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Please help me find a circuit schematic

  1. Jan 3, 2010 #1
    Please help me find a circuit schematic :)

    Hello, First let me say i am NOT an expert and have very little knowledge of electronics. I am able to solder and build simple circuits, but I don't have any training and I understand almost nothing about how electricity works.

    I am looking for an electrical schematic. My goal is to turn a key on my computer keyboard on and off repeatedly at an adjustable rate (between 1-10 Hz.) I already have lead wires connected to the key, and it turns on and off correctly if I connect the wire leads.

    Ideally, the power source for the circuit would be 5.2 volts from a USB plug, but I can get an external power source if that is not enough. So far, I have tried using a relay connected to an LM3909 LED flasher. Unfortunately, since this was designed to power an LED, there was not enough power to trip the relay.

    Please let me know if you can direct me to any solutions on the web, such as premade schematics that could be easily adjusted for this.

    Thank you very much!
  2. jcsd
  3. Jan 3, 2010 #2
    Re: Please help me find a circuit schematic :)

    What is this key for if you don't mind asking, there might be a better way.

    If you want to use a relay what are the specs on the relay? Can you turn the relay on if you apply 5V from an external power supply, if so what is the current draw?

    A USB port generally supplies 500 mA. If you measure how much of it is drawn by the keyboard you will know how much is left.
  4. Jan 3, 2010 #3
    Re: Please help me find a circuit schematic :)

    The key is for a game I play and I would like to simulate repeated key tapping (I would like to avoid software solutions and would prefer to use hardware.)

    I have tried several relays. The lowest power one I could find was a reed relay, 5 VDC, 20 miliamps for the coil. http://www.radioshack.com/product/index.jsp?productId=2062478"

    The relay worked correctly when current was applied, however it didn't have enough power when I ran it through the oscillator chip.
    Last edited by a moderator: Apr 24, 2017
  5. Jan 3, 2010 #4
    Re: Please help me find a circuit schematic :)

    Your IC is probably running out of drive. Buffer it with a transistor.
  6. Jan 4, 2010 #5
    Re: Please help me find a circuit schematic :)

    sorry, i dont know what an IC or drive are and I don't know how to buffer. I am just looking for help finding some plans on the internet.

  7. Jan 4, 2010 #6


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  8. Jan 4, 2010 #7
  9. Jan 4, 2010 #8
    Re: Please help me find a circuit schematic :)

    Hello pookshuman,

    ICs (Integrated Circuits, aka microchips) are possibly the most important components in modern electronics. Your LM3909 LED flasher is an IC - and it seems surprising that you would not have come across this term already through your interest in building circuits.

    Drive is the necessary voltage and current required to make something operate. Buffering can have many meanings, but in this case it means converting the output from one part of a circuit (the LM3909) to match the requirements of another (the relay).

    These are very basic terms which you ought to understand even if you only want to build things from kits. If you really know as little about it as you have said, then you should understand that people may be reluctant to send you circuit ideas.
  10. Jan 4, 2010 #9
    Re: Please help me find a circuit schematic :)

    Are you trying to activate a relay with the led flasher or the led flasher with a relay?
  11. Jan 4, 2010 #10
    Re: Please help me find a circuit schematic :)

    Thank you for your response.

    Yeah, this is something else I don't understand. I am here to learn and I was very upfront about my ignorance in this subject area .... If people are reluctant to help people that are looking for help, then why are they here? Just to poke fun like the previous poster?

    Either people can help or not .... and if they are not interested in helping, I would just as soon they pass over this post and reply to someone else's thread. If someone has a link to a schematic which resembles what I am looking for, I would really appreciate it.

  12. Jan 4, 2010 #11
    Re: Please help me find a circuit schematic :)

    This was a dead end, i was just explaining what I had tried. I tried running the relay off an oscillating chip, but not enough power came through to work correctly. I am looking for any schematics using any parts that will get the job done, which is: an adjustable oscillating circuit which can open and close another circuit (meaning the wires connected to my key) and which will run on reasonable power (3-9 volts, would be perfect and could be accomplished with a few batteries or a USB power cord.)
  13. Jan 4, 2010 #12


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    I didn't realize it at the time, but evidently, you seem to think the whole point of my post was to have fun at your expense. The first link was meant to be facetious (and not insulting) while the second was serious--the on-board macros probably do exactly what you want them to do, hit a key every few seconds, on an adjustable basis.

    I do apologize if my post came across as condescending; that was not my intent.

    EDIT: Pursuant to what Phrak said, I found these as the second and third results when I googles for 'transistor as switch':
    Last edited: Jan 4, 2010
  14. Jan 4, 2010 #13


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    Re: Please help me find a circuit schematic :)

    relay driver.JPG

    This is a relay driver. (Click on it to make it bigger). Your oscillator chip's output would connect at the left of the resistor marked R.
    This resistor would have a value of about 47000 ohms

    Your switch would connect to the two relay contacts at the top of the relay.

    The transistor would be any small NPN power transistor such as a BD139 although you would have to know the current drawn by the relay and its rated voltage. This may be written on the body of the relay.

    The diode across the relay is there to protect the transistor from high voltages generated in the relay. It should have a peak inverse rating that is high enough to do this. Diodes are cheap, so a 1000 V diode would be suitable.
    A 1N4007 would be OK.

    The keys on my keyboard repeat if I hold them down for about a quarter of a second. Would that be a suitable repeating rate for you?
    If you go to START / CONTROL PANEL / KEYBOARD, there is a window there that lets you set the initial delay and the repeating rate if you just hold a key down. This is in Windows XP.
    In any case, you would have to allow for it in case the relay holds the switch closed for any length of time and the computer considers that someone is holding the key down.
    Last edited: Jan 4, 2010
  15. Jan 4, 2010 #14
    Re: Please help me find a circuit schematic :)

    thanks vk6kro, I will look into this and see if it works

    take care
  16. Jan 5, 2010 #15
    Re: Please help me find a circuit schematic :)

    I am sorry if you found my post upsetting - that was not what I intended. As for reluctance to give advice, what you had just posted really did made me think better of advising you any more - for your own safety.
    If your lack of knowledge meant you were unable to understand what you were told, and this led to accident or injury, where would this leave your advisor? What if, for example, you were still a minor and electrocuted yourself trying to follow my advice?
  17. Jan 5, 2010 #16
    Re: Please help me find a circuit schematic :)

    I appreciate your concern for my safety and I completely agree. After all, I have no way of knowing if you are a minor as well. As for not giving me any more advice, that shouldn't be too much of an issue since you didn't give me any advice.

    Only one poster, vk6kro, has been kind enough to answer my question, and I thank him for that.

    Thanks again and take care :)
  18. Jan 5, 2010 #17
    Re: Please help me find a circuit schematic :)

    Thanks to all those that replied, I will not be monitoring this thread going forward. TY
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