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Water Level Alert Circuit: Build Your Own
Description
This circuit will generate a sporadic beep (or will flash an LED) when the quantity of water contained within a container reaches the desired level. It should be positioned atop the container (such as a plastic tank) using two crocodile clips, which also function as probes. If a more extensive sensing depth is required, the clips can be extended with two pieces of inflexible wire (refer to the illustrations).
A 555 CMOS timer chip, designated as IC1, is configured as an astable multivibrator, and its operational frequency is determined by C1, R1, and R2, combined with the resistance offered by the water across the probes. When the resistance across the probes is zero (meaning the probes are shorted), the output frequency will be approximately 3Hz, and the sounder will emit a sound or the LED will flash about three times per second. Due to water usually presenting a certain resistance, the actual oscillation frequency will be lower: less than one beep/flash per second. As the probes are increasingly immersed in water, the resistance across them will diminish, and the oscillation frequency of IC1 will increase.
This provides a rough auditory or visual indication of the water level reached. If a LED is chosen as the alert, C2, D1, and D2 must be incorporated into the circuit to amplify the output voltage, ensuring proper LED operation (see the rightmost section of the schematics). Notable characteristics of this circuit include a 1.5V supply and ultra-low current consumption: 40µA in standby and 0.5mA in operation. This enables a single AAA alkaline cell to last several years, alongside the convenience of a power on/off switch.
Circuit operation:
IC1, a 7555 or TS555CN CMOS Timer IC, is wired as an astable multivibrator whose operating frequency is set by C1, R1 and R2, plus the resistance presented by water across the probes. If the resistance across the probes is zero (i.e. probes shorted), the output frequency will be about 3Hz and the sounder will beep (or the LED will flash) about three times per second. As water usually presents a certain amount of resistance, the actual oscillation frequency will be lower: less than one beep/flash per second. As probes will be increasingly immersed in water, the resistance across them will decrease and the oscillation frequency of IC1 will increase.
Pictures of the project:
Circuit diagram:
Parts:
- R1 = 1K - 1/4W Resistor
- R2 = 100K - 1/4W Resistor
- C1 = 2.2uF-50V Electrolytic Capacitor
- C2 = 220µF - 25V Electrolytic Capacitor
- D1 = 5 or 10mm. Ultra-bright red LED
- D2 = 1N5819 - 40V 1A Schottky-barrier Diode
- IC = 7555 or TS555CN CMos Timer IC
- BZ = Piezo sounder (incorporating 3KHz oscillator)
- B1 = 1.5V Battery (AAA or AA cell etc.)
- Two small crocodile clips
- Two pieces of stiff wire of suitable length
- Battery socket, etc.
Notes:
- If a LED alert is needed instead of the beeper, R2 value must be changed to 10K, the Piezo sounder can be omitted and D1, D2 and C2 must be added, as shown in the rightmost part of the schematics.
- A common red LED can be used for D1, but ultra-bright types are preferred.
- Any Schottky-barrier type diode can be used in place of the 1N5819, e.g. the BAT46, rated @ 100V 150mA.
- Wipe the probes regularly to avoid excessive resistance variations due to partial oxidization.