Magnetic reed switches
	Glass reed switches are very fragile so please take note: If you apply heat to the wire it will expand and shatter the glass. What you must do is to hold the wire near to the glass (but NOT touching the glass!) with a pair of "needle-nose" pliers. This will suck heat away before it reaches the wire in the glass. You can then use a very hot iron to quickly "tin" the end of the wire with solder.
Then tinned copper wire can be soldered to the reed switch wires, still using the pliers. If you need to bend or shorten the reed switch wires (not recommended) you must again grip the wire close to the glass with pliers and bend or cut the wire furthest from the glass. The pliers will prevent any strain on the glass. Once it's prepared, the reed switch can be glued in place using hot-melt adhesive or similar. If you use Evo-Stick DO NOT let it touch any connections because it corrodes metal. How to test a reed switch Connect a multimeter to the reed switch with the lowest resistance (Ohms) range selected. The meter should indicate infinity. Now bring a magnet close to the reed switch. The meter should indicate zero Ohms as the switch reds click together. The orientation of the magnet is important. Hold it so that its North pole is close to one wire and its South pole is close to the other. Reed switches for fun and security These handy devices are used with magnets to detect doors and windows opening. They can also be used for amusement in conjunction with electronic counters, lights, LEDs and other devices. 560-492 Magnetic reed switch £1.95 Qty

Car Lamp Fail Warning Here's a useful gadget that you can make with a reed switch. The concept is simple. You fold a length of insulated wire in half and wind it around the reed switch. Cut the folded wire and connect ends A2 and B1 to the 12 volt supply to a pair of bulbs (eg. brake light bulbs). Connect ends A1 and B2 to the left and right brake lights. When current passes through the wire coils to the bulbs, it goes through each coil in opposite directions so that the magnetic field of one cancels the field of the other. The result is that there is no magnetic field and the reed switch remains OFF. If one bulb fails, the current flows through only one coil and the reeds are pulled together by the magnetic field. With the switch thus ON, the current can flow through resistor R1 to light the LED. (A small 12v bulb can be used in place of the resistor and LED). The warning LED is positioned on the dashboard (or in the rear window where the driver can see it in his mirror). Use 22 SWG enamelled copper wire. You will need about 30 (twin) turns to generate enough magnetic field to operate a small reed switch from a brake light. Secure the wire with heatshrink sleeving or suitable glue or tape. A 21 Watt brake lamp takes about 2 Amps so if you are detecting lower Wattage bulbs, increase the number of turns accordingly. A 5 Watt bulb will need around 30 x 4 = 120 turns but you can use thinner wire (try 24 SWG). Have fun experimenting! Note: On modern cars, left and right bulb circuits are on separate fuses (apart from the brake lights) so the coil circuits must be kept separate, too.

©2004 SatCure

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