Fluid or Water Level Indicator with Reed Relays
The Reed Relay based Float Switches are used in
Automobiles and even corrosive and volatile
fluids. This can also be used to indicate level of
water or operate pumps.
The reeds are hermetically sealed in glass, Like
the Incandescent bulb. The signal switched by reed
also should be a few mA or even uA. This makes is
safe in Volatile fluids, where sparking can cause
This is a way by which you can monitor the level
of fluids like water in a tank. Based on data from
the sensor you can control things like solenoids
or motors as required or even turn on a buzzer,
hooter or annunciator in a control panel.
Retro-Reflective Proximity Switch
Look at the picture on the right, the float can
be a Styrofoam type with a couple of ceramic
magnets stuck inside. The float must move freely
up and down a long plastic pipe of around 10 mm
dia. If the fluid is not water but a solvent then
design a non-soluble float or coat the float you
made with something like an epoxy resin.
Measurement and Control
Then you need to assemble a small circuit within
the long plastic pipe. Many reed relays with a
chain of resistors in series has to be inserted in
the pipe. The resistor and the glass reed relays
should be in different levels of the pipe where
you need an alarm or indication. Seal the pipe
hermetically at both ends with epoxy resin after
taking out a shielded cable of just 2 wires + 1
When the fluid level changes, the float moves up
or down with it, when the float moves over a
section of pipe where the reed relay is mounted,
the magnets on the float make the reed operate and
the resistance of the sensor changes.
The resistance is measured and the alarm can be
operated as you like. The advantage of this
arrangement is the electric circuit does not come
in contact with the fluid, no sparking risk.