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Digital Water Wave/Tide/Level Meter (LM324)

2015-01-12 07:59  
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A simple probe can be made using insulated wire. The
insulation is then the dielectric of an annular capacitor with
the inner conductor as one plate and the water as the other. The
probe is not suitable for distilled or very pure water because it
is not conductive enough. It works well even in relatively
clean water such as roof runoff water in a plastic tank. PVC
plastic coated multistrand hookup wire will work but is porous
and may eventually give trouble. Better to use the PTFE (teflon)
equivalent. Enamelled copper wire gives a high capacitance per
unit length because the insulation is very thin and this is
suitable for short probes of up to about 200mm. For long probes a
pipe with a sliding clamp to tension a single wire works well.
Keep the wire away from the pipe to avoid turbulence
effects. The problem with a single wire is sealing the end under
water, especially the PTFE type. It is better to make a loop to
form a U shape so that both ends are out of the water, of course
only one end needs to be connected to the electronics, with the
metal frame being the other connection to the electronics common
and to the water. A side benefit is that the capacitance per
unit length is doubled but in the case where the probe is
being used to measure wave height it must be aligned so that both
arms of the wire are normal to the wave, otherwise an error
could result. Light springs can be used at the top to maintain
wire tension. Make sure there are no sharp edges at the bottom of
the U where the wire is held to the frame.

The probe needs to be attached to something fixed like the
side or top of a water tank, or to a pier. If the water depth is
not too great a stake can be driven into the bottom of a river or
dam and the probe attached to it.

The electronics should be above water in a waterproof box that
is attached to the metal probe frame. The box cannot be
separated some distance away from the probe because the
capacitance of a cable would be too great compared to the probe
capacitance. The output cable can also bring low voltage
unregulated DC power to the unit. It can be long and even be
under water back to dry land if required.

nalog Electronics Unit

A simple analog unit can be made using a LM324 quad op. amp.

This article describes the Digital Water Wave / Tide / Level Meter (LM324). The content is very simple, very helpful. Components in this article can help you understand better understanding of this article. For example, in this article, you can go to find and buy these components:LM324.

One op. amp is configured as a 5kHz oscillator and a fraction
of the output from it is fed to the non-inverting input of a
second op amp. This is configured as a non-inverting stage with
the probe connected to the inverting input and ground and a
“range” capacitor connected from the output back to the
inverting input. The output level from this stage starts at the
same as the input level and goes up with wave height or depth.
tw1o 10M ohm resistors in series are placed across the range
capacitor to reduce 50Hz/60Hz pickup.

The output is high pass filtered and rectified to DC. The
third op. amp is a differential stage that subtracts the
rectified DC from the output from a potentiometer connected to
5V for zero adjustment. The output from this stage is low pass
filtered to 20Hz and the final op. amp is a non-inverting
stage with amplification.

Total current from a 9V battery using 5V and -5V micropower
regulators together with a ICL7662 charge pump inverter is about
1.8mA.

Output is in the range of /- 3.5 volts.

Digital Electronics Unit

The capacitance probe is connected to an ICM7555 running in
astable mode. The 7555 has the advantage as an oscillator for
this application because the capacitor is grounded. The resistor
values are chosen so that the minimum frequency is above 5kHz.
The minimum occurs at the maximum water level.

The probe and it`s connections have some capacitance out of
water and will determine the maximum frequency. The ICM7555 has a
maximum frequency of about 500kHz.

The period of oscillation is proportional to the capacitance
and therefore to the water level. The period is measured using
the timer/counters of a microcontroller, the Microchip PIC
16F876. The output of the ICM7555 is connected to pin RA4 which
is timer 0 clock input via a prescaler.

Timer 0 byte is preloaded with a value and an interrupt is
generated when it overflows. A fixed number of 7555 pulses are
therefore counted.

There is some jitter in the 7555 frequency so it is necessary
to count a large number of pulses to get the average period. One
thousand is sufficient, but it is a tradeoff. For tank water
level measurement the time taken can be long but for wave
measurement it is necessary to have a higher overall sample rate.

