Typically, pressure switches in washing machines have a large sensing diaphragm approximately 60mm in diameter and three male quick connect terminals. One connection is common, while the others are for normally open (NO) and normally closed (NC) connections. A set point adjustment mechanism is incorporated (controlled directly by the ‘water level’ knob) and the switch opens and closes with audible clicks.

(Note that if your washing machine has digital controls, rather than a mechanical switch, it will almost certainly use an electronic variable pressure sensor to detect the water level. These three-wire sensors are easy to connect, but we’ll leave them for another time).

Washer pressure switches directly control the hot and cold water inlet solenoids and are therefore rated for fairly high currents – 15 A at 230 VAC is typical. So for low voltage DC applications the breakers can certainly cope with (say) 5A. This means that, for most loads, a relay will not be needed.

Dishwashers use pressure switches that are similar to those used in washing machines. However, instead of having a switch point, they have multiple levels. For example, a two position switch can switch at 10 cm and 15 cm of water, while a three position switch can trip at 15 cm, 25 cm and 35 cm of water.

These switches also look a lot like pressure switches on washing machines, except they have multiple electrical terminals (six or even nine) and have no external level adjustment. However, some have screwdriver adjustment for both trigger levels and hysteresis.

When removing the pressure switch from a washing machine, be careful not to inadvertently do it too.

Remove the adjustment mechanism, it is often part of the bracket that holds the switch in place. As for identifying the pressure switch, it’s easy. Nine times out of 10, it is directly behind the ‘water level’ adjustment knob on the top control panel of the machine. If the machine is upside down or partially destroyed, follow the sensor tube from the base of the wash tub. And while you’re at it, it’s generally worth looking for the tube, too, which is often a high-quality plastic hose. You never know when it might come in handy. In contrast, dishwasher pressure switches are typically buried under the stainless steel drum. Since they cannot be adjusted externally, they do not need to be located near the control panel.

Depending on your application, you may want your reclaimed pressure switch to operate at pressures other than its standard range. That’s not difficult to do if you have a washing machine pressure switch, as they are fairly easy to modify. But if you remove the setting

bracket, you can access the internal spring that sets the sensitivity. For example, by using a very light spring (that is, one that provides enough force to return the diaphragm to its unactivated position when the

pressure is removed), it is possible to make a switch trip with as little as 5 cm of water (~ 0.5 kPa or 0.08 psi). External adjustment would normally give a range of about 5cm to 7.5cm of water, but of course this will vary depending on the unit and spring used. Attach a stiffer spring and adjust

the range becomes larger. While we haven’t tested it, it could probably prevent the switch from closing until it has 15-20 kPa pressure. However, note that the rubber diaphragm is not designed to withstand these

pressure levels, so there may be some long-term reliability issues.

So what uses can be made of these switches? That depends on your imagination, but here are some suggestions:

(1). Control an electric water pump

– for example, to keep a container full of water. As in a washing machine, the depth of the water can be detected with a hose connected to the base of the container.

(2). Providing a low water level warning, for example, the switch could be used to activate a buzzer or a light if the liquid level in a tank falls below an adjustable point.

(3). Provide water level indication, for example by using a dishwasher pressure switch to activate LEDs or lights to indicate the water level in a tank. By using two dishwasher switches and adjusting their individual set points, it’s easy to have six levels displayed. However, you will need a lot of cables to connect the switches to your display.

(4). Vehicle movement detection by washing machine pressure switch. The switch would be activated by air pressure when the vehicle crosses a hose. Just remember to connect the end of the hose that is not connected to the switch! So there are four applications but there are many more. These sensitive switches are certainly worth saving!