Piezo's and LED's

[Dylan]

Hey guys, long time no post.

Many many moons ago there was a thread (I'm fairly certain on this board) asking if anyone knew how to light up an LED, controlling the brightness using a piezoelectric transducer. I can't find that thread anymore, and I cannot remember if there was an answer, so if you asked how to do that, and never got an answer, I can help you out.

I was building a lighting rig for my band when I discovered I could do exactly what I explained. I drew a schematic of what I built. When you don't put any pressure on the piezo, it is off. When you apply pressure, the LED comes on. Apply only a little pressure, and it's a little on. Experiment with different resistor values to get the effect you prefer.

[Circuitbenders]

When you don't put any pressure on the LED, it is off. When you apply pressure, the LED comes on.

I'm assuming you mean the piezo there? Putting pressure on an LED probably won't help  

How much maximum voltage does your average piezo supply?

[Dylan]

oops! On the transducer I'm using, I was getting a maximum of about 1.5V, averaging at about a volt. I was thinking this might be a cool was to control VCO's.

[SineHacker]

this is a very cool idea! will try it out later, I've been spending a lot of time working with ways to produce light lately for my performances, so I'm always looking for stuff like this, I wonder how many LED's you could string to one disc - or imagine having a cello or something with piezo's stuck in different positions triggering a load of LED's as the instrument is bowed or plucked!

[SineHacker]

imagine a giant wall of LEDs!!

[Dylan]

this is a very cool idea! will try it out later, I've been spending a lot of time working with ways to produce light lately for my performances, so I'm always looking for stuff like this, I wonder how many LED's you could string to one disc - or imagine having a cello or something with piezo's stuck in different positions triggering a load of LED's as the instrument is bowed or plucked!

That's exactly what I was experimenting for! I joined my friend's band a couple months back and he wanted me to think of some lighting rigs. So I thought it would be cool to have some lights flashing to the beat of the tunes.

[Circuitbenders]

presumably you could wire up a huge load of LED's if you used the transistor to switch something like a relay or optocoupler that could deal with a lot more current.

[SineHacker]

what is the other component on the sheet, is that a transistor?

[Dylan]

what is the other component on the sheet, is that a transistor?

The component on the left (SG1) is the piezo. The one on the right is a basic NPN transistor. The one I'm using is a MPS2222A, you could use any decent gain NPN though I would imagine.

[SineHacker]

cool i think i have one

[Dylan]

presumably you could wire up a huge load of LED's if you used the transistor to switch something like a relay or optocoupler that could deal with a lot more current.

I'll be breadboarding that in a moment!

[SineHacker]

Boo I just have some 2n3819 jfet's will have to order something else

[Dylan]

Eh, you'll live. They're only $0.05 a pop at jameco https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_38359_-1

[Gordonjcp]

FETs are fun.  Because the gate impedance is so high they pick up noise from nearby sources easily.  You could wire up an LED to your 2N3819 rather like in the piezo circuit there and make it light as you bring your finger *near* the floating gate pin.

[SineHacker]

well me being me, i did set it up and mess around, I deconstructed it afterwards and I can't really remember how I had the pins connected, but basically using the 2n3819, the LED was constantly brightly lit and tapping the piezo caused it to fade out fade in smoothly depending on how hard you tapped the piezo