The time taken for timer 0 to overflow is measured by the 16
bit timer 1 counting the crystal frequency divided by four. The
crystal is 4MHz so the value in timer 1 is in microseconds. Any
overflows of timer 1 are counted by counting the overflow
interrupts.

The tw1o resistors needed on the 7555 are both 21k ohm and are
low temperature coefficient types from the RC55 series from
Farnell or equivalent. They have coefficients of 15 parts per
million per degree C. The Philips ICM7555 itself has a
temperature coefficient of around 0.02% per degree C. This means
that for a depth of 2000 mm and a 50 deg C. temperature change
on the 7555, the output will change by 20 mm. Depending on
the application, this may be a significant error.

In the present unit the ambient temperature near the 7555 is
measured using a digital temperature sensor, the Dallas DS1624,
and the measured temperature is used to immediately correct the
7555 period measurement to that at a reference temperature of
0 deg C. The correction can be set for a particular 7555 for
best results because there is some variation in the temperature
coefficients.

The timer 1 count is scaled to millimetres and output to the
RS232 serial port in ASCII format so that the data can be viewed
and saved to a file using a terminal program such as
Hyperterminal in Win 95/98. Temperature is also output with a
resolution of 0.1 deg C. and accuracy of 0.5 deg. C.

prototype Digital Water Wave/Tide/Level Meter

A photo of the prototype board hooked up to a rainwater tank.

water PCB Digital Water Wave/Tide/Level Meter

A photo of the complete board.

Water pcb layout Digital Water Wave/Tide/Level Meter

Layout of PCB.

Circuit Board Description

There are Protel files for a printed circuit board for this unit. The board is 67mm by 64mm.

In addition to the 7555, 16F876, and DS1624 chips mentioned
earlier the board has provision for components that may be
omitted for some applications that dont need them.

Where a component is available with dual-in-line pins as well
as in surface mount both footprints are provided, one within the
other.

The Philips PCF8583 IIC bus clock/calendar chip can be used to
date/time stamp the level and temperature data. Also, the alarm
output is connected to the external interrupt pin (RB0) of
the16F876. This can be used to wake the microcontroller from a
low power sleep state after an interval of 1 to 100 seconds or 1
to 100 minutes. When the external interrupt occurs, the handling
routine clears the timer flag on the PCF8583 and also reloads the
PCF8583 timer register with (100 – interval needed).

The Microchip 24LC256 IIC bus 256K bit serial EEPROM can be
used to store data for later retrieval. More of these devices
with different addresses up to a total of eight can be added by
connecting them externally. Other IIC bus devices such as
an lcd display can be added by connecting them to the IIC bus
connector.

There are jumpers to either connect a device permanently to
5V or to have it powered by bit RC5 only when required. For
example, the MAX232 consumes about 10mA, but can be powered only
when data is to be sent out.

The MAX666 micropower 5V regulator has a /LOBAT output that
is connected to pin RB1 to flag a flat battery. In addition the
battery voltage can be monitored and output in the data if
needed.

A high value resistive divider is buffered by an op. amp. that
is connected to pin RA0, A/D input 0. The op. amp. is needed
because to maintain 10 bit accuracy there a 10k ohm inpedance
limit on the A/D input and low value resistors would consume too
much power. If the LM324 op. amp. is omitted a compromise is
possible using 100k ohm and 47k ohm resistors because only low
precision measurement to 0.1V is needed.

The measurement is only valid down to 7V because below that
the MAX666 regulator drops out, and the A/D reference is 5V.
Below 7V the /LOBAT signal should be used to flag invalid data.
The program adds 0.7V to compensate for the reverse voltage
protection diode D1.

PWM can be output if needed, to control a pump motor for
example. An analog output can be derived by filtering the PWM1
output with R12 and C12.

RB2 is brought out to a pin as a control bit. For example it
can be used to turn on a pump to add water to a tank when the
level falls below a level set in softw1are. Variables like this
and the probe calibration constant can be stored in the EEPROM
area of the 16F876.

 


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