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Ultrasonic remote control switch problems.

Discussion in 'Electronic Basics' started by Robert C Monsen, Jul 23, 2004.

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  1. One problem is that the power of your sender is limited by the voltage
    you put across it. 50m is a pretty far distance. Consider how much
    energy you would use to yell intelligably to somebody 50m away...

    One way I've gotten more energy across the transducer is to use an
    inductive spike. You have an N-MOSFET attached to the gate of your
    40kHz oscillator. The source is grounded, and the drain is attached to
    one side of an inductor, possibly 1000uH or so. Your transducer (which
    is, in effect, a capacitor) is in parallel with the inductor. In order
    to limit current through the transducer, you'll need a resistor of
    about 100 ohm in series with it.

    Assume you are generating a 40kHz square wave. Then your mosfet is on
    for 12.5us. That means that at the end of 12.5us (with a 9V supply)
    your current will be about 112mA. Thus, the energy across the inductor
    on each cycle is now

    1000uH * (112mA)^2/2 = about 6.3uJ. The power is thus

    6.3uH / 25us = about 1/4 W

    You might be able to hear that with your receiver circuit at 50m.

    Make sure the power mosfet can handle 100V, since the spikes will be
    something like 80V.

    Note that some transducers are voltage limited; if so, you'll need to
    increase the inductor, or decrease the duty-cycle of the oscillator.

    Bob Monsen
  2. Rubicon

    Rubicon Guest


    My intention is to build a reliable ultrasonic remote control switch
    powered by 9V batteries with a desired range of 50M.

    I've had a poor time of it so far.

    I started with one of the few circuits seemingly available on the net.

    Replacing the transmitters unavailable SL100 and SK100 transistors
    with BD139 and BD140 transistors I achieved a range of 5cm. Asking for
    some advice in this group I changed them to BC548/558 added 220uF caps
    across the battery supply rails and tried to reverse the transistors
    but couldn't get that to work. I also added 20% 0.1 and 0.001 ceramic
    caps across the transmitters supply rails, 20% IC decoupling caps, 1%
    resistors and used 10% caps for the other caps along with shielded
    cables and managed to achieve a 1.5M range. The 1.5M range was with
    the transducer driven directly from the 555 and not by the transistors
    which only managed less then half that range.

    Finding the seemingly only available kitset (24kHz) on the net I see
    that it uses few components with an advertised 50ft range. The kitset
    uses a 16pin Dip, three resistors, a cap, trimmer, LED and push button
    switch for the transmitter.

    Can anyone help me build an ultrasonic tx/rx switch like it or better?

    One particular electronic kitset site that sells the 24kHz kit also
    sells the LM388 14Pin (now obsolete) as an ultrasonic driver.



    Better photo of the kitset

    This led me to search National Semi for ultrasonics and I found the
    LM386 to replace the LM388

    At first I thought a sine wave was required and then found that a
    square wave is fine and in the LM386 PDF datasheet, page 5, there is a
    square wave circuit but for only 1kHz.
    I don't know how to convert this circuit to 40kHz for the combined
    transtmitter/receiver transducers I have or how the transducer is
    connected. I think the circuits Vo should go through a low value
    resistor and capacitor before the transducer to V- but I don't realy

    Any help is greatly appreciated.

  3. Rubicon

    Rubicon Guest

    Thankyou for the information.

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