Want to drive relay from LED on VOX tape recorder

Discussion in 'The Projects Forum' started by DerekD, Sep 24, 2010.

  1. DerekD

    Thread Starter New Member

    Sep 23, 2010
    5
    0
    Hello,
    New to the forum, seems like a great place! I am trying to run a relay from a Voice activated tape recorder that I have taken apart. It runs on 4AA batteries and has an LED that lights up when activated by sound. With the LED in place, the voltage across the wires is 1.8V DC. Without the LED, it reads 4.95V DC. I will be driving a 5 or 6V relay (I have both). the 5V is part #JW2EN

    From the datasheet:

    Nominal operating
    current, mA
    (​
    ±10%) 106
    Coil resistance,
    W
    (
    ±10%) 47

    I've tried using a 3904 transistor with a 1K resistor and a 4148 diode across the relay, but it didn't work. I've thought about using a 4n35 optocoupler to drive a transistor, but don't know how to get any values for resistors or how to determine which transistor to use. I'm just looking for input on what to consider or how I can make this work. If there is any other info anyone needs, please let me know. Thanks for taking the time to read this.​

     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,792
    948
    is the 1K resistor in the base leg of the transistor? Or is it in the collector or emitter legs? Where is the relay in the transistor circuit?
     
  3. DerekD

    Thread Starter New Member

    Sep 23, 2010
    5
    0
    Yes, the resistor is on the base leg. I'll have to check the other legs when I get home to see where I have them leading. I don't think it's backwards. But my other question is, do I need to run ground from a 5 or 6 volt source on the board, since it seems to only be putting out 1.8 volts across the led? I'm confused whether to use the negative lead going to the LED as ground, or both. I don't have the LED in the circuit as I'm trying to do this. Thanks
     
  4. DerekD

    Thread Starter New Member

    Sep 23, 2010
    5
    0
    Well, I have the red and black wires coming from the tape recorder going to the led. The red wire connects to the 1k resistor to the base of the transistor. the collector is running to ground and the emitter is going to the relay, then on to 6v from the batteries. Ground from the transistor is running to the batteries. I now have a n4001 diode between the relay coil. It's like the transistor is stuck on. The relay closes as soon as power is applied, it doesn't matter if current to the led is flowing or not. Is this a saturation issue because of my resistor value? Any help would again be appreciated. Thanks.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    With the LED in place, the voltage across the wires is 1.8V DC. Without the LED, it reads 4.95V DC. I will be driving a 5 or 6V relay

    There's an internal current limiting resistor for the LED.
     
  6. DerekD

    Thread Starter New Member

    Sep 23, 2010
    5
    0
    If I put a 100k resistor in place of the 1k, the relay does not come on. If I put a 100k pot there, I can turn it on and off that way. So I'm guessing I just need to keep trying until I find a resistor value that works. Does this sound like it should take care of it?
     
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