inverting resistance?

Discussion in 'The Projects Forum' started by ale2121, Jun 17, 2009.

  1. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    Hi, i want to make this: http://www.instructables.com/id/DIY-Force-Sensitive-Resistor-FSR/?ALLSTEPS

    I want to use it with sound frequencies that come out of a 555 timer. However, I want the sound to be normally off, and the harder i press the force sensor, the lower the resistance I get, therefore the louder the noise/ higher the pitch. How would I invert the resistance?
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The output level of a 555 oscillator does not change. You could make a simple highpass filter to make it less loud at lower frequencies.
    The frequency of a 555 oscillator increases when the timing resistor decreases.
    Look at Step 4 (the test of the sensor) to see that its resistance decreases when it is pressed harder which is what you want.
     
  3. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    well, the schematics I have (from the forest mims catalog) do have pots in place for volume. couldn't i also replace these with these force sensors? have one for pitch and one for volume?
     
  4. Audioguru

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    Dec 20, 2007
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    You forgot to post the schematic of the volume control pot connected to a 555 oscillator. Usually a volume control has 3 wires. The force sensor has only 2 wires. A resistor must be added to make it almost the same as if it has 3 wires.

    A volume control is fine to feed an amplifier that drives a speaker.
    But a volume control is not designed to drive a speaker directly. The resistance of the force sensor is much too high to directly drive a speaker.
     
  5. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    http://www.flickr.com/photos/purplepandarobotdance/3638880688/[​IMG]
    here is one of them. Instead of the 8 ohm speaker, I have this connected to my amp. I have to be honest, I couldn't get the volume knob to actually work. It did at one point, but stopped ( I think when I connected it to the amp instead of the speaker). I figured it would just be a matter of troubleshooting why the volume knob didn't work, then replacing it with a force sensor.

    My end goal here, is that I don't like how the tone is all or nothing. I want to ease into it, more like an actual instrument. Like if you play a guitar, you can play the strings light or heavy.

    here's the other circuit I'd like volume control with (this doesn't have the pot in place).

    http://www.ecelab.com/circuit-toy-organ.htm
     
    Last edited: Jun 18, 2009
  6. Audioguru

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    Dec 20, 2007
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    It is a horrible way to adjust the volume of a speaker by using a pot in series with it.
    A pot is fragile and will soon burn out unless it is a big power variable resistor. Then it is used as a voltage divider, not as a series rheostat.

    You need a circuit to adjust the envelope of the waveform so that it sounds like an instrument and not like an on-off buzzer. The envelope has a certain rise time and a certain fall time.
     
  7. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    Last edited: Jun 19, 2009
  8. Audioguru

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    Dec 20, 2007
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    You do not want a simple volume control. You need a very complicated circuit that adjusts the volume of each sound continuously, changing its envelope. The circuit uses voltage controlled amplifiers and was used in the first Moog Synthesizer in about 1967. Now a computer and software do it.
     
  9. ale2121

    Thread Starter Active Member

    Mar 20, 2009
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    there must be a way? isn't the second link i sent a voltage controlled amplifier? i've seen lots more circuits that use a resistor in line with the speaker. I mean, technically, couldn't i just replace the volume knobs from my mixer with a force sensor? (that's the crude version, I know)
     
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