impedance.

i am planning for an arduino with 16 piezos as drum surfaces.
i hope to use the 4051 multiplexer in order to get the extra ins.

the 4051 multiplexer has an input impedance of 1.2k ohms.
arduino’s dital pins have an impedance “equivalent to a series resistor of 100 Megaohms” in front of the pin.
i couldnt find specs on arduino’s input impedance on its analog pins.

in the comments of this thread: http://todbot.com/blog/2006/10/29/spooky-arduino-projects-4-and-musical-arduino/, todbot says that the multiplexer…

must have really high input impedance (around 10 Mohms) so you can sense the piezo’s output voltage.

is it possible to raise the input impedance of the 4051, with, as the arduino literature describes, “a series resistor of 100 Megaohms in front of the pin?”

any guidance on the issue would be much appreciated.

nym

the 4051 multiplexer has an input impedance of 1.2k ohms.

Why do you say that? I can find no reference in the data sheet to input impedance. You are not mixing it up with any other parameter? I would expect the input impedance to mirror what the output is connected to. (If you can talk about inputs and outputs with a bidirectional buffer)

While that second circuit is excellent it does use negative supplies and as such is not suitable for direct connection to an arduino.

grumpy - i found out by emailing Jameco and asking them to test.

you say you would expect the input impedance to mirror what the output is connected to. that would mean that when connected to the Arduino, the input impedance of the 4051 should = the input impedance of the Arduino.

thanks for the link for the piezo pad. i figured i’d have to create some sort of smoothing/capture frame for this. i thought i’d just do it with code but i see i should make a some sort of peak detector.

now i gotta find out how to get the capacitor to flush in a timely matter

todbot says that the multiplexer

todbot is mistaken: muxes like the 4051 are not exactly like mechanical switches, but they're close enough in this case.

The piezos will see the impedance of the analog input the mux connects them to.

found this awesome solution. even has a pot for analog sensitivity adjustments. that's a handy solution.

so: of the problems encountered so far, this:

1- converts signal to dc 2- generates a more predictable signal from the piezo 3- protects the arduino input from overload 4- flushes the capacitor (hopefully fast enough)

my arduino code will probably do something like read the signal after this circuit, wait until it stops growing, and register it as a midi note the second it begins to decrease.

this slight delay is because the 100uF capacitor will cause the arduino to read a signal something like this:

and the 1M Ohm will cause it to quickly drop off.

The piezos will see the impedance of the analog input the mux connects them to.

this is excellent news.

last night after seeing the complexity of Wyatt Olson's "Drum Master" arduino project, i began to think this was too much for me to do. i got a little down on the project (and on my limited knowledge and experience) but today i know that the urge to make this thing is too strong.

this is all just an opportunity to step my electronics game up. thank you all so much for your help.

grumpy - i found out by emailing Jameco and asking them to test.

Worrying they know so little about what they sell.
Did they say how they measured it? Was it the DC resistance of an un-powered chip?
Might be worth a laugh asking them.

found this awesome solution.

Those capacitors look awfully big, I would have thought they would simply swallow what the transducer puts out.
You can clear the capacitance on the input by programming it to be an output, pulling a logic zero on it then turning it into an input again.

the email was simply "we tested 1.2k ohms"

these guys probably don't like having to answer questions about orders under 10 USD.

if the capacitors are too big, i'll experiment with smaller ones.

hey, richard - got that schematic here:

http://petervieth.com/index.php?page=8-input-midi-drum-pad

he describes what each component is doing in detail. his purpose and equipment is the same as mine (5v max input AVR, piezo, etc)...i wonder why he is using such large capacitors.

If you look at the photo that accompanies that page you will see that there is nothing like a 100uF capacitor in the circuit. My guess is that he got the units mixed up and perhaps he means a 100nF not 100uF.

richard - i certainly don’t disbelieve you. as you can probably tell, i’m relatively new to component relations and circuit diagrams. i will indeed be implementing a capacitor discharge as that seems to be the most sure way to get a sure 2nd reading from the piezo.

the good news is that this means i won’t have to go to radioshanty and buy capacitors, as i’ve got plenty of smaller ones.

thanks for your comments.

did a little research. i will definitely be working with smaller capacitors in order to reduce the RC time. the large capacitor will likely be a 100n or 10n, which will reduce my RC time to 10ms or 1ms, respectively.

http://www.massmind.org/images/www/hobby_elec/e_capa.htm

this site taught me a lot about capacitor value and type identification. however, i don't know which i should be using. i have tons of ceramic caps of the proper values, but it says they shouldn't be used for analog circuits "because they can distort the signal." it says that electrolytic capacitors, on the other hand, are made to function at a range of [u]>[/u]1uf.

can someone tell me which capacitor type should i use and why?

thanks nym

“because they can distort the signal.”

Is a bit of a rubbish statement.

Use ceramic capacitors when you can and electrolytic when you must. You won’t see an electrolytic lower than 0.1uF and most are in the range of 1uF and upwards. You won’t see many ceramics above 10uF and most are 0.33uF and below.

grumpy - you're an excellent false statement detector. thank you.

i should have some results at some point this weekend. my rbbbarduinos came in the mail today.

I'd be careful with the piezo stuff as it may can produce a voltage pulse that could blow semiconductor materials unless the peak voltage is dampened with a capacitor or similar. Just thinking of the lighter on my grill.

Some varieties of capacitors (ceramic and otherwise) can indeed cause distortion when used improperly

Trick is not to use them improperly.

Google: capacitor distortion

I did and the first credible hit said:-

A great deal has been made of the 'sound' of capacitors. Dielectric losses (dissipation factor, dielectric absorption) feature heavily, with some fairly outrageous claims made as to the importance of these losses in amplifiers and other audio equipment. There are sites on the Net showing that different caps have different properties, and this is often used a 'proof' by many people that the differences are audible. There are sites that seem to have impeccable credentials, but have managed to create nothing but FUD (fear, uncertainty & doubt) with wild claims of irreparable damage to the signal by using the 'wrong' kind of cap ... even as a supply bypass (yes, it's true - this claim has been made). In some cases you will read things like "listening tests have indicated ... (blah, blah, blah)". But where is the data? Who conducted the test? How was it conducted? Was the test ever really conducted at all? Most claims of this nature indicate that there is a hidden agenda, so beware.

Which I whole heartedly agree with.

Full text at:- http://sound.westhost.com/articles/capacitors.htm#2.3