Piezo momentary switch Help!

Discussion in 'General Electronics Chat' started by scscorpion, Apr 14, 2017.

  1. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    Looking to make a momentary switch using a piezo disc. Would like to do this with no power source or microcontroller. Looking to make a impact target that would switch on a prop that comes with a try me mode that uses a momentary switch. I think I know how to do this but not all the components or correct wiring. If I am correct I would hook up a piezo disc to a mosfet with a resistor on the ground from the disc to the mosfet. If someone could help with a diagram of the circuit with the information on the components needed (a BOM would be a big help) I would be grateful. I am a newbie and have some variable knowledge on electronics.
     
  2. #12

    Expert

    Nov 30, 2010
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    You're going to have to try a little harder.
    Piezo transducer: What part number? What size? Any specs at all? Found it in a gas grille ingiter? Was it supposed to be a sound producer or a spark igniter when it was born?
    Impact: With what? A BB? A hammer? Little sister's head?
    Switch on a "prop": A propeller? A prop for a magic show? A proposition for a business contract?
    No power supply: Mosfets don't work with no power supply, and neither do, "props".

    You do have some of it correct. A piezo transducer will produce a voltage pulse. A mosfet has nearly infinite impedance on its gate but they have rather delicate voltage limits. A piezo igniter can produce 1000 volts but a mosfet gate will punch through at about 20 volts. You need to have enough capacitance to hold the charge on the gate long enough to activate a switch of some sort. At the same time, you need a voltage limiter below 20 volts and a bleed off resistor in harmony with the capacitor to make a time constant related to what you want to switch.

    That's a lot of typing for, "I don't know" but you get the point. I don't know because I can't see what you see.
     
  3. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    Prop a animated figure that has it own power supply that uses a normal push type momentary switch to activate the circuitry inside the prop. Like a Halloween witch that laughs and eyes glow when button is pressed. I want to use a piezo disc that would be glued to the back of a 4 in round plastic outlet cover that when shot with a nerf dart or airsoft bb it would power the mosfet to complete circuit taken place of the momentary switch on the prop. I do not have the knowledge on which piezo I need or mosfet or resistors. That is some of the information I am trying to get if possible. Plus a diagram of the circuit so I can build it. There is a post on here for a piezo trigger a guy made to activate a flash when it felt the vibration from something being shot no battery or anything. when the piezo registered vibration it would trigger the flash. But the post had no information on parts used.

    I hope this helps you help me.
    Thank you
     
  4. #12

    Expert

    Nov 30, 2010
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    There have been Threads here about target indicators. I see you don't already have the piezo thing, only an idea. There are other ways. With a 4 inch target and the energy of an Airsoft gun, you could go directly to a microswitch behind a flexible membrane. Maybe cut up a rubber glove intended for washing chores, stretch it across the 4 inch frame, and place a microswitch behind it. Use about a 3 inch diameter cardboard stiffener behind the 4 inch membrane target. Just about zero electronics involved. No kilovolt pulses trying to kill a mosfet gate.
     
  5. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    Yes I've seen the forms with target indicators but they all require some type of controller which I could do but it would limit my idea to smaller area and run the cost up. Was trying to find away to use momentary switch as the trigger but didn't think it would be sensitive enough with out being very accurate hit on the switch. The plan is to make a shooting gallery inside a gym with props in different location for kid 5 to 12. Just thought the piezo as a impact sensor would be the most sensitive way to register the hit from the nerf guns.
     
  6. #12

    Expert

    Nov 30, 2010
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    That's why I suggested NOT doing it that way.
    It isn't. A piezo needs a harsh impact and an Airsoft is, by definition, a soft hit.

    You just heard my best idea for converting the energy of an Airsoft projectile into a switch movement.
    Other people will soon give their ideas.
     
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  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    My 2c is that he could use a mercury tilt switch, along with a de-bouncing circuit that would have to be calibrated in accordance to the level of motion imparted on the target. A bit more complicated, but a lot more sensitive than a microswitch.
     
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  8. #12

    Expert

    Nov 30, 2010
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    The debounce seems simple to me. A 12 volt DC supply with diode and a capacitor will be much more able to fire a mosfet without the danger of a piezo punching its gate out.
     
