Digital Water Level Indicator

This digital water level meter circuit indicates up to 65 discrete water levels in the overhead tank (OHT). This
helps to know the quantity of water in the OHT quite precisely. The circuit is specially suited for use in  apartments, hostels, hotels, etc, where many taps are connected to one OHT. In such cases, if someone for gets to close the tap, this circuit would alert the operator well in time.

 IC's Used:

• IC1 & IC2 : CD4033B
• IC3 & IC4 : NE555

Normally, for multi water level readings, one has to use complicated circuits employing multi-core wires from the OHT to  the circuit. This circuit does away with such an arrangement and uses just a 2-core cable to monitor various water levels. Fig. 2. shows various water levels and the corresponding readings on 7-sement displays.

Circuit diagram of  Digital Water Level Meter: Click on image to enlarge

IC1 and IC2  shown in Fig.1 are CD4033  (decade up counter cum 7-segment decoder) which form a two-digit frequency counter. The CK pin 1 and CE pin 2 of IC1 are used in such a way that the counter advances when pin 1 is held high and pin 2 undergoes a high-to-low transition. 

For water level reading, this frequency counter needs two types of inputs. One  of the these is a continuous 30Hz clock (approx.), which is applied to pin 2 of IC1. The other is a timing pulse, which is applied to pin 1 of IC1. It comprises a positive pulse of variable duration (0.5 second to 3 seconds, depending upon the water level in the tank) followed by 4-second low level. Thus, during  positive duration of timing pulse at pin 1 of IC1, the frequency counter is allowed to count the number of negative going pulses which are continuously available at its pin 2. At the same time the pnp transistor T1 remains cut-off ndue to the positive voltage at its base, and so the displays (DIS.1 and DIS.2) remain off. However, at the end of positive pulse at pin 1 of IC1 (and base of transistor T1), the frequency counter is latched. During the following 4-second low level  period,
transistor T1 conducts and displays DIS.1 and DIS.2 show the current count. At beginning of the next positive timing pulse the frequency counter resets (as the reset pin 15 of IC1 and IC2 receives a differentiated positive going pulse via capacitor C1) and starts counting afresh.

Thus, this digital water level meter shows water level reading for four-second duration and then goes off for a variable period of 0.5 second to 3 seconds. Thereafter the cycle repeats. This type of display technique is very useful, because if there is ripple in water, we shall otherwise observe rapid fluctuations in level readings,  and shall not get a correct idea of the actual water level.

Timer IC4 is used as a free-running astable multivibrator which generates continuous 30Hz clock pulses with 52 % duty cycle. The output of IC4 is available at its pin 3, which is connected to pin 2 of IC1.

The other timer, IC3, is used as timing pulse generator wherein we have independent control over high and low duration (duty cycle) of the timing pulses available at its output pin 3. The different duration of high and
low periods of the timing pulses are achieved because the charging and discharging paths for the timing capacitor C2 differ. The charging path of capacitor C2 consists of resistor R19, diode D1, and potmeters VR1 for OHT (or VR2 for sump tank, depending upon the position of slide switch S1) and VR3.  However, the discharge path of capacitor C2 is via diode D2 and variable resistor VR4. By adjusting preset VR4, the low-duration pulse period (4 seconds) can be set. The low-pulse duration should invariably be greater than 3 seconds. Potmeter  VR1 (or VR2) , a l inear wirewound pot, is fitted in such a way in the overhead tank (or sump tank) that when water level is minimum, the value of VR1 = 2 kilo-ohm. When water level in the tank is maximum, the in-circuit value of potmeter VR1 increases to 340-kilo-ohm (approximately). This is achieved by one third movement of pot  shaft. The change in resistance of VR1 results in the change in charging period of capacitor C2. Thus, depending upon the level of water in the tank, the in-circuit value of VR1  (or VR2) resistance changes the charging time of capacitor C2 and so also the duration of positive pulse period of  IC3  from 0.5 second to 3 seconds. Adjustment of presets for achieving the desired accuracy of count can be ac-
complished, without using any frequency counter, by using the following procedure (which was adopted by the author during calibration of his prototype):

1. First adjust the values:  VR1 (or VR2) = 2 kilo-ohm, VR3 = 64 kilo-ohm, VR4 = 90 kilo-ohm, VR5 = 23.5 kilo-ohm.
2. Now switch on the circuit. The 7 segment DIS.1  and  DIS.2  blink. If necessary, adjust VR3  such that the display goes ‘on’ for 4-second period.
3. If display readout is 15, increase value of pot  VR1  from 2-kilo-ohm to 340 kilo-ohm. Now, the display should show 90. If there is a difference in the displayed count, slightly adjust presets VR5 and VR3 in such a way that when VR1 is 2-kilo-ohm the display readout is 15, and when VR1 is 340-kilo-ohm the display readout is 90.
4. If 15- to 90-count display is achieved with less than one-third movement of pot shaft, increase the value of  VR5 slightly. If 15- to 90-count display is not achieved with one-third movement of pot shaft, decrease value of VR5.

If you need this digital water level meter to monitor the levels of water in the overhead as well as the sump tanks, it can be done by moving  DPDT slide switch to down (DN) position. The indication of  the position selected is provided by different colour LEDs (refer Figs 1 and 2) or by using a single bi-colour LED. To monitor  water level in more than two tanks, one may use a similar arrangement in conjunction with a rotary switch. For timing capacitors C2 and C4 use tantalum capacitors for better stability.

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