using digi-pot with 555 timer

Discussion in 'The Projects Forum' started by reinvention, May 1, 2014.

  1. reinvention

    Thread Starter New Member

    May 1, 2014
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    Hi all, i searched but couldn't find anything similar enough.
    i'm looking to interface a capacitance touch panel (TTP224) with a digital potentiometer (MCP4011) with a 555 to pulse the output.

    I need to pulse the +V or the 0V but not together, how can i make the MCP4011 digi-pot see only high or low pulses from a 555 timer? will a simple blocking diode be sufficient? this is just at the planning stage so far. Thanks all. (i'm a bit of a newbie)
     
  2. reinvention

    Thread Starter New Member

    May 1, 2014
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    if you were wondering.... i'm making a non-contact volume control, the pulse output (about 7-10Hz) from the Cmos 555 after the touch panel will adjust the volume quickly instead of me tapping the touch sensor. Thanks.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Can you sketch out what you're planning? This makes no sense to me.

    You can't pulse voltage without changing voltage, so I don't follow what you asked for.

    OK, I see you want multiple pulses after a single touch. How will the device know when to stop?
     
  4. reinvention

    Thread Starter New Member

    May 1, 2014
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    sorry for being vague, the capacitance touch panel puts a high output i figure that can power the Cmos 555 (or using a transistor), i want to set the 555's to astable mode at about 10Hz. i will use one touch output as +volume and another output as -volume each having a seperate 555.... so i need one to oscillate as high/open for +vol and the other as low/open for -vol.... the normal 555 output is high/low which means the digipot will just bounce back and forth instead of up or down.
    if i read the spec sheet properly, the digital potentiometer changes on reading a high or a low on the input pin, this needs to be released before additional changes apply.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Ah, OK, thanks. I think the first thing I'd do is experiment with that to confirm that a diode will in fact give the result you need.

    Even if you had two timer ICs, your pot only has one input pin, right? A single 555 can generate the pulses you need, but you need a way to choose which stream - sink or source of current - is hitting that pin.

    It occurs to me this is similar to an H-bridge that is used to send power to a DC motor. In one state it goes forward and in the other it reverses the motor. Just thinking out loud.
     
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  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You don't use only the high or low level. It is the relative timing of the pulses on two pins which is important. Are you using the 555 for pulsing the U/_D pin or the _CS pin? Are you familiar with the serial protocol necessary for incrementing/decrementing the pot value?
     
  7. reinvention

    Thread Starter New Member

    May 1, 2014
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    Thanks Alec_t.
    i missed the CS (chip select) when i looked at the PDF, figured i could simply pulse the U/D pin...... now i feel stupid.
    so.... another look (when i'm not so tired) suggests i need to hold the U/D pin before grounding the C/S pin to instruct the device in a certain direction, and the commands on the U/D pin react on a rising edge.
    if i use a pull-up resistor on one 555 output, then a pull-down resistor on the other 555 i just need to delay the oscillating until the CS is held low...... i think i'll make the order and just breadboard it and have a play until its what i need, i really don't want to be defeatist and end up with a manual pot on my project...... i was hoping to have a minimalist look to my DIY bluetooth speaker.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Don't think that's going to work. The 555 output is a low impedance sink and source.
     
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  9. reinvention

    Thread Starter New Member

    May 1, 2014
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    quite right..... giving myself more work than needed (as usual), would need 2 digi-pots for stereo, at least 4 or more Cmos 555's and god only knows how many other components :confused: ....... i've just found some PT2256's that'll do the job cheaper and save me the headache..... can still use the touch panel IC (TTP224) with it too...... super-simple even for me...... i'll use the 4 outputs of the TTP224 as follows, on/off, bluetooth sync, vol-up, vol-dn..... thats no visible switches or knobs on the speaker box. :D

    Thanks both for the initial help and push in the right direction..... feel a silly for troubling you guys now.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Well if you still wanted to go the digi-pot route, here's an option for generating the CS and UP/-DOWN pulses with the correct timing:
     
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  11. reinvention

    Thread Starter New Member

    May 1, 2014
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    Thanks Alec, i've seen the 4093 really cheap somewhere (some Chinese site), and thats a handy circuit to keep because i intend to make a super-miniature, audiophile quality, class D portable hi-fi at some point..... thanks also for taking the time to simulate the pulse timings i wanted..... i've got ALOT to learn.
     
  12. reinvention

    Thread Starter New Member

    May 1, 2014
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    Thanks Alec, thats a pretty handy circuit.... i'll save that for another time.
    i appreciate you taking time to do the simulation & thanks for your suggestions..... i'm just waiting for some parts from China then i'll breadboard my circuit to iron out any problems..... i know i'll at least have to adjust some capacitors on the TTP224 touch sensor.
     
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