Hello, I'm trying to build a circuit where I need to send a one single input to a keyboard, and then every key on that keyboard has it's own output. So, at that output, each key has a trimmer to control the voltage. (I'm trying to build a 1V per octave analog keyboard). So, in order to be able to activate multiple keys at once without messing up the voltage supplied to each I (presume) need a buffer. The problem is that if I use an op-amp buffer in this case, while I'm not pressing the key, the op-amp sends a higher voltage instead of 0v (which is what I need, the circuit to be at 0v if no key is pressed). So my question is if I can use the SN74HC244N ic instead of an op-amp, and if it would produce that result. Thank you in advance!

 

Please post the schematic of the op-amp that is not working the way you want.
A digital IC like a SN74HC244 will not work with analog voltages. It has an output that is low or high and will not be at 1V or 2V.

 

Please post the schematic of the op-amp that is not working the way you want.
A digital IC like a SN74HC244 will not work with analog voltages. It has an output that is low or high and will not be at 1V or 2V.

Hello ronsimpson, thank you for the reply.
But the high or low will be the input voltage and zero right? Could I not use a trimmer at the output to get it to my desired power?

I'm using the simple op-amp buffer scheme with negative feedback with the tl074acn (Data sheet -> https://www.allaboutcircuits.com/electronic-components/datasheet/TL074ACN--Texas-Instruments/ )

 

What you have there is a buffer known as a follower, where the output follows the input.

You should never leave the input floating (no connection)

Try placing a pulldown resistor from the + input to ground.

It may also be better to control the output of the Op-Amp at the input rather than the output. (using a voltage divider)

 

Because I don't understand, I drew a schematic.

The voltage is key to your project. I used a LM7805 voltage regulator to get a good solid 5V. Keep it close to the Pots. Use caps C1, C2.
I have a Pot (trimmer) for each key (SWx). There are 10 turn Pots that will allow you to set the frequency easy.

If you only wanted one voltage (tone) at a time then all the switches could be wired together to one oscillator.
But you want to press two or more switches. I can't see where you are going.

Please send more information.

 

If you are trying to build your own piano keyboard, there are other solutions.

1) There is something called a Top Octave Generator IC MOSTEK MK50240. This chip is obsolete but you can search around for it.

2) You can program an MCU to generate the desired frequencies across all octaves.

 

The description in post #1 is unclear. "a voltage" could be a DC level or an AC signal with a specific frequency. And there might even be an octave's worth of notes being supplied to the keyboard.
So to avoid random guesses based on not enough information more information is needed.

 

I don't know if that amp you are using will do the job or not, I don't have a model for it, but I did find one that will go from "0" volts to whatever output you need depending on supply voltage. It comes as a single or double but not quad.

In all honesty I have no idea how a 1V per octave analog keyboard works but perhaps this information can be useful nonetheless.

The switch represents the key and is connected to a voltage divider that delivers 1 volt to the follower and in turn outputs 1 volt.

The voltage I used is 12 but can be anything from 5 to 24 with this amp (AD8065/AD8066) with the proper adjustment of the divider for different voltages.

Good luck with your project.

 

Hello, I'm trying to build a circuit where I need to send a one single input to a keyboard, and then every key on that keyboard has it's own output. So, at that output, each key has a trimmer to control the voltage. (I'm trying to build a 1V per octave analog keyboard). So, in order to be able to activate multiple keys at once without messing up the voltage supplied to each I (presume) need a buffer. The problem is that if I use an op-amp buffer in this case, while I'm not pressing the key, the op-amp sends a higher voltage instead of 0v (which is what I need, the circuit to be at 0v if no key is pressed). So my question is if I can use the SN74HC244N ic instead of an op-amp, and if it would produce that result. Thank you in advance!

OK, here is my guess, which is that the keys will select voltages provided by what will function as a multiple tapped voltage divider. Given that octaves most often refer to musical frequencies, my guess is that the voltage controls an oscillator.
What is not mentioned is if more than one key will be operated at a time, and what should happen if more than one key is operated at any time. What do the voltages cause to happen?? Or is this simply a switching voltage source??
One option is to sum the voltages in a single op-amp, another is for each grouping to be summed per some plan.
Making every output buffered so that multiple outputs can be isolatedsimply requires one amplifier per key. Complex but easy to do.

 

Hello everybody! I'm really grateful for the time and effort you gave to try to understand and answer my question. Yes, the objective of the keyboard is to control an oscillator with voltage. Probably I didn't make the problem clear enough, but the issue was that I was trying to buffer each one of the keys in order to isolate them, but I was leaving the input (+ side of the op-amp) floating. So the solution from ElectricSpidey worked out perfectly.
I used 100k pulldown resistors and everything is now working as expected (I hope, because I just ran some tests and haven't yet built the full circuit). Although I had to change the op-amp from a tl074 to a lm324 to work.
Once again thank you all for the answers and the support

 

So the system will be a "one note at a time" keyboard. At least that is what is described. If only one key at a time is ever operated then one buffer for all is all that is required.

 

So the system will be a "one note at a time" keyboard. At least that is what is described. If only one key at a time is ever operated then one buffer for all is all that is required.

I don't think that would work, because I want to give them different values. There is only one input, and that input voltage is sent to every key. After I press the key that same voltage is sent to the buffer. But I need different values in each one.
I'm using a piano keyboard from an old piano I had Laing around. So I'm stuck to the existing circuit. That's why I want to buffer the key output and then make the voltage divider to have the right voltage.

 

Creating a multi-tap series string voltage divider will provide all of the different voltages you seek. And if a potentiometer is used for each tap point then you will need 88 of them. Each adjustable voltage will be above the one before it, no overlap possible.
Certainly somebody wit a good drawing program can create the drawing, A string of potentiometers with the ends i a series string so the total resistance does not change, but all of the wiper connections are available.