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Salvage It!
BY JULIAN EDGAR
Washing machine pressure switches
Want to control the water level in a tank,
or detect a vehicle passing over a smalldiameter hose? You can do both these jobs
and much more, using pressure switches
salvaged from old washing machines.
A
LL OLDER WASHING machines
and dishwashers use mechanical pressure switches to detect the
water level inside the tub. A hose is
connected from the base of the tub to
the pressure switch, which measures
the air pressure being applied by the
column of water.
The pressure at the base of a column
of water 27.69 inches high is 1psi, or
to put it another way, 10.2cm of water
exerts a pressure of 1kPa (kilopascals).
So, as you can imagine, a switch
that’s designed to detect when the
water level in a washing machine has
reached (say) a depth of 15 inches must
be capable of accurately measuring
pressures of just 0.5 psi (~3.5 kPa)!
In other words, the pressure switches in washing machines are amongst
the most sensitive that you can buy.
Well, you don’t really have to buy them
– these switches can be scrounged for
literally nothing wherever washing
machines are being discarded.
Washing machine pressure switches
have three main characteristics:
(1) Very high sensitivity;
(2) A “snap” action, to ensure sharp,
decisive switching – they’re either on
or off; and
(3) Excellent power handling.
How they work
Typically, washing machine pressure switches have a large sensing
diaphragm that’s about 60mm or so
in diameter and three quick-connect
male terminals. One connection is
common, while the others are for
normally open (NO) and normally
closed (NC) connections. A setpoint
adjustment mechanism is built in (it’s
directly controlled by the “water level”
knob) and the switch opens and closes
with audible clicks.
(Note that if the washing machine
has digital controls, rather than a mechanical switch, it almost certainly
uses an electronic variable pressure
sensor to detect the water level. These
three-wire sensors are easy to interface
with but we’ll leave them for another
time).
Washing machine pressure switches
directly control the hot and cold water
inlet solenoids and so are rated for
quite high currents – 15A at 240V AC
is typical. So for low-voltage DC applications, the switches can certainly
cope with (say) 5A. This means that,
for most loads, a relay won’t be needed.
Dishwashers use pressure switches
that are similar to those used in washing machines. However, instead of
having one switching point, they have
multiple levels. A 2-position switch
may switch at 10cm and 15cm of water, for example, while a 3-position
switch may trip at 15cm, 25cm and
35cm of water.
These switches also look a lot like
washing machine pressure switches,
except they have multiple electrical terminals (six or even nine) and
don’t have external level adjustment.
However, some have screwdriver adjustment for both the trip levels and
the hysteresis.
Salvaging pressure switches
Washing machine and dishwasher pressure switches can be easily scrounged
for nothing – you’ll find a pressure switch inside nearly every one of these
appliances when they’re discarded. They are extremely sensitive and their trip
points are externally adjustable.
90 Silicon Chip
When extracting the pressure switch
from a washing machine, be careful that you don’t also inadvertently
remove the adjustment mechanism
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This Goyen Controls Model SRM40 washing machine pressure switch is typical
and has single-pole normally open (NO) and normally closed (NC) contacts.
When set to minimum pressure, it closes at about 13cm of water and opens at
4cm of water. Conversely, when set to maximum pressure, it closes at about
20cm of water and opens at 14cm of water. Its current rating is 15A at 240V AC.
– it’s often part of the bracket holding
the switch in place! As for identifying
the pressure switch, that’s easy. Nine
times out of 10, it’s directly behind the
“water level” adjustment knob in the
top control panel of the machine.
If the machine’s upside down or
partly destroyed, follow the sensing
tube from the base of the wash tub.
And while you’re at it, it’s also usually
worth scrounging the tube, which is
often a high-quality plastic hose! You
never know when it might come in
handy for something!
By contrast, dishwasher pressure
switches are normally buried beneath
the stainless steel drum. Because
they’re not externally adjustable, they
don’t need to be located close to the
control panel.
Modifying pressure switches
Depending on your application, you
might want your salvaged pressure
switch to operate at pressures that are
different to its standard range. That’s
not hard to do if you have a washing
machine pressure switch, as they are
quite easy to modify.
In standard form, turning the ad-
Rat It Before You Chuck It!
Whenever you throw away an old TV (or
VCR or washing machine or dishwasher
or printer) do you always think that surely
there must be some good salvageable
components inside? Well, this column is
for you! (And it’s also for people without a
lot of dough.) Each month we’ll use bits
and pieces sourced from discards, sometimes in mini-projects and other times as
an ideas smorgasbord.
And you can contribute as well. If you
have a use for specific parts which can
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easily be salvaged from goods commonly
being thrown away, we’d love to hear from
you. Perhaps you use the pressure switch
from a washing machine to control a pump.
Or maybe you have a use for the highquality bearings from VCR heads. Or
perhaps you’ve found how the guts of a
cassette player can be easily turned into
a metal detector. (Well, we made the last
one up but you get the idea . . .)
If you have some practical ideas, write
in and tell us!
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August 2005 91
Dishwasher switches
have multiple contacts
that switch at different
pressure levels. This
Eaton 78376-002
Dishwasher Level
Switch has three
single-pole, normally
closed (NC) contacts.
Level 1 closes at 18cm
and opens at 8cm of
water; level 2 closes
at 20cm and opens at
18cm of water; and
level 3 closes at 34.5cm
and opens at 30.5cm of
water. Its total current
rating is 18A at 250V
AC.
justment control typically allows the
trip pressure to be set to detect water
depths anywhere from 9-20cm. But if
you remove the adjustment bracket,
you can access the internal spring
which sets the sensitivity.
For example, by using a very light
spring (ie, one that provides just
enough force to return the diaphragm
to its un-triggered position when the
pressure is removed), it’s possible to
get a switch to trigger at just 5cm of water (~0.5kPa or 0.08psi). The external
adjustment would then typically give
a range of about 5-7.5cm of water but,
of course, this will vary depending on
the unit and the spring used.
Put in a stiffer spring and the adjustment range becomes larger. And while
we haven’t tried it, you could probably
stop the switch from closing until you
had 15-20kPa of pressure. Note, however, that the rubber diaphragm isn’t
designed to withstand these pressure
levels, so there may be some long-term
reliability problems.
Using pressure switches
So what uses can be made of these
switches? That depends on your
Brand New From
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imagination but here are some suggestions:
(1). Controlling an electric water
pump – eg, to keep a container full of
water. Just as in a washing machine,
the depth of the water can be sensed
from a hose connected to the base of
the container. The advantage over a
float switch is that the level is easily
adjustable.
(2). Providing a low water level warning – eg, the switch could be used to
activate a buzzer or light if the liquid
level in a tank drops below an adjustable point.
(3). Providing water level indication
– eg, by using a dishwasher pressure
switch to activate LEDs or lights to
indicate water level in a tank. By
using two dishwasher switches and
adjusting their individual setpoints
appropriately, it’s easy to have six
levels indicated. However, you’ll need
a lot of wires to connect the switches
to your display.
(4). Detecting vehicle movement
using a washing machine pressure
switch. The switch would be triggered by air pressure when the vehicle
crosses a hose. Just remember to plug
the end of the hose that’s not connected
to the switch!
So there are four applications but
there are lots more. It’s certainly
worth salvaging these very sensitive
SC
switches!
160 PAGES
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