making a relay out of transistors?

Discussion in 'The Projects Forum' started by microzee, Nov 14, 2012.

  1. microzee

    Thread Starter New Member

    Nov 14, 2012
    10
    0
    Hey everyone, i really do not know anything about electronics. i know the very basics if even that.

    if you dont wanna read all of the below, heres my question: i need to make a relay out of transistors that when powered will complete another circuit.

    but still, read below to get a basis for what im trying to do :p


    I took apart a broken remote control helicopter (nothing wrong with circuitry), and was going to hook it up to whatever i wanted to make it wireless. so i decided i wanted when i press a button, it applys voltage to nichrome heating wire. first of all i realize that the helicopters use infrared LED's to communicate so this isn't the best way to do it remotely. but heres what i had so far. you press a button on remote, it outputs 3v to a nichrome heating wire that heats up and lights a fuze. the problem? it causes a short on the circuit board. so what do i do? i know you could have it connected to a relay that connects and have another battery pack powering the wire, but i would prefer to do it with transistors. so pretty much my question is, how do you make a "relay" using transistors? or is there something else i can do?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    It sounds like all you really need is a transistor to function as a switch. That's easy. Given the low voltage of your signal, I'd use a regular BJT (not a MOSFET). Ground the emitter pin and put the collector on the low side of your load, the wire. Put a resistor from your signal to the base pin. That's it. A voltage on the base will allow current to flow thru the collector-emitter circuit.

    Now, you need to choose a transistor that can more than handle the current needed to heat the wire. Choose the base resistor to give you 10% of that current level. (Assume the base is at ground + 0.7V).
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    One good thing about transistors is that they can replace relays in low power systems :)
     
  4. microzee

    Thread Starter New Member

    Nov 14, 2012
    10
    0
    so you mean like this??



    [​IMG]
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    No, that puts the 9V directly (well, via the nichrome) onto the helicopter supply, and that's not good. The two grounds can share, but not both poles. Run the V+ from the 'copter through R1 to the base of the transistor. That will be the low current "control" signal. The nichrome should run from battery + to the collector pin, and the emitter will be tied to the common ground.

    In other words, relative to your drawing, rotate the battery 180°, connect R1 to V+ instead of V-, and sever the connection of V+ to anything other than R1.
     
    Last edited: Nov 20, 2012
  6. microzee

    Thread Starter New Member

    Nov 14, 2012
    10
    0
    ahhh so you mean like this? ohh if you see the same picture, clear your cache.

    and it should be the same as the pic above as i edited this schematic, so same link.

    [​IMG]
     
    Last edited: Nov 20, 2012
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You mention lighting a fuze - what is on the other end of the fuze?
    The remote is likely just fine for a model helo, but very doubtful that it would be safe for use with pyrotechnic devices. Stray interface may cause accidental ignition at a very inopportune moment.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Moderator, please close this thread because the OP might be a terrorist.
    When I was a kid I made sure my bombs did not hurt anybody.
     
  9. microzee

    Thread Starter New Member

    Nov 14, 2012
    10
    0
    well if thats not safe then ill connect it to something else. anyways though, heres my main point. (quoted for ease of reading o_O)

    so i found that one easy way would be to use an old helicopter. but i want the something to have its own power source. so would this work:

    [​IMG]
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your new circuit will blow up the transistor because the current is probably too high for a little low current 2N3904 transistor.
    Maybe not because the transistor is connected backwards (upside down).
     
Loading...