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Auto power on/off switch for RF signal

Discussion in 'General Electronics' started by Julie, May 12, 2004.

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  1. Julie

    Julie Guest

    I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF
    output. Currently, the monitor is always on, however, I'd like to be able to
    automatically switch the monitor off after a period of RF inactivity.

    I'm looking for a circuit design that will monitor the RF signal, and if
    present, switch (12v) power on; if not present, switch power off.

    Any simple circuit that I can construct, or is there anything out there

    Thanks for any info.
  2. John Fields

    John Fields Guest

    If you've got enough RF, probably the easiest thing to do would be to
    build a simple AM diode detector and use its output to trigger a
    comparator wired to turn on a relay(?) and hold the relay on until a
    little while after the RF goes away.

    1. What frequency are you talking about?

    2. What RF amplitude do you have available?

    3. What's the output impedance of the RF source?

    4.Do you want a schematic?
  3. Julie

    Julie Guest

    RF TV channel 3/4.
    Don't know, I haven't scoped it and it is from a consumer appliance, so it
    isn't in the specs. I'd suspect 1v p-p (?).
    No idea. It is the output from a Sony TiVo SVR-2000 DVR.
    Yes, please!
  4. Julie

    Julie Guest

    Anyone have any ideas on this one? Still looking for some suggestions or
    potential solutions.

  5. Rich Grise

    Rich Grise Guest

    When the power's off, how does it know that there's a signal?

  6. John Fields

    John Fields Guest

    From your previous reply you indicated the device which is generating
    the signal is a SONY SVR-2000. Since you say your monitor's plugged
    into the RF output, that leaves the video output vacant, which you
    could use to determine when there's signal coming out of the RF port
    without actually having to tap into the RF, which I doubt is 1VPP.

    Try this: (View it in a fixed pitch font like Courier)

    | | | |
    | | | +---+---+
    | | | |K |
    | | | [1N4001][RELAY]
    [RS] | | | |
    | | [RL] +---+---+
    | | | |
    VIDEO>--[100pF]-+-[1N4148>]-+-----+----|------|+\ | C
    |K |+ | | | >--+--[1K]--B 2N4401
    [1N4148] [10µF] [1M] [100K]<--|-/U1 E
    | | | | | |

    +V can be whatever you have lying around, up to about 36V, the max
    allowable for U1, (1/2 of an LM393; short the inputs and the output of
    the other half to ground), and size Rl to allow no more than about 4
    mA to flow into the comparator's output when it's low. For example,
    with a +V of 5V, RL = E/I = 5V/4mA = 1.25k ohms. 1.2k would be OK, and
    it would dissipate P = I*E = 4mA*5V = 20mW, so a standard 1.2k +/- 5%,
    1/4 watt resistor would be OK.

    With 1VPP of video in (6MHz sinewave) the + input of the comparator
    sees about 400mV of DC, and with the 10µF cap and the 1 megohm
    resistor shown, it decays to about 150mV in 10 seconds.

    You can use the 100k pot to adjust the timeout to much longer than 10
    seconds if you want to by cranking it closer and closer to ground.
    Use RS to adjust the sensitivity of the pot so you don't waste a lot
    of rotation just to get to where the pot starts to work. For example,
    1 volt across the pot would probably work pretty well, so you can
    figure out RS by considering that the pot and RS look like a voltage


    Now, if E1 is +V (and it's 5V), R1 is RS, and E2 is the voltage we
    want with the pot (R2) cranked all the way up, we can write:

    R2 (E1-E2 100E4 (5-1)
    R1 = --------- = ------------ = 400E4 = 400000 ohms
    E2 1

    430k is a 5% value and would put E2 just a little under 1V (943mV), so
    that would be OK.
  7. Earl

    Earl Guest


    The RF channel 3/4 output isn't likely enough to run a diode detector. It's
    meant to attach to the input of a TV.

    Maybe you could take a tuner from a TV set. There are plenty of those in
    the junkyard with working tuners. You could just look at the AGC line, or
    video output, or something. An RC circuit could provide the turn-off delay.
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