Depending on how strong the magnetic field is in your water meter, amplification might need to be adjusted (resistor R1), and/or the DC offset (resistor R2). Channel 2 (blue) shows the output from the Schmitt trigger (pin 7). Channel 1 (yellow) shows the amplified sine wave coming out of op-amp (pin 1). The oscilloscope screen shot shows exactly how the signal looks when water is flowing through the water meter. With an amplification of 1000x, there is bound to be some noise, so a second channel of the LM324 op-amp will be turned into a Schmitt trigger. Resistors R1 and R3 set the amplification, in this case roughly 1000x, so a fluctuation of 2-3mV would become 2-3V on the output (pin 1). Capacitor C1 removes the DC quiescent output voltage from the Hall effect sensor, feeding only the tiny sine wave output from the sensor into the op-amp. At the core of the circuit is a 4 channel LM324 op-amp running in an ac-coupled inverting configuration. This is a fairly simple schematic of the amplifier and filter. In my experiments I have used a widely available and cheap (less than $0.25) LM324 op-amp, and the schematic in the next step is based on the LM324 op-amp. To amplify the signal coming from the Hall effect sensor, a general purpose operational amplifier like the LM324 will work just fine. So, to use an Arduino to measure such tiny voltage fluctuations, and with good resolution, amplification will be required. If you were to measure field strengths of 1 gauss or less, the Honeywell SS494B will deviate no more than ~5mV from the quiescent output voltage. But if you hold a magnet in front of the sensor, the magnetic field will pull the output voltage either towards ground(0V) or Vcc(5v), depending on the polarity of the magnetic field. This is called the quiescent output voltage, or in other words, this is the voltage the Hall effect sensor will output when no magnetic field is present. If you were to look at the output from this Honewell Hall effect sensor, simply powered from a +5v source, you would see that the output sits at around +2.5v, or roughly half of Vcc. The sensor has 3 pins: power (Vcc), ground, and output. But before tackling the challenge of measuring very small magnetic fields, let's take a look at how the Honeywell SS494B Hall effect sensor works. This presents a challenge, since 1 gauss or less is at the extreme low end of the Hall effect sensor's measuring range. Depending on the construction of your water meter enclosure, if it's made of metal or plastic, the magnetic field strength outside the meter can be as low as 1 gauss or less. The Honeywell SS494B is a very sensitive Hall effect sensor, as far as low cost hall effect sensors go, but its measuring range still reaches over 400 gauss. For a frame of reference, the earth's natural magnetic field measures around 0.5 gauss, a refrigerator magnet is about 50 gauss, and a neodymium magnet is in the 1000's of gauss. The Honeywell SS494B promises to be sensitive enough to provide around 5mV per 1 gauss. Other sensors should work also, just pick an analog model, not latching, with comparable or better sensitivity. In my experiments I have used a Honeywell SS494B Hall effect sensor, which is available online for $3-$4. To monitor magnetic field fluctuations you will need a Hall effect sensor. If the compass needle fluctuates when the water is flowing, then there is a very good chance that you will be able to monitor those fluctuations. Hold it close to where the meter and the register join, this is where the magnetic field will be strongest. To test this, hold a compass next to the water meter. Since the wheel has magnets attached to it, the magnetic field from the magnets will almost certainly be detectable from the outside of the water meter enclosure. So to measure water flow, you just need to monitor whether this wheel is spinning, and how fast. This magnetic drive between the water meter and the register is used in virtually all residential water meters. The register is the part that sits on top of the water meter, analog or digital, that shows you how many gallons of water you have used. When water is flowing through the meter, this wheel spins, driving the register. This type of water meter design usually incorporates a wheel, as part of the internal mechanism, with one or more magnets attached to the wheel. The most common water meter type that is installed by local municipalities is a positive displacement water meter. First, let's take a look at what is going on inside a typical residential water meter. The solution described here is non-invasive, uses your existing municipal water meter, and the total project cost can be kept under $5-10. If you'd like to monitor your water usage, as part of a DIY smart home automation and monitoring project, then this instructable might help you to achieve that goal.
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