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Description of requirements /thoughts of circuit design.
In brief: We will use 4 inputs to trigger an output of 24V DC. When any of the inputs are active,
output is active (a logical OR gate).
The
circuit
will
have
input
of
water
detection.
When
water
is
detected,
the
output
will
always
be
off.
In addition, and audible and visible alarm will be set. Audible alarm can be shut off when pressing a
button, but will re‐engage after 60 minutes or if the water sensor has been “low” in the meantime.
DETAILS:
In our opinion, a microcontroller is suitable for this purpose. Since the inputs must be electrical
isolated from the circuit, and the voltage that should be sensed varies, 4 opto‐couplers might be the
best solution. 2 of the inputs will be fed with 230V AC, and the remaining two must be able to handle
both 12 and 24 V. In the drawing we have 2 resistors for each input, giving us the possibility to
connect the inputs to either 12 or 24 V. None of the inputs needs to be secured via fuse. All inputs
have a small
led
to
give
visible
control
that
power
is
going
into
the
opto
‐coupler.
By using Google, we have found a schematic for a water detector that uses a PIC microcontroller, but
has no knowledge about it’s functionality. If it can reliable be used as drawn than this is an option,
but possibly an amplification with suitable transistors is necessary.
The output of 24 V DC must be able to deliver 0.5A. Can a Mosfet be used for this? And, since the
load is inductive, suitable protection against back‐EMF must be provided, and also a fuse.
Indication of water detected, should be short beeps with 5 – 10 seconds interval, and a blinking LED.
By
pressing
a
“quiet‐
button”
the
audible
alarm
will
shut
off,
but
visible
remains.
The
audible
alarm
shall reactivate after 60 minutes, or if the water detector had been low and then high again (ie: no
water sensed, and then water sensed again).
When designing the PCB, than power to the circuit and output can be external. Connections for input
can be via screw‐terminal on the pcb, this also includes water probes, and output. Adequate distance
between 230V and low voltage as required by European safety standards. Quiet‐switch must be
accessible via enclosure, but the switch itself can be on the pcb, but with enough distance to 230V
connections.
For the 12/24 volt input, a tree‐way terminal is ideal. Note that in use, there will only be either 12 or
24 volt
on
one
input,
never
both
at
the
same
time.
Despite
that
the
input
will
be
by
technician,
maybe a protection against wrong polarity is necessary.
On the drawing:
1 & 2: Optocouplers for 230V input.
3 & 4: Optocouplers for 12/24 V input
5: Microcontroller
6 & 7: Current limiting resistor, led and diode for the 230V AC input
8 & 9: Current limiting resistors for both 12 and 24 input, with led.
10: Water probe (water sensing electronics not drawn)
11: Quiet‐button
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12: Buzzer
13: Water alarm LED
14: Output circuit. 24 V DC 0.5A output into inductive load. Fused and protected against back emf.
15: Power supply for circuit. Input 24 V DC regulated from external power supply.
NOTE: No
ground
connections
are
shown
in
the
drawing.
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5
IN/OUT
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1
2
3
4
15
6 7
8
9
10
11
12
13
14
Untested water detection drawing for use on PIC
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