Hobbyist Question: 3V to trigger 12V circuit

Thread Starter

KevinSRussell

Joined Mar 11, 2019
40
First post - go easy!

I am building a Floor Piano using the innards from a Music Keyboard. I have reverse engineered the Keyboard PCB (attached) the switches are the Keys on the Keyboard. the circuit is 3 Volts DC.

When the switch is closed (key pressed) I need to light the Key up. I plan to have a 12V DC LED circuit to do this.

My question is - how to trigger the LEDs? Independent circuit or incorporate it into the attached Schematic? Relays?

I will have a further question based on the answer! While the switch is obviously On or Off, I'd like the LED lights to dim slowly after turning off (1 second delay). If I'm remembering correctly from my Apprenticeship - I'd need a Capacitor somewhere.

Any advice would be gratefully received.
Kevin
 

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Ya’akov

Joined Jan 27, 2019
8,973
Welcome to AAC!

The matrix switching of the keyboard means the "right" way to do this is with a microprocessor. While this is relatively complicated, thanks to Arduino and its IDE (Integrated Development Environment), it is attainable.

Roughly, you'd be running the output of the keyboard switching matrix to the MCU, then sending it to the mainboard of the instrument as well as using it for your own purposes. It also means you'd be able to do fancy things, even beyond fading, including, for example, prompting key presses with lights, changing colors, etc.

I am sure you can get plenty of help with such a project here if you want to try it.

Good luck!

[EDIT] A nice side effect of this approach is that you'd also be able to have the MCU play the keyboard.
 
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pmd34

Joined Feb 22, 2014
527
Hi Kevin! Welcome to AAC! It sounds like the easiest way to activate your LEDs, would be to use N-MOSFET switches:
http://www.learningaboutelectronics.com/Articles/N-channel-MOSFET-switch-circuit.php

(You just need to make sure that the mosfet you choose, has a "threshold voltage" just below 3V so its well switched on at the voltage levels you are using.)
You could maybe do it with relays, but I think you would get a bit P'd off with all the clicking noises!
 

Thread Starter

KevinSRussell

Joined Mar 11, 2019
40
Thanks Chaps - great answers.

Yaakov, thanks for the answer. I have seen a Floor Piano Designs using Arduino and while I am not averse to this approach (or Raspberry Pi) and granted that this may be the best solution - I was hoping to create a simpler solution without programming.

PMD34 - The MOSFET solution is s simple one to test. I guess that a 2N7000 MOSFET is the way forward?

Point taken regarding Relays, although the clicking of relays on a Floor Piano may not be the most annoying aspect!!

Thanks again,
Kevin
 

Ya’akov

Joined Jan 27, 2019
8,973
Thanks Chaps - great answers.

Yaakov, thanks for the answer. I have seen a Floor Piano Designs using Arduino and while I am not averse to this approach (or Raspberry Pi) and granted that this may be the best solution - I was hoping to create a simpler solution without programming.

PMD34 - The MOSFET solution is s simple one to test. I guess that a 2N7000 MOSFET is the way forward?

Point taken regarding Relays, although the clicking of relays on a Floor Piano may not be the most annoying aspect!!

Thanks again,
Kevin
While the discrete approach can work, you will have to be sure the circuit switching the LEDs has a high impedance so it doesn't booger up the intended use of the switches.

It will also require a lot of devices. You will end up with a mess unless you make PCBs to deal with it. Your fading will also require circuitry. The "without programming" part may be false economy. Brute force can work, but wow... I think you might find that investing the time in the Arduino approach will not cost more of it, and will have a great deal of knock on benefit.
 

Thread Starter

KevinSRussell

Joined Mar 11, 2019
40
Yaakov,

You are probably right. Each key of the Piano is 600mm X 200mm so there is plenty of space in the cavity to hide the mess! As for teh impedance, now that you mention it, that may be an issue, especially when multiple keys are pressed.

I think I'll try the MOSFET solution first with a single key then come back with my tail between my legs if it doesn't work!

