E00cH06.PDF

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Rainwater Storage Gauge

Dr U. Pilz

Not only on ecologically grounds but also eco-

nomically it makes sense to collect rainwater for

use in the garden and increasingly for `grey

water` domestic use. People who take rainwater

collection seriously use large underground tanks

for storage. The problem now arises, how can

the water level be determined without lifting

the tank hatch and peering in? One solution is

to use float switches mounted at different

heights in the water tank, and to use a row of

LEDs mounted remotely to show the water level

in the tank. Transferring this information to a

remote display will involve long cable runs so it

is of interest to reduce the number of cables to a

minimum.

The circuit here shows how information

about the water level in a tank can be sent over

two wires to a remote LED display. R1 together

with the resistor chain made up of R2-R6 form a

voltage divider, ﬂoat switches are wired across

the resistors R2-R6 in one arm of the voltage

divider. As water flows into the tank and the

level rises, switch S5 closes followed by S4, etc.

Each time a switch closes, it will short out its

parallel resistor in the chain thereby changing

the output voltage of the divider. When the tank

is full, all ﬁve switches will be closed and all the

LEDs will be on.

The voltage output from this divider chain is

applied to the inputs of five op amps that are

configured as comparators. A voltage chain

comprising R7-R12 supplies the reference volt-

age for each of the five comparators. Both

divider chains use the same supply so they will

be insensitive to supply ﬂuctuations. The maxi-

mum supply current for the circuit is less than

25 mA.

Some of the resistors chosen to make up the voltage

dividers are not standard values but can be easily made up

12V

IC1

IC2

7V9

SSR 1

S202 S01

S202 S02

R13

2k2

IC1a

8V4

R18

R14

2k2

7V1

IC1b

6V7

R15

2k2

I = 1mA75 (LEDs off)

I = 22mA8 (LEDs on)

5V9

IC1c

5V0

R10

R16

2k2

3V9

IC1d

3V4

R11

R17

2k2

IC2a

1V7

R12

IC1, IC2 = LM324

004012 - 11

from combinations of 10 k

resistors.

If you need to expand this ﬁve level display to give a better

Ω

and 100 k

Ω

12/2000

Elektor Electronics

SMALL CIRCUITS COLLECTION

resolution of the tank contents, it is a simple job to add more

float switches and to expand the voltage divider chain. IC2

also has three spare op amps; these can be pressed into serv-

ice as further comparators.

Underground tanks inevitably require a pump to move the

water to where it will be used. An optional feature of this

design is the pump protection circuit. When LED D1 goes off

indicating that the tank is almost empty, the solid state relay

SSR1 can be used to switch off the mains power to the pump.

This will prevent damage to the pump when the tank runs dry.

The S202 SE1 solid-state relay (SSR) from Sharp has an isola-

tion voltage between its input and output of 3000 V (Class 1).

It is important to note here that any mains equipment near the

water tank installation must be supplied from an RCD safety

socket for the sake of your own health!

(004012-1)

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