Water level indicator

This water level indicator was designed after a recent mishap with the pump in a water tank on a rural property. It went unnoticed that the water supply had fallen below the level at which the pump could operate properly, with the result that it burned out and had to be replaced.

This circuit depends on the fact that there is a fairly low and constant resistance between a pair of electrodes in a tank of water, irrespective of the dis-tance between them.

The circuit is based on the LM3914 linear LED bar/dot driv-er. It drives five green LEDs to indicate level, with the added feature of displaying a red LED when the water falls below the lowest sensing point; ie, when all the green LEDs are extin-guished.

To give a clear on/off indication, the red LED is driven by a 555 with the control voltage on pin 5 pulled towards the supply voltage by a 1kΩ resistor. This causes the 555 to switch at 0.46 and 0.92 of the supply voltage instead of the usual 1/3Vcc and 2/3Vcc thresholds.

A BC548 transistor (Q1) is used as a buffer to provide a reason-ably low impedance drive into pin 5 of the LM--3914 (SIG) while keeping the current through the water sensors below the level at which electrolysis could become a problem.

The sensor assembly is made by threading six lengths of 1mm enamelled copper wire through an 8mm OD clear PVC tubing, long enough to reach the bottom of the tank and with sufficient addi-tional length to fasten the top end securely. The reason for using 1mm wire is primarily to make it easy to thread it through the plastic tube.

The top sensor (S6) is about 10 to 15cm below the overflow outlet at the top of the tank and the other sensors are spaced evenly down the tube. Using a 1.2mm drill, holes are made through the tube wall at the appropriate points, including that for the bottom contact (S1) to hold it in place more securely.

The cut end of the wire should be smoothed to make it easier to push it through the tube and to avoid any danger of scratching the enamel of the wires already in the tube. The wire goes in more easily if the PVC tube is bent more or less at a right angle so that the drilled hole is in line with the bore of the tube.

About 15cm of wire is left outside the tube at each point, scraped clean of enamel and close-wound firmly around the outside of the tube. A 30mm length of 12.5mm copper water pipe can be pushed over S1 to add weight and increase the surface area if desired.

On no account should solder be used on the submersible part because corrosion will result from galvanic action.

At the top end of the assembly, the resistors are soldered to their respec-tive wires (double checking is recommended!), with insulating sleev-ing over each join, This is then covered with heatshrink tubing after attaching the two leads to run to the indicator unit.

A. March,
North Turramurra, NSW. ($35)

Do-it-youself car battery charger You can easily buy a charger for your car’s battery but some people like to "roll their own" and may have some or all the necessary parts on hand. I found that a 12V halogen lamp trans-former was ideal, as it’s fully enclosed with thermal protection and needs only a 35A bridge rectifier on a small heatsink to function as a charger. I fitted a voltage indicator in the form of a red LED, 10V zener and 100Ω resistor across the battery connections. The in-line 10A blade fuse in the positive output lead will blow if the battery leads are re-versed. The transformer I used was rated at 63VA (Jaycar MP-3050) but other similar ones should be just as suitable. Do NOT use the electronic switchmode or toroidal types. The rectified DC current is about 3A average. P. King, Croydon, NSW. ($20)

Hi/Lo pulse indicator uses a 7-segment display

A common cathode 7-segment display and a single NPN tran-sistor can be used as a pulse polarity indicator.

As shown in this circuit, Q1 is used to invert low signals and its collector drives diodes D1, D2 and D3 which are connected to segments d,e & f. Hence, when the input signal is low, the collector of Q1 will be high (Q1 is off) and so the display will indicate "L".

When the input signal is high,
diodes D4-D8 drive all segments of the display except a & d, to indicate "H".

Note that the circuit loading is quite low, as set by the 1kΩ input resistor. This is a compromise between circuit loading and segment brightness.

Raj K. Gorkali,
Kathmandu, Nepal. ($20)