Normally-On JFET

Discussion in 'The Projects Forum' started by DanRilley, Mar 25, 2008.

  1. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Hi I am trying to combine two electronics projects. One is an LED sequence which lights 10 LEDs in sequence by using a 555 and a divider. The other is a little sound recorder device I have that when you press a button it plays a sounds. My idea was to simply hook the button of the sound player to the LED light voltage and create a kind of sound sequencer. I have successfully done this using a JFET so that when the voltage goes into the LED, it allows the button to conduct and starts the sound. The only problem is that the JFET is normally-on. So when I disconnect the power, the button to the sound player is constantly down and the sound just goes on and on. Do you guys know of another way of doing this, or if you can buy Normally-Off JFETS? I'm attaching a diagram of the way I have hooked these up:

    [​IMG]
     
  2. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    This should do it:
    [​IMG]
    If it doesn't work 1. time, try swapping the leads to the players switch.
    Almost any small signal (bipolar) transistor will do.
     
  3. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Cool, I'll try it out. For some reason someone had suggested a JFET for this over a transistor, are there any disadvantages to using a transistor over a JFET?
     
  4. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    They probably suggested it since it can be wieved as a variable resistor and bidirectionally close to a short when fully on, while the BjT will only conduct in one direction (hence the "try the other way around if it doesn't work the first time").
    A quick way to establish whether it will work with a BjT is to short the switch with a diode (eg. 1N4148, 1N914, 1N400x or whatever you have in the junk box). It will probably only work with the diode in one direction and this will be the direction the BjT should be used (with the collector where the anode of the diode was when it worked).

    Advantages in this app. is that a Bipolar Junction Transistor (BjT) usually comes cheaper than a Junction Field Effect Transistor (JFET) and it doesn't need a negative gate signal to "pinch off" (close).
     
  5. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Great, I tried it out and it works like a charm thanks!
     
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