Need help sending two signals to the same output without combining!

Thread Starter

megaben300

Joined Aug 10, 2021
5
Okay, obviously electronics is not my strong suit. I hope I can explain my problem well enough that someone can help me.

I am working on an electronic drum project. I have two piezo's that need to connect to a tip/ring/sleeve jack. Both piezo's share ground on the sleeve.
I need the signal from piezo#1 to go only to the tip on the jack. The signal from piezo#2 needs to connect to both the tip and the ring to work correctly. When I connect piezo#1 to the tip, I get the expected signal for that input. When I connect piezo#2 to only the ring, I get nothing, which is also the expected signal for that input alone. When I short the tip and ring, I get the expected signal from the ring input, but I lose the "tip only" signal from piezo#1 because now both piezos obviously trigger the ring signal because of the short.

My idea was to use a diode between the tip and ring so that piezo#1 triggers only one input, but the piezo#2 triggers both. Will that work as expected? If so, how do I determine what diode type and value to use? According to my meter, the piezos put out around 350mv when activated at max.
If this won't work, can anyone tell me a better solution? I need to keep this passive if possible.

Thanks in advance, and sorry I don't know the correct terminology.
 

BobTPH

Joined Jun 5, 2013
3,821
I think the piezo is going to produce an AC signal, so a diode will not isolate it properly.

I think you need to send both signals through a high impedance mixer circuit to get the combined signal for the ring. Piezo 1 would also be connected directly to the tip.

The mixer circuit would use an op amp.

Bob
 

LowQCab

Joined Nov 6, 2012
1,189
It would help tremendously if You would state what You are trying to accomplish
and supply at least a Block Diagram, (if not a proper Schematic), of what You want.
And Pictures as a last resort .............
.
.
.
 

Audioguru again

Joined Oct 21, 2019
3,670
A drum resonates with a smooth AC output. A diode would rectify the signal and make it sound something like a buzzer.
But you mentioned "trigger" then maybe you need only a pulse output to activate a drum sound?

A Schottky diode works from an input as low as 0.2V. Connect piezo #1 directly to the tip and connect piezo #2 directly to the ring. Then connect piezo #2 in series with a Schottky diode to the tip.
 

Thread Starter

megaben300

Joined Aug 10, 2021
5
A drum resonates with a smooth AC output. A diode would rectify the signal and make it sound something like a buzzer.
But you mentioned "trigger" then maybe you need only a pulse output to activate a drum sound?

A Schottky diode works from an input as low as 0.2V. Connect piezo #1 directly to the tip and connect piezo #2 directly to the ring. Then connect piezo #2 in series with a Schottky diode to the tip.
Thanks. Here's a little info that may help explain:
I have a "drum module" which is basically a simple computer with drum sounds stored in it. I also have drum "pads" which are drums that each have 2 brass/ceramic piezos in them. The drums themselves make no usable sound when struck. The two piezos give each drum 2 "zones" so that you could trigger two different sounds depending on where the pad is struck. If I hit in the center it vibrates piezo#1 and triggers sound#1 from the module. If I hit the drum on it's edge it vibrates piezo#2 and triggers sound#2. The drum module has a stereo 1/4" jack for each "drum" that can be attached. It is common to "split" the inputs on drum modules so that you can connect two separate pads with one piezo each, instead of one pad with two piezos. This can be done easily with a simple splitter that splits a stereo trs into separate left and right channels. However, my module has a few inputs that work differently, as described above, and I am looking for a passive way to split those into two separate pads. Instead of using a piezo connected to the tip to trigger sound#1 and a piezo on the ring to trigger sound#2, these inputs expect to see a signal on only the tip of the jack to trigger sound#1, and a signal on both the tip and the ring to trigger sound#2.
I hope this makes more sense. I'm a machinist, and it's not likely that I'll be able to come up with a schematic or anything useful for a visual aid.
 

