fast, low voltage SPDT relay or alternative circuit

Discussion in 'The Projects Forum' started by julienrl, Aug 10, 2009.

  1. julienrl

    Thread Starter Member

    Aug 10, 2009
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    Hello, I am looking for a very low voltage trigger SPDT relay that could be triggered by 1milivolt. I have found a few of these on the digikey site for anywhere from 20-140$. Now to make things even more complicated, I would need it to be able to be driven at 20khz+

    Does anyone know if there are ICs that can do this? and if not, what kind of circuit would be able to do this?

    thank you
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Can you post the circuit you are trying to design?

    Ken
     
  3. julienrl

    Thread Starter Member

    Aug 10, 2009
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    well I have an oscillating output at about 0.004V at 19khz from my phone and I want to amplify it, but don't want to risk damaging it so I want to use a separate circuit with its own power supply so that there is no amplification to do, but I want to use the relay to drive the new circuit so that it keeps its original properties.

    The other option I am looking at is a relay that still needs to be sensitive to 0.004V but with a 3khz capacity would suffice if I have a oscillator crystal driving the powered half of the circuit at the proper frequency.

    I do not have a schematic yet as I am still trying to find the most appropriate solution.

    thank you
     
  4. julienrl

    Thread Starter Member

    Aug 10, 2009
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    bump................
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I took at quick look at DigiKey and couldn't find any mechanical relays that operated at 1 mV and/or anywhere near those frequencies. Can you provide the part number for what you found?

    Do you mean mA, not mV?

    John
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Please be more specific as I suspect it's not a relay that you need at all. Is this a 20KHz sine wave or digital wave form and what is its purpose?
     
  7. julienrl

    Thread Starter Member

    Aug 10, 2009
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    the goal is to create hardware-based post-processing of signal being output by the phone. The wave files being used has the "peaks" where the 38khz signal has to be active (a sin wave, but I do believe that the receptor would not discriminate from a square wave). When this "peak" shows up, the voltage over the audio output goes up to a measly 4-6 millivolts meaning we can not simply drive an oscillator and IR LED directly from the audio output. The only real thing that can be done with this voltage is to use it ans an indicator as to when the properly tuned and powered circuit should be on or off.
     
  8. julienrl

    Thread Starter Member

    Aug 10, 2009
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    could this be the same as a millivolt trip relay?
     
  9. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
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    I believe this 40$ relay allows for millivolt triggers (please correct me if I misunderstood).

    thank you
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    You mention pulses, 38KHz and IR.
    It sounds like you are trying to produce an infra red remote control signal from a demodulated remote code stored as a .wav file.

    Forget relays for this.

    Presumably you are taking the audio from the headphone connector on the phone?
    I'd try using a 100 Ohm resistor as a load for the audio output (to ensure no DC component), then feed it to something like an LM311 comparator. Have a look at the data here:
    http://www.national.com/ds/LM/LM311.pdf

    Look at the example 'Detector for Magnetic Transducer' on the first page. Where it shows the magnetic pickup, substitute the 100 Ohm earpiece load resistor with the phone audio connected across it.

    Add 10uF electrolytic caps across the supply and R2 for stability and run the circuit on a 4.5 or 6V battery for safety.
    The output at pin 7 will be the switching signal, at the battery voltage, to control your IR transmitter oscillator. If the On/Off polarity is wrong, swap the connections from the phone out to the comparator circuit, or re-encode the .wav inverted.

    Ideally the phone ground would be connected to the r1/r2 junction.

    Once you get the basic thing going, you should really mod it so the phone ground connects to the external circuit ground. All you need for this is one extra capacitor and resistor, I'll let you think about that.

    ps. If your signal is really only a couple of mV, you may need to add the Offset Balancing pot shown on page 1 of the data sheet, then adjust that while watching the output on a 'scope to get the cleanest results.
     
  11. julienrl

    Thread Starter Member

    Aug 10, 2009
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    SWEET! thanks Robert, I was starting to loose hope... I am having trouble with PDF files lately on my computer, but I will give you an update once I get my hands on all this! :)
     
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