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Zero Cost Pressure Switches

Zero cost, super-sensitive pressure switches.

by Julian Edgar

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If you're like me you get excited when you go to the dump. Nope, not because of the unwanted things that you're getting rid of - but instead because of all the things that you can bring home! Yes, what other people blithely throw away can provide the foundations for a variety of automotive modifications. Amongst the simplest things to use are pressure switches taken from washing machines.

Sure, we've covered some pressure switches in the past but the ones covered in this article have two significant advantages. They're snap-action (I'll give the definition later) and they cost nothing!

Water Level Detection

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All older washing machines use mechanical switches to detect the water level inside the tub. A hose is connected from the base of the tub to a pressure switch, which measures the pressure being applied by the column of water. If you've read our previous stories on measuring low pressures (see the whole bunch of articles cited at the end of this story) you'll be familiar with the concept of pressure being measured in 'inches of water'.

[I read a forum discussion once about one of our muffler tests, in which we showed the pressure differential across the muffler during the flowbench testing, measured in inches of water. The poster thought that we'd flowed water through the mufflers.....and, hell, that couldn't be as good as using air, could it?!]

The pressure at the base of a column of water 27.69 inches high is 1 psi, or to put it another way, 10.2cm of water = 1 kPa. So, as you can imagine, a switch that is designed to assess when the water level in a washing machine has reached (say) 15 inches of depth must be capable of measuring pressures of just 0.5 psi (~3.5 kPa)!

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Hysteresis is a fancy name for a simple concept. In all switches that sense a variable level, the switch-on and switch-off occurs at different points. If they were at the same level, the switch would chatter at the switch-on (and switch-off!) point forever.

A pressure switch with high hysteresis that's being used to detect car speed (or more correctly, the different air pressures that act on the body at different speeds) might trip at 100 km/h and then not turn back off until the car has slowed right down to 20 km/h. With less hysteresis, the turn-on point would remain the same - but the turn-off point might be at a much higher speed, like 75 km/h.

Some of the dishwasher switches mentioned in this article have adjustable hysteresis as well as an adjustable switch-on point.

The washing machine switches have three characteristics:

  • As mentioned, very good sensitivity
  • A 'snap' action, where there isn't any trembling at the point of switching - they're either on or off
  • Excellent power handling
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Typically, the pressure switch uses a large diaphragm 60mm or so in diameter, with the switch having three quick-connect male terminals. One is 'common', while the others are for 'normally open' and 'normally closed' connections. An adjustment mechanism is built in (often 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, it almost certainly uses an electronic variable pressure sensor to detect the water level - rather than a mechanical switch. However, that probably leaves 20 or 30 million washing machines to pick pressure switches up from....)

The switches control the washing machine water solenoids directly, and so are rated for quite high currents - 15 amps at 240V AC in the case of the one shown here. So for 12V DC applications, the switch can certainly cope with (say) 5 amps - a relay won't be needed for many loads.

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This Singer pressure switch was removed from the guts of a washing machine at my local dump. Be careful that when you extract the switch you don't inadvertently also remove the adjustment mechanism - it's often part of the bracket holding the switch in place. This switch was removed from the machine by the quick and easy expedient of wriggling the bracket back and forth until it fractured - note that if the shown screws had been undone to remove the switch, the adjusting mechanism (arrowed) would have been lost.


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Depending on the actual switch, turning the adjustment control typically allows the trip pressure to be set anywhere from 3.6 inches to 8 inches of water pressure.

But here's where you can get tricky. If you remove the adjustment bracket you can access the internal spring that has an affect on the sensitivity. Using a very light spring (ie just enough force exerted to return the diaphragm back to its un-triggered position once the pressure had been removed) we were able to get a switch to trigger at just 2 inches of water (~0.5 kPa or 0.08 psi). With this light spring in place, the adjustment screw gave a range of 2 - 3.2 inches of water.

And, very importantly for practical applications, the switch has in-built hysteresis (ie difference between the turn-on and turn-off pressures - see breakout box). With it set at its most sensitive (with a turn-on pressure of 2 inches of water), its switch-off point was just under 1 inch of water.

But it gets even better! (Boy, am I ever getting enthusiastic about washing machine pressure switches!)

When you add a long, small diameter sensing hose to the switch, the switch develops a switch-off delay (as the air needs to get pushed back out the hose before the switch turns off). It takes very close to 2 seconds for the switch to turn off when it is being triggered through a two metre length of 3mmm bore tube. Double the length of hose and the switch-off delay increases proportionally. And that characteristic, along with the hysteresis, overcomes the need for an electronic 'anti-chatter' circuit...

