( QTY 5 ) Miniature Magnetic Reed RELAYS

First, I tested (12) relays to characterize the pull-in / drop-out voltages. All closed by 3.5 volts going up and opened by 2.8 volts going down. Based on the dozen samples, allowing for hysteresis, I believe they will all pull in by 4 volts and drop out below 2.5 volts.

Now here's the neat stuff. With a variable power supply, I carefully biased the coil just below the drop-out point. Using the same magnet, the sensitivity increased to about a foot. The magnetic field has to be in phase with the coil's field. That's at least 10x the distance unbiased.

But it gets better! Next I tried the obvious and increased the coil bias until the switch closed. Turning the refrigerator magnet around, to buck the electro-magnetic field, I could then turn the switch off with the magnet at a distance of about a foot. So this N.O. relay can be used as a sensitive N.C. proximity switch too. Cool.

Remember: Results may vary from relay to relay. Allow for hysteresis. Particularly, avoid setting the bias current too close to the thresholds for either the N.O. or N.C. states. A buffer zone may reduce the sensitivity, but reliability and repeatability will be improved.

FREEBIES, but only if you ask when making payment. Our tiny micro-magnet (right) will switch at about ¼". Biasing the coil just barely on, or barely off and provided the magnet was properly oriented for the task, I could flip the switch from about an inch away. They are small enough to be easily hidden and virtually undetectable: under a watch band, in a ballpoint pen, sewn in a shirt sleeve, in a soda straw, a book binding - you get the idea. We give you (2), just for the asking, at no charge per lot of relays. Magnets are NOT part of the auction and will NOT be shipped with the relays unless you ask for them when making your payment. Just ask for the free Micro-Magnets.

Now this is way out on the fringe but you may find it interesting.
Miscellaneous findings (may or may not be reproducible): As the bias current is slowly adjusted, each relay performs differently at it's magnetic threshold. Some switch rather abruptly, while others go through a very brief transition where there is noticeable contact resistance. My test circuit switched 5 volts through a 1K resistor to a LED. I found some relays have a critical bias setting where the LED glows dimly. I carefully adjusted one relay to the point where the LED was at about half intensity - the contact resistance acted as a current control. Incredibly, with some relays, I could vary the intensity of the LED with the refrigerator magnet at a distance of 1 to 2 feet. (26 inches was the most sensitive. But that was certainly the exception.) At that great distance, the permanent magnet's strength wasn't strong enough to either fully actuate or deactuate the switch, but only vary the LED brightness. It wouldn't be accurate to say it would switch with the magnet at 1 foot, but is fair to say it could sense the magnetic field at foot or more. That's amazing. But other samples worked much differently. With careful biasing of the coil current, most switches tripped solidly with the magnet at about 4 to 6 inches.

Most relays will probably fall into the later category since there is a 'capture' effect with the reed contacts as they close. Their mutual attraction increases exponentially as the gap closes. My theory is that a slight misalignment of the contacts causes them to close gradually with a slowly changing magnetic field. Without a sharp snap it produces a very brief actuation "gray zone".

One more thought. You can place a magnet just out of range of the switch and then bridge the gap with some ferrous material. For instance, you could count nails on a conveyor belt. The switch on one side would be unaffected by the magnet on opposite side until a nail passes in between to conduct the magnetic flux across the belt.