Sound board without On/Off switch powering

Discussion in 'The Projects Forum' started by ttttrigg3r, Oct 14, 2016.

  1. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    Hello, I'm a new member and didn't see a section to introduce myself, so I'll do it here in my first post. Hope everyone is having a good week.
    I'm starting a new project which is a 30 button sound board. You press any one button, and a custom sound plays.
    I've come across this Adafruit Audio FX Sound board which has 11 I/O ports for buttons. Three of these boards give me 33 buttons.
    The boards have a Voltage In and Ground and I plan to power it using batteries kind of like the provided picture below.
    However, this setup means a power switch needs to be present to turn it ON/OFF or else the circuit will continue to run.
    I want this sound board to not have to be turned on and off. i.e. if I press a button, it powers on, plays the sound, then after playing it, powers off.
    Staples do this with the Easy buttons. I've seen it in other portable sound boards. How do I do it to my circuit?
    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    You can use a 555 in a one-shot circuit to power the board when a button is pushed, then turn the power off after the sound is done.
    You set the one-shot time for the maximum sound length (which is what?).
    How much current do the modules take?

    Does that sound (pun intended) like what you want?
     
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  3. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    I don't know how much current the module takes. I'm planning to use the module along with a 3W+3W amplifier and have all units to a common battery. I don't know what a one shot circuit is, so I'll be researching that now. Thanks.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    What powers the 3W+3W amplifier?
    It will need more power than the three 1.2V batteries you showed.
     
  5. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    I'm running a single 18650 Li-Ion battery. I've had good experience with the 18650 and this amplifier board in a separate bluetooth speaker project. When I crank the speakers up to max, the current draw is close to 3 Amps. I'm limiting this sound board to 1 speaker, so I'm fairly certain the battery is good enough to power 3 boards and 1 amp.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Below is the LTspice simulation of a 555 timer one-shot circuit that should do what you want.
    It uses a CMOS version of the 555 since the bipolar version is not specified below 5V.

    The MOSFET can be just about any logic-level device with a specified ON resistance (Ron) of ≤10mΩ at a gate-source voltage (Vgs) of 3.5V or less (as shown in the data sheet spec table).

    It uses thirty three 50kΩ resistors (only 3 are shown) for isolation to connect to the 33 push-buttons.
    You could also use one resistor and 33 diodes for isolation.

    The R2C1 time-constant determines the ON time of the output after one of the PBs is pushed.

    The circuit draws no quiescent current when off.

    Edit: This circuit assumes the PB signal to the sound board is a positive going pulse.
    If not then the circuit will need to be modified.

    upload_2016-10-14_23-35-12.png
     
    Last edited: Oct 15, 2016
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  7. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    Thank you. I have some questions about the MOSFET. I'm looking for a P-Channel MOSFET right? The closest I could find is 25mOhm @ 25A, 5V. What is the importance of this rating.
    Also, if my supply source is 4.05V, what is the important spec that I'm looking for?
    I'm on Digikey trying to find a suitable one, and there's so many. It's a little overwhelming.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,794
    1,103
    Look for Vgs(th)(max) lower than 3.5V in magnitude.
     
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  9. crutschow

    Expert

    Mar 14, 2008
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    The Vgs(th)(max) is only a rough indication of the MOSFET's suitability at low voltages.

    What you need to look at is the voltage where the Rds(on) value is tested, as shown in the data sheet.
    So you should look for a transistor where the Rds(on) value is specified for a Vgs of 4V or less.
    These are commonly called "logic-level" types.
     
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  10. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    Would this work? FDS4465.png
     
  11. crutschow

    Expert

    Mar 14, 2008
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    3,232
    It should be, assuming the current rating is sufficient for your application.
    It will have 8.5mΩ of ON resistance with a gate-source voltage of ≥4.5V at 13.5A maximum current.
    Note that the dissipation will be 1.55W when fully on and carrying 13.5A so it should be mounted as shown in Note 1A.
    When mounted with no copper pad underneath for heat sinking, the maximum current would be about 10A.

    Be sure and use all the pins in parallel for the drain and source connections to minimize the ON resistance.
     
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