BY PAUL FLORIAN
■ PHOTO 1
Q2 drives the piezo alarm and Q3 energizes the pump switch relay coil. D4 is used to eliminate the
back-EMF when the relay coil voltage is turned off. C4 remains
charged when the probes detect water. If the water level falls
below the probe tips, C3 discharges through R3, providing a
time delay in turning off Q1. This means that the relay will not
chatter when the probes are just barely in contact with water.
The unit is powered by a 12 VDC 200 mA wall
transformer. However, the device can operate from a
lantern battery as a backup power source (optional). The
Water Detection Alarm (WDA) will draw power from the
greater of the voltages at the anodes of D1 and D2. When
power is applied from J1, the voltage at the anode of D1
will generally be greater than the 12V battery. If the voltage
from the wall transformer fails, a lantern battery can power
the circuit through D2.
■ PHOTO 2
crimp housing connector mated to a . 100” male
header soldered to the PCB. Next,
construct the probe assembly. Start
with a 1.5“ by 2” piece of single
sided, copper clad PCB. Put 3/4”
pieces of tape on both sides and
then etch. After etching, there should
be copper foil as shown in Photo 3.
(the probe assembly). Drill the three
holes as indicated in the photo. Take
two 4” lengths of 12 Ga wire and solder to the PCB. It may be necessary
to use a 100 watt soldering iron to do
this. Bend the wire at an angle such
■ FIGURE 2
Construction
PARTS LIST
Photo 1 shows the top
side of the PCB (printed
circuit board) and Photo 2
shows the reverse side of the
PCB. The Parts List shows the
component references. Be
sure to use an eight-pin socket
for U1. Install the parts on the
perfboard. Transistor pinouts
are shown in Figure 2. Attach
8” of red wire to the TIP
terminal of J1, and 8“ of black
wire to the RING terminal of
J1. Then connect two 8” white
wires to switch S1. Also
connect a 12” red wire to the
+ terminal of the 12V battery
and another 12” black wire to
the – terminal of the battery.
These wire colors are shown
on the schematic in Figure 1.
The connections can be
made by soldering the wires
directly to the PCB, or they
can be made using a female
PART
❑ U1
❑ R1, R5, R6
❑ R2
❑ R3
❑ R4
❑ C1, C2
❑ C3
❑ C4, C5
❑ Q1
❑ Q2, Q3
❑ D1, D2, D4
❑ K1
❑ J1
❑ PZ1
❑ TB1, TB2
❑ S1
❑ Case
❑ Eight Pin Socket
❑ Terminals Ring tongue connector
❑ Four Standoffs 4/40 1/2” Nylon standoffs
❑ Ten Screws 4/40 1/4” screws
❑ Two Screws 4/40 3/8” screws
❑ Four 4/40 nuts
❑ Hookup Wire
❑ Wall Xfmr
PART NO.
LM555CNNS-ND
10KQBK-ND
1KQBK-ND
470KQBK-ND
1MQBK-ND
478-3187-ND
478-2462-ND
P1188-ND
2N3904FS-ND
2N3906FS-ND
578-1360-1-ND
275-248A RS
274-249 RS
273-057 RS
276-1388 RS
275-624 RS
270-1805 RS
ED3308-ND
64-3032 RS
1902CK-ND
Varies
Varies
Varies
Varies
12 VDC 200 mA transformer Varies
DESCRIPTION
LM555CN
10K 5% 1/4W
1K 5% 1/4W
470K 5% 1/4W
1M 5% 1/4W
100 nF
10 nF
22 µF 35V radial electrolytic
2N3904
2N3906
1N4002
10A SPST relay
1/8” phone jack
Piezo buzzer
PCB terminals
SPST toggle switch
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All part numbers are DigiKey except for RS part numbers (RadioShack) and HWS (your local
hardware store).
May 2007 43