know how...
     
  8. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    ive got a avr microcontroller that i can use on this project but ive got more additional components to add then and thats what im trying to avoid. (avr mc, dip socket, decoupling caps, crystal or resonator, etc..).

    any ideas guys?
     
  9. KMoffett

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    Dec 19, 2007
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    So what are the voltage and current requirements of the data logger?

    Ken
     
  10. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    +5vdc ~80mA for just the gps data logger
     
  11. KMoffett

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    Are you powering it from a battery?

    ken
     
  12. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    car battery. i built a voltage divider circuit to drop down the input volatge to the 7805 regulator so it wont have to dissipate the extra voltage above 7.5V as heat. (yes i put a heatsink on the T0-220 just incase). it will outup 1A which is way over what i need. i just need a neat little way to turn off and on the circuit and i thought a momentary switch would be perfect. seems more trouble than its worth.

    check this out. just found it...
    http://todbot.com/blog/2010/01/02/momentary-button-as-onoff-toggle-using-555/

    what ya think...
     
  13. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    this is the schematic for my power supply to the gps data logger...

    [​IMG]
     
  14. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    the cn2 and cn3 are for the usb headers to connect the data logger to and an extra port to connect to the pc to read the data without disassembly of the enclosure.
     
  15. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    i used my circuit simulator software and built the circuit and it didnt work to begin with so i followed it through and found you need a PNP instead of a NPN and you also had to change the cap to a 10uf to work.

    anyways here is the correct schematic for using a momentary switch as an on off with a simple 555 timer...


    [​IMG]
     
  16. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Is the intent to have the push-on/push-off circuit replace SW1? The 555 circuit will always have to be powered in order to work. Though it could work with a PNP output transistor, it will always need to be drawing some current.

    The 7805 connected directly to +12V will only have dissipate about 0.7 watt...well within its spec's. The R1/R2 divider is a big waste of power and will dissipate more power than just the 7805 alone.

    Ken
     
  17. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    yeah but the car battery can go to around 14.4 VDC when running and charging by the alternator. it will be connected to a fuse that has power when the car key is on the run or on position so i think it will be ok. the switch in the schematic is a momentary. you press the switch once and the led lights up. push it again and the led turns off.
     
  18. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    i may do away with the voltage divider if the t0-220 lm7805 can dissipate enough power from the car battery when it is charging @ around 14.4VDC. i dont have the data sheet in front of me at the moment to see how much heat it will produce from the excess voltage.
     
  19. KMoffett

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    That is something you will have to determine with your installation. How much temperature rise you can accept.


    Yes, but your on-off circuit will continue to draw current to power itself even after it shuts off power to the data-logger circuit.

    Ken
     
  20. WonHungLo

    Thread Starter New Member

    May 9, 2011
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    ok... i got what you are saying the 555 will still draw power as long as it has power avaliable. that will be ok bc when the vechicle turns off it will cut the data logger off and the 555 circuit. so no concern about power consuption. good point. thank you
     
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