Kevin
 

pmd34

Joined Feb 22, 2014
527
Hi Kevin, there are actually 1000's of different fets about, in all sorts of packages. You have to choose which one suits you best.. the 2N7000 is just an example really.. if you look for the data sheet, you get all the infomation about what it can do:
https://www.onsemi.com/pub/Collateral/2N7000-D.PDF

The "drain sorce voltage" - 60 V is more than enough for your application (at just 12V)
(This maximum current goes hand in hand with the "RDS(On)" effectively the switch resistance when its closed.. the lower this value the higher the current it can switch.)
on page 1 "on characteristics" there is a bit more information on the resistance at different voltages, it starts switching at just 0.8V which should be ok,
However the "drain current" is just 200mA continuous, so this maaaay be a little low for your needs?

If so you can have a look for some more powerful ones (always better to over engineer). In this case just search your "supplier" ebay, amazon, aliexpress etc.. for something like "N-Mosfet to-92" the TO-92 is this particular style of package, its fairly small and has easily solder-able wires, so should be fine.
 

Ya’akov

Joined Jan 27, 2019
8,973
Yaakov,

You are probably right. Each key of the Piano is 600mm X 200mm so there is plenty of space in the cavity to hide the mess! As for teh impedance, now that you mention it, that may be an issue, especially when multiple keys are pressed.

I think I'll try the MOSFET solution first with a single key then come back with my tail between my legs if it doesn't work!

Kevin
It can certainly be made to work, but if you are going to do that, plan on having PCBs fabricated.
 

Thread Starter

KevinSRussell

Joined Mar 11, 2019
40
20190313_102126.jpg
Actually, can I ask another basic question. The attached image is the SMD Diodes on the Keyboard PCB. There are no markings but I narrowed it down to TZMB10-GS08 SMD 10V Zener. I have found what I think is the ewquivalent leaded Zener: 5.1V Zener Diode 5% 500 mW Through Hole 2-Pin DO-35

Is this correct?

Cheers
Kevin
 

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pmd34

Joined Feb 22, 2014
527
Hi Kevin, you would have to try and measure the voltage drop across the diode with a multimeter to be sure, its a standard "sod-80C" diode package, and they can be of many flavours, whether it is a zenner, or just a simple diode, its not usually possible to tell.
If you take a photo showing more of the circuit board it might be possible to get a better idea of what they do.
 

pmd34

Joined Feb 22, 2014
527
I was thinking it might be something like that... then I guess they are normal diodes.Something like a "1N4148" would probably do the same job. They are probably used something like this:
http://www.openmusiclabs.com/learning/digital/input-matrix-scanning/index.html

Its a way of "multiplexing" a large number of switches with a minimal number of data lines, you "scan" the switches row by row to see if they are active, so long as you do it quick enough you won't miss any pushes.
 

Ya’akov

Joined Jan 27, 2019
8,973
Another thought strikes me. If you designed discrete matrix decoder that could read the key presses just as the main board does, you could use it to address LEDs, all with discrete logic, no programming. It would cut back dramatically on the number of boards you need.

After all, the keyboard is designed the way it is because wiring each key as you intend to do is... a little excessive, let’s say.
 

Tonyr1084

Joined Sep 24, 2015
7,777
MOSFETS: When you turn them on they stay on unless you have a way of draining the capacitance within them. That means that if you strike a middle C you can light an LED that represents the middle C note. However, when you release the key the LED may stay on for a substantially long time. Unless you use a high resistance to drag the gate voltage back to zero volts and turn the LED off. Depending on the resistance you can possibly set a decay rate so that you get something similar to what you described early on - the note decay and the light decay to match. However, doing so may interfere with the electronics that generate that middle C note. You COULD change the way the circuit works.

I hate to be the guy who finds problems with the solution. I'd prefer to find solutions to the problem.
 

ElectricSpidey

Joined Dec 2, 2017
2,756
If you are building the floor piano from scratch, I would suggest using a second switch inside each key to do the lighting, and skip the interface with the keyboard pcb.

I don’t know what kind of switches you have in mind, perhaps something with a magnet and Hall Effect, or just micros, but having a separate switch to light the keys, makes the circuits a piece of cake.
 
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