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A circuit that offers visual indication
of fluid level in a vessel, with a switchable audible
alarm. Example uses would be to monitor the level of
water in a bath or cold storage tank.
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Circuits
Conductance is the reciprocal of
resistance. The conductance of fluids vary with
temperature, volume and separation distance of the
measurement probes. Tap water has a conductance of about
50 uS / cm measured at 25 � C. This is 20k/cm at 25 � C.
See this site for more details about the conductance of
fluids.
This circuit will trigger with any fluid
with a resistance under 900K between the maximum
separation distance of the probes. Let me explain
further. The circuit uses a 4050B CMOS hex buffer
working on a 5 volt supply. All gates are biased off by
the 10M resistors connected between ground and buffer
input. The "common" probe the topmost probe above probe
1 in the diagram above is connected to the positive 5
volt supply. If probe 1 is spaced 1 cm away from the
common probe and tap water at 25 � C is detected between
the probes (a resistance of 20k) then the top gate is
activated and the LED 1 will light. Similarly if probe 2
at 2 cm distance from the common probe detects water,
LED 2 will light and so on. Switch 1 is used to select
which output from the hex buffer will trigger the
audible oscillator made from the gates of a CMOS 4011B
IC.
As 7 wires are needed for the probe I
reccommend the use of 8 way computer ribbon cable. The
first two wires may be doubled and act as the common
probe wire. Each subsequent wire may be cut to required
length, if required a couple of millimetres of
insulation may be stripped back, though the open "cut
off" wire end should be sufficient to act as the probe.
The fluid and distance between probe 6 and the common
probe wire must be less than 900k. This is because any
voltage below 0.5 Volt is detected by the CMOS IC as
logic 0. A quick potential check using a 900k resistance
and the divider formed with the 10M resistor at the
input proves this point:
5 x (0.9 / (0.9+10) = 0.41 Volt.
As this voltage is below 0.5 volt it is
interpreted as a logic 0 and the LED will light. If
measuring tap water at 25 � C then the distance between
top probe and common may be up to 45 cm apart. For other
temperatures and fluids, it is advisable to use an
ohmmeter first. When placing the probes the common probe
must be the lowest placed probe, as the water level
rises, it will first pass probe 1, then 2 and finally
probe 6. |