  9. Alec_t

    Expert

    Sep 17, 2013
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    So your piezo/whatever switch could make use of this if needed?
     
  10. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    If needed with a little bit of work most of the power supply could be hacked into. The only issues is most of the props have 3 Volts systems some do 6 volts but its rare. The debounce would require a power source that would be to costly for what I'm trying to do.
     
  11. scscorpion

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    Apr 14, 2017
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  12. Reloadron

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    Jan 15, 2015
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    Really depends on the sensor. I used several accelerometers which look like these:
    Accel1.jpg
    Not visible is a 10-32 mounting stud on the bottom. I also used some with a magnetic base. Even a gentle bump or bang would produce a signal. I amplified the signal using a INA (Instrumentation Amplifier) which I fed into a comparator circuit where I could adjust the threshold for sensitivity. This in turn, for my application, triggered a D Flip Flop. The accelerometer merely detected a "bang" or "thump" and worked quite well. Pictured are some old Endevco Meggit types but others make similar types. All you care about is enough sensitivity since you are not looking to measure any actual G Force or anything, you merely want to use an accelerometer to detect a small bang or thump.

    Actually, depending on the target you could likely use an el' cheapo microphone of sorts and amplify the output and feed that signal into a simple comparator circuit which in turn would trigger a D Flip Flop. Even the acoustical pick up from a cheap toy kids electric guitar would work.

    Ron
     
  13. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    Think I'm going to order these and try them out. SM-18010P, SM-18020P, SM-18030P. Think one of theses will work. They are the same just different sensitive level. Wish there was an electronic store that carried them around here so I could try them out before buying in bulk.
    SW18020.jpg
     
  14. Reloadron

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  15. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    Yes I've already seen them. Don't know which one I need. Would like to buy one of each to try out and see which one is right. Then order them. $3 on eBay for 10 $9 for 50.
     
  16. Kjeldgaard

    Member

    Apr 7, 2016
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    I once a few years ago had the task to develop vandal resistant push button assembly instead of a tactile push button. Initially I have some pictures of the mechanical structure.

    First I built a proof of concept setup with a 22 mm piezo buzzer:
    IMG_8598_SCALE.JPG

    Turned upside down and disassembled you can see the buzzer, here it's glued to the 2 mm thick aluminium plate with Epozy glue:
    IMG_8599_SCALE.JPG

    Here is a prototype of the final switch, with 12 mm buzzer on 0.8 mm stainless steel plates. The two steel plates and the buzzer is glued together with a single piece of double-sided adhesive film, having the same outer dimensions as the steel plates:
    IMG_8600_SCALE.JPG

    Both mechanical structures have an output of more than 150 mVolt peak at a pressure corresponding to the tactile push button.
     
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  17. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Why did you use two layers of steel instead of just one?

    Also, shouldn't it be activated by impact instead of pressure?
     
  18. scscorpion

    Thread Starter New Member

    Apr 14, 2017
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    Nice but how was the circuit configured. Did it act as a momentary switch or hooked up to a controller?
     
  19. Kjeldgaard

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    Apr 7, 2016
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    The whole project was about replacing some door open push buttons, with something far more robust. The original buttons were tactile switches with a plastic foil front.

    We discussed how to solve the task and chose to do some experiments with a piezo buzzer.The outcome that the first tests with 22 mm piezo elements and the two square aluminum plates and just one oscilloscope were so good that we continued the construction of a two-wire comparator circuit that would connect to a PLC.

    The slightly special outline is taken directly from the original push buttons design, so you just had to mill a little more hole in the middle of the piezo element. The original design was milled 1.5 mm down in an aluminium profile. However, as a single 1.5 mm stainless steel plate would be too rigid to press, we chose to use two 0.8 mm steel plates and the thinnest double sided adhesive we could get and got a solution where the front of the piezo button was about 0.2 mm above the aluminium profile.

    I have attached the schematic of the circuit we used, where I have added some of the important currents and voltages. As mentioned earlier, it is a two-wire solution where the idle current must be low and when the button is pressed, an impulse must be sent at which the residual voltage on the terminals must be low.
    PiezoButton_1a.jpg
     
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