Simple way to measure wheel spinning speed with Adroid phone + magnet (and maybe Reed Switch)

If you want ease of implementation, value for money, and good accuracy - then a cheap commercial 2 kg scales interfaced by whatever means is possible is liable to seve you well.

You can buy load cells and instrumentation amplifiers and roll your own, but the Chinese already make just what you want by the millions at a price which is liable to be far less than you can buy the parts for new.

The best (and in some cases also the cheapest) kitchen scales use a "real" load cell with 4 strain gauges in a bridge plus hopefully a 5th temperature compensating strain gauge (places on the load cell where stress/strain is caused only by temperature changes. The photo below shows a typical load cell used in low cost scales.

These can be converted to "hanging" operation either by removing the load cell from the scales body and custom mounting it (not very hard)

or by placing the scales on a beam and using a "question mark" shaped stiff wire to convey the force (weight) out around the scale body and onto the pan. I use a similar arrangement with the scales described below to suspend objects for weighing in an out of water for 'Archimedes' density measurements with good success.

Performance oc cheap kitchen scales varies from not-overly-marvellous through to unbelievably superb.I bought some end of line 2 kg kitchen scales here a few years ago for about $10 each, that were so good that I went back and bought the rest that were available - about 10 sets.

Mine looked very like these. Similar appearance no guarantee.

These are now about 'used up' and hopefully some equally good and cheap ones will turn up shortly.

Factors which relate are accuracy, linearity, repeatability and temperature independence. The first 3 are close travelling companions but not identical.

Obtain several hundred one and two cent coins. These will probably weigh close to 1.000 gram and 2.000 gram each. If not that then some other fixed mass. This is partially so banks can check amounts by weighing. They are amongst the best value for money calibration wights you can but. Make a few larger test weights by using eg coins to calibrate them. Say 100 200 400 800 gram would be easy. These can be made out of almost anything stable. Even eg plastic screwtop jars with water in - as long as they are airtight.

My scales would track linearly for any number of coins aded or removed and whether removed or added one by one or N at a time. Superb. Some scales are not so good.

Some scales are poorly temperature compensated. Mine can have a hairdrier waved over them on high until they are toasty hot (50C plus) with little of no display deviation. Superb.

Once you have found ones which are accurate enough for you, you nee to interface them to the Arduino. Most will have an anlog voltage(amplified strain gauge signal) which is converted to digital by the display controller. Signal level should be in the volt or few range - easy for an Arduino ADC to measure.

If you can't get at the analog signal you may be able to access the controller at the digital stage. Long ago people have gone so far as decoding 7 segmont multiplexed display signals for processor interface - but if it's that hard then finding ones with analog signal access would beeasier.

Some may use a V to F (Voltage to frequent) converter. These are easy to read by either pulse counting in finite time or by seeing how long it takes to make N clock cycles.

you have several ways to measure the capacity : 1- make a oscillator and measure the period or the frequency (use a CMOS Inverter with resistor connected form output to input and connect the sensor between input and GND) or use LM555 in astable circuit. The period of output depend directly on the value of sensor capacity (T = K * C) where K = Constant . 2- If you apply a step function on R-C network and measure the time t until the C voltage to a reference (Vref) you can use the equation : Vref = Vin * (1 - Exp(-t/Taw)) ; [Taw = R*C]. then t/Taw = -Ln(1 - Vref / Vin) ==> C = -t / [R * Ln(1 - Vref / Vin)] 3- if you use a constant current source to charge the capacitor, the C voltage become : Vref * C = I * t ==> C = I * t / Vref . You can use a LM317 to make a current source, a MCU internal analog compactor to compare the sensor voltage to Vref, a MCU internal voltage reference or TL431 as external voltage reference.