And of course, if you fit a stiffer-than-standard spring, you can increase the pressure at which the washing machine switch trips. While we didn't try it, you could probably stop the switch from closing until you had a few pounds of boost - but the rubber diaphragm isn't designed to withstand these pressure levels so there may be some long-term reliability problems. (Having said that, we know of at least one turbo car application where a switch of this type was used to activate a second fuel pump. It was fed full boost - and happily worked for years doing this.)


So what uses can we make it of this little beauty of a switch?

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Firstly, as covered in the story linked to below, you can switch on and off devices on the basis of road speed. For example, you could use the 'normally closed' terminal to activate a fan on an intercooler, where the fan will operate when the road speed is below a pre-set level. When you get to sufficient road speed, the switch will open, turning off the fan. However, the sensitivity of these switches (even with the 'light spring' mod) is less than the pressure switch sold in the AutoSpeed Shop - the one used in the previous AutoSpeed story on this idea. The speed-dependent pressure signal can be increased by using a small funnel aimed forward (eg painted black and positioned behind the grille), but realistically the earliest a washing machine switch will trigger on a still day is about 90 km/h. The funnel approach is diagrammed here.

That trigger speed is still slow enough if you drive quickly, or want to use it in a racing application. As well as switching an intercooler fan on and off, you could also control the movement of active aero aids (something that we didn't recommend with the Shop pressure switch, due to the difficulty in getting its hysteresis right), or even turn on a brake water spray if the straights at your local race track are fairly short (ie by the time you exceed - say - 150 km/h you gonna need to be braking very soon!), where the early switch-on will be more effective that waiting for the brake light switch.

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While we wouldn't recommend using one of these switches directly with heaps of boost, there's no reason that you can't run the pressure switch off the manifold of a turbo or supercharged car. All that you'd do would be to tee a bleed into place in front of the switch, as shown here. Adjust the amount of bleed and the switch should be able to be made to switch at variable boost pressures - all the time without the switch having to see full boost. In addition you could use the switch to trip an exhaust butterfly, with the switch sensing exhaust backpressure.

Typical Standard Specs
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Goyen Controls Model SRM40 Washing Machine Level Switch

  • Single Pole Normally Open and Normally Closed contacts
  • Set to minimum (standard spring): closes at 5.0 inches of water and opens at 1.6 inches of water
  • Set to maximum (standard spring): closes at 8.0 inches of water and opens at 5.6 inches of water
  • 15 amps at 240V AC
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Eaton 78376-002 Dishwasher Level Switch

  • 3 x Single Pole Normally Closed contacts
  • Level 1 - closes at 7.2 and opens at 3.2 inches of water
  • Level 2 - closes at 8.0 and opens at 7.2 inches of water
  • Level 3 - closes at 13.6 and opens at 12.0 inches of water
  • 18 amps (total) at 250V AC

Dishwasher Switches

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And just when you were getting used to the idea of washing machine pressure switches, here's the next level of sophistication. Dishwashers use a switch that is very similar to ones described so far, but instead of having one switching point, they have multiple levels. A two-level switch may switch at 4 and 6 inches of water, for example. A three-position switch that we sampled tripped at 6, 10 and 14 inches of water. These switches look very similar to washing machine switches, except they have multiple electrical terminals - six or even nine of them! You'll also find that they have more adjustments - the two level switch that we bought was adjustable both for trip levels and hysteresis.

So with these switches you can start getting really tricky! How about three trim positions for an electrically-controlled spoiler? Or (with a bleed in the pressure line to the switch) a green light for boost and a red light for overboost? Or three positions (four positions if you include 'off' as one) for an exhaust butterfly?

Getting Them

While - as already said - the first switch mentioned in this article was gathered gratis from the local dump, in the course of researching this story I put together a collection of no less than ten appliance water level switches.

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Most were gathered from washing machine repairers, located through the Yellow Pages. However, note that some repair places will want to charge you exorbitant amounts - like $18 a switch! Make sure that you tell them that you're not collecting parts in order that you can go out and fix washing machines (this seems to be a preoccupation of repairers), and point out that most washing machines are junked with the switches still in place. At one repairer I paid $10 for three switches (including a rarer dishwasher multi-level switch), while at another I was simply ushered towards the pile of discarded machines out the back and told to "go for it"!

Whichever way you do it, don't pay more than $5 a switch...

Happy switch gathering - and if you use a switch in a way not covered in this article, we'd love to hear from you.

AutoSpeed Pressure Switch stories:

"Powering Up the Pressure Switch"

"Aero Flow Switch!"

Junkyard Dawg - Part 2
Junkyard Dawg - Part 3

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