LowQCab

Joined Nov 6, 2012
1,189
Diodes probably won't work, the Piezo-Elements just don't have enough Voltage
to overcome even a 0.25V Voltage-Drop.
What is needed is to lower the Impedance of the Piezo-Elements with an Op-Amp.
Then the low Impedance Op-Amp Outputs can be mixed together any way You like.

Do You think that You could make a very simple Bread-Board Project
that runs on 2- AAA-Batteries ?
If yes, I'll make You a Schematic with part numbers.
.
.
.
 

Thread Starter

megaben300

Joined Aug 10, 2021
5
Do You think that You could make a very simple Bread-Board Project
that runs on 2- AAA-Batteries ?
If yes, I'll make You a Schematic with part numbers.
.
.
.
I'm sure I could assemble it if you're willing to go to the trouble to draw up a schematic. I appreciate the help!
 

LowQCab

Joined Nov 6, 2012
1,189
I didn't get to looking-up part-numbers yet.
If You have any questions along the way, just ask here. ............
.
.
.
Piezo Amplifier FLAT .png
 

Thread Starter

megaben300

Joined Aug 10, 2021
5
I didn't get to looking-up part-numbers yet.
If You have any questions along the way, just ask here. ............
.
.
.
Thanks for that. I really appreciate it.
I don't think I'll have any problem building this. I should have everything on hand except for the op amp. I'm a little confused as to how I will implement this to achieve what I'm trying to do though. I probably just don't understand exactly what is happening in this circuit, or I may be explaining my end goal in a confusing way. In case it is the latter, I have attached a hasty ms paint drawing (sorry). This is what I'm trying to achieve, but the short between the tip and ring needed by piezo#2 causes both piezos to act the same. I have tested this, and verified that each piezo connected on it's own works fine, but the short needed by piezo#2 also causes a short for piezo#1. Piezo#2 must connect to both the tip and the ring, but piezo#1 can only connect to the tip. Assuming the circuit in the schematic you sent will work, where in this chain does it go?
diagram.png
 

Audioguru again

Joined Oct 21, 2019
3,670
The piezo transducer is being hit with drum sticks. The voltage will probably be high enough that a protection circuit is needed at the opamp input. E-drum circuits use a diode to the + supply and another diode to the - supply.
 

LowQCab

Joined Nov 6, 2012
1,189
You need to think of the Piezo-Elements as Microphones.
So basically You need several simple Mic-Mixers.

You can drive the Inputs of 2 separate Amplifiers with the same Piezo-Element,
this will create 2 completely isolated outputs,
one of those outputs will be "mixed" with
one of the other Amps from a different Element,
the other isolated Output can go to a separate input on your Controller.

I'll draw it out when I get some time.
.
.
.
 

LowQCab

Joined Nov 6, 2012
1,189
The piezo transducer is being hit with drum sticks. The voltage will probably be high enough that a protection circuit is needed at the opamp input. E-drum circuits use a diode to the + supply and another diode to the - supply.
Interesting point.
I didn't imagine that the Piezo-Elements could have that much Output-Voltage.
.
.
.
 

Audioguru again

Joined Oct 21, 2019
3,670
Doesn't a piezo BBQ lighter produce a high voltage spark without a transformer? Of course a load will reduce the voltage, maybe the 10M of the voltmeter reduced it.
 

LowQCab

Joined Nov 6, 2012
1,189
I've tinkered around quite a bit with Piezo-Electric Horn-Tweeters,
they pretty much act just like a really crappy Microphone, as far as Output goes,
and I'm leaning towards the Drum manufacturer using the same
sort of "round-wafer-on-a-Brass-Disc" design.
No radical "clicking" just smooth bending, (very rapidly).

The Drum Processor would be destroyed by any accidental high-Voltage-spikes.
.
.
.
 

Audioguru again

Joined Oct 21, 2019
3,670
One e-drum circuit uses a series resistor feeding the base of a transistor and a parallel reverse diode.
Another circuit uses a LM324 opamp powered by a single 5V supply and uses a Schottky reverse diode to protect the input from going too far negative and uses an input volume control and a 47k shunt input resistor for positive input voltage protection.
 

Attachments

Top