Electronic – How to choose a reed switch and magnet

Bipolar transistors and FETs work about the same on the output side when you are using them as switches in low power applications like you mentioned. Both come in two flavors to make either high side or low side switches. The NPN bipolar and N channel FET are the low side switches, and the PNP bipolar and P channel FET are the high side switches.

The difference between bipolar and FET is mostly how they are turned on and off. Bipolars are turned on by running some current thru the base. This allows a much greater current to flow thru the collector. The ratio of the possible collector current to base current is the gain of the transistor. FETs are controlled by voltage instead of current. A basic N channel MOSFET might require 12-15 volts on the gate to be fully on, with 0V fully off. There are things called "logic level" FETs that can switch between on and off well enough to be driven directly by 3.3V or 5V logic outputs.

I'm going to disagree with Starblue and say that for very simple switching applications for hobby use, get a handful of good logic level FETs like the IRLML2502. They will cost of 10s of cents each, but are very fogiving as long as everything is limited to 20V. I would certainly not start with that for a volume design done by a professional, but if you just want to get started with one answer that serves the most needs, this is a good first part to get used to.

Once you're ready to experiment with bipolars get 100 each of 2N4401 and 2N4403. They are dirt cheap, widely available, and all around pretty robust for their size. They would also be fine for switching 200mA loads, but you need to know a little more to use them well. You can't just connect them directly to a microcontroller ouput in a switching application.

1. The most popular types of magnets are Neodymium-Iron-Boron (sintered or bonded), Samarium-Cobalt, ferrite (often bonded), Aluminium-Nickle-Cobalt (often called Alnico), and ceramic magnets. They all have different properties in terms of magnetic field strength, resistance to high and/or low temperatures, resistance to demagnetization, cost, etc.
2. The magnet in the photo is most likely a sintered Neodymium-Iron-Boron (NdFeB) magnet with a Nickel-Copper-Nickel coating to help protected it from corrosion. NdFeB magnets are produced in a variety of different strengths and temperature grades. This will be called out as a number and a letter, like 35UH or 50M. The number tells you the energy product of the magnet (which gives you a rough idea of the strength of it) and the letters tell you what the Intrinsic Coercivity of the magnet is (which gives you a rough idea of the maximum temperature the magnet can be used at).
3. The equivalent for a magnet is called flux density and it is measured in Tesla, Gauss or sometimes "lines per square inch." Flux density is a measure of flux per unit area. "Typical" magnetic field strength at a certain distance is going to depend on the specific grade of NdFeB and the thickness of the magnets. Once you have those, you can find calculators online that will give you the answer.
4. Once you have a material picked out, the main things you need to know are shape (square, rectangle, circle, etc), area and thickness of the magnet.
5. Sintered NdFeB magnets are made in the following way: Raw materials are melted down and then formed into a fine powder. The powder is pressed into a shape (this can be done in a couple different ways -axial pressing or isostatic pressing). Then they are sintered (heated). Then they can be and often are machined, which could be slicing a bigger block into smaller parts or grinding in order to meet a certain tolerance. Then they are coated to prevent corrosion. The coating could be an epoxy, Nickel-Copper-Nickel, or some other method. Once this is done, they are magnetized by being placed in a strong magnetic field.