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Small NPN 3904 relay style circuit

  1. Oct 6, 2014 #1
    I am currently trying to make a small circuit work, and more than just trying different things until it does work, I wanted to hopefully get an explaination to why it is not working. I am a novice, I only started dabbling in my free time recently and thought I had a decent if only vague understanding of how a NPN3904 Transistor would work. I have two separate circuits. One is a 5 volt circuit with a button on it, the second is a 6volt circuit with a coin meter on it. The button triggers a keyboard press that will eventually trigger that a coin dropped through my coin door, at this time I would like the NPN3904 turn on and flip the coin meter. For whatever reason, when I measure voltage at the emitter of the transistor there only seems to be 1 volt passing through and there for not enough to trigger the coin meter. I am using a 1k ohm resistor on the base of the transistor. What am I missing?

    I am attaching a crude drawing, please explain what I am missing or how I can design this so it will work?

    http://i1370.photobucket.com/albums/ag263/nevian2/RANDOM/npn_zpsf9e63613.png~original [Broken]
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 6, 2014 #2


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    Firstly, welcome to PF. Now the questions;

    Is the coin meter a two terminal device? Does it use a relay solenoid driving a mechanical counter?
    Or is it a three terminal electronic counter?
  4. Oct 6, 2014 #3
    It's a mechanical coin meter with only two leads. Once 6v is supplied it clicks over and will only do so again once power is removed and reapplied.
  5. Oct 6, 2014 #4


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    doesn't tell us a lot

    is it a relay/solenoid ?
    at the moment ... where are your 5V and 6V rails being supplied from ?
    does the 5V side really need to be 5V or can it also be 6V ?

    also a big part of the problem is you haven't shown complete power supplies
    I will redraw the circuit for you

  6. Oct 6, 2014 #5


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    OK a pair of circuits one for all 6V supply and one for 5V and 6V supplies
    take your pick

    Till you respond, I will assume the coin thing is a relay/solenoid type of thing and is denoted by the coil and the relay switch in my diagram
    as such, because it's an inductive load, you will need the 1N4007 (or similar) diode across the coil as shown


    The base resistor value isn't too critical somewhere around 2k2 - 5k6 will be fine

  7. Oct 6, 2014 #6
    I am not certain this is the coin meter I have, as I am not at home, I am at work, but it is similar to this.

    Sorry about the late response, I work nights on the Pacific Coast. Thanks for the help.

    As for either ckt being supplied from the 6v, I have two separate power sources and I would like to keep them that way, I do not want to accidentally fry the iPac that I am using in my MAME.
    Last edited: Oct 6, 2014
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