Electronic – arduino – Read force sensors of the keys an existing electronic keyboard

You want a signal range from 0V to 5V. Don't we all :-)? Let's go for a different approach and see where that gets us.

Starting point: cheapest and most simple solution.

That would be a series resistor to create a voltage divider. That's the absolute minimum. I've noticed that people don't give that resistor much thought, the just pick a nice round value like 10k\$\Omega\$. But I found that there's an optimal value for this.

The curve shows the voltage difference between the minimum and maximum reading (9k\$\Omega\$ and 20k\$\Omega\$ resp.) as a function of the series resistor (in k\$\Omega\$). See, it indeed has a maximum. That's easy to find if you remember that

\$ \left(\dfrac{f(x)}{g(x)}\right)' = \dfrac{f'(x)\cdot g(x) - f(x) \cdot g'(x)}{g^2(x)} \$

The difference \$V_{MAX}\$- \$V_{MIN}\$ has an extremum for

\$ \dfrac{d}{d R_X} \left(\dfrac{R_{MAX}}{R_{MAX} + R_X} - \dfrac{R_{MIN}}{R_{MIN} + R_X}\right) = 0 \$

Solving for \$R_X\$ gives

\$ R_X = \sqrt{R_{MIN} \cdot R_{MAX}} \$

A beauty!

So in our case the series resistor will be 13.42k\$\Omega\$, you can check this on the graph. Placing the resistors between 0V and +5V this will give us an output range of [2V, 3V]. That's the maximum range you can get with 1 resistor(*).

Is it enough? The Arduino has a 10-bit ADC, so this range will give you a range of 200 discrete levels. That should give a sufficient accuracy for a DIY sensor. So no other components like opamps needed.

(*) The accepted answer gives a 1.9V range, but it has the wrong equations. It's impossible to get a higher range than 1V with 1 resistor and only a +5V supply.

Depending on the type of your door and key, you might have some success with inductive measurement from the inside. If your door, lock, and cylinder are mostly diamagnetic, and the key is ferromagnetic , you can measure a change in inductance of an inductor mounted on the inside of the lock when a ferromagnetic key is inserted. This should work for paramagnetic keys as well.

If your key is ferromagnetic, you might successfully magnetise the key and measure the inducted electromagnetic field when the key is inserted.

If the door cylinder is useable from both sides, you might insert a pin into the lock from the inside that will be moved when a key is inserted on the outside. Not a good idea because it might possibly damage your lock, though.

A totally different way would be to force people to put their keys on a specific hook, and alert if the key is not there.

Or make the door keyless.