Magnetic reed switches are an interesting way to turn things on and off. I’m currently using one to turn on an aiming laser on a handgun when I grasp the weapon with a magnetic ring on my finger. Reason: pressure switches can drain the batteries if you manage to get the weapon crunched up against the holster, like while bending over.
A magnetic switch can be activated (closed) by a magnetic ring (like the scammers sell to “relieve” arthritis or what have you) if you get the ring within an inch or so of the reed switch. You could also do this to light a flashlight for “instant on” capability.
Check the power of the ring you buy (in gauss). The more strength, the higher the price, generally. I suggest getting more than one type of ring until you find the perfect combination of function and appearance (some look fairly cool) because they don’t always live up to their advertised strength. Avoid the copper ones with little pill magnets glued on. Also, you can by stainless steel ring blanks (like crafts people use) and raw magnets in many shapes and sizes if you want to make your own ring.
Anyway, reed switches are small and fragile, a glass cylinder about the diameter of a round toothpick and an inch or so long, with a semi-stiff uninsulated wire protruding from each end. One weakness is that wires tend to want to pull out of the switch if subjected to much stress. So,
Tip#1: epoxy the wires to the glass cylinder.
Tip#2: treat the switch with care lest you break it. Buy more than you need.
Tip#3: Clamp an alligator clip between the cylinder and your soldering, as a heat sink.
Tip#4: if, like me, you decide to secure the switch with some Plastic Dip (rubbery stuff) or epoxy, those can conduct electricity until they cure, so don’t glue the switch with a battery connected or you’ll drain it.
Lastly, (at least the reed switches I got) tend to care about the magnetic polarity. So, the magnet has to align properly North-South along the axis of the switch or the switch won’t close. If you need a more uni-directional switch, try crossing two switches (in the shape of a +, or an L) and wire them in parallel. Thus, if either switch closes, the circuit works.
I have bought some different reed switches some years ago, I didn’t liked the bare glass cylinder switches because they break so easy. But I found some which came in a plastic case they worked really good.
I have searched after these a few days ago but couldn’t find them anymore…
In combination with the now available neodymium magnets it is really cool to build some invisible switches.
The one they used in that video is big, but some of them can be quite tiny and are very durable since they’re built like a typical transistor. I’ve used the very small 3 pin ones like in this video to make a DIY RPM sensor on a small RC car over 20 years ago. Most reed switches I’ve seen are much larger than that. The only caveat with your application is that you’d need a transistor to control power to the laser, but the tiny sensor can be placed just about anywhere. You can even use 2 sensors in opposing directions to achieve switching regardless of magnetic polarity.
Thank you KuoH. So, help me understand the purpose of the transistor since I’m more electromechanical. Is the Hall effect sensor limited to very lower power? In other words, is the transistor functioning like a relay, to engage a higher power circuit to feed the laser? The laser wants 6v and about 600ma. Would the Hall effect sensor not handle that?
Transistors and relays both have current gain and can be considered current controlled current sources. Base current is analogous to coil current and collector current to relay contact rating. Transistor current gain may vary between 10 and 100 and you can see from catalogs what kind of current gain a relay can give you.
Today most people uses fet transistors, they do only need power to change state. Holding a on or off state does not require any power and they have a very low loss, even when handling many amps. Like old fashion transistors they also last a very long time.
Yes, when I was playing with them, the hall effect sensors were only capable of low current. Probably only a few mA or less, but I was using it for signaling, not switching, so it didn’t matter to me. The sensor demonstrated in the first video probably has switching circuitry built in judging by its size and the large wires. The one in the second video appears to be driving a fairly bright LED so they may have improved, though 600mA is probably still a big stretch. Although you could use a FET, as someone else suggested, I don’t think that’s necessary given the low current the laser needs and the fact that you’d have to build additional logic circuits to switch modes. The transistor would be a simple on/off proximity switch when coupled just with a hall effect sensor and maybe a current limiting resistor or two.
an FET is voltage controlled resistor like a vacuum tube, so along with a power supply it could be thought of as a voltage controlled current source. But, IIRC the input capacitance is way higher than vacuum tubes.
A relay is not voltage controlled because even with superconducting coil wires it’s the current through the coil that supplies the Magneto Motive Force. It’s MMF being proportional to amps x coil turns.
Thanks guys. When I was 17 I went to work for the telephone company, initially as a lineman then through various technical grades, installing and services PBX switching systems. I learned a lot about electromechanical stuff, then went into sales, then computer software development. So I never really had any working experience with solid state electronics. Maybe I should take a basic short course.
I think I understand most of what you just told me, but I do have a question for Dim Bulb. Are you talking about solid state relays?
If I want to trigger a solid state relay with 3v (like a CR123A) and I wanted five sets of dry contacts to control five circuits, each of differing power values from 1.5V to 9V what nomenclature would I use to specify that? In a mechanical relay it would be a 5PST (or 5PDT) with a coil rating of 3V.
Thanks again, fellas. I really appreciate your time. This old dog loves learning new tricks.
