Nuts & Volts - May 2007

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

SUPPLIER

COST $1.04 5 for $0.27 5 for $0.27 5 for $0.27 5 for $0.27 $0.24 $0.24 $0.31 $0.16 $0.18 $0.04 $4.49 $2.99 $10.49 $2.29 $2.99 $3.79 $0.42 $1.99 10 for $5.17