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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 siliconchip.com.au Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & P 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 siliconchip.com.au 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! These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ 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 SILICON CHIP 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 23 CHAPTE RS Mail order prices: Aust: $A22.50 (incl. GST & P&P) Overseas: $A26.00 via airmail From the publishers of Completely NEW projects – the result of two years research & development • Learn how engine management systems work • Build projects to control nitrous, fuel injection and turbo boost systems • Switch devices on and off on the basis of signal frequency, temperature and voltage • Build test instruments to check fuel injector duty cycle, fuel mixture and brake and coolant temperatures We explain the why as well as the how to! Intelligent turbo timer I SBN 095852294 - 4 9 780958 522946 $19.80 (inc GST) NZ $22.00 (inc GST) TURBO BOOST & nitrous fuel controllers How engine management works Available direct from Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. Phone (02) 9979 5644; Fax (02) 9979 6503. Email silchip<at>siliconchip.com.au or visit our website: www.siliconchip.com.au 92  Silicon Chip siliconchip.com.au