I sometimes end up having someone's unwanted old electronics, usually faulty ones. Does it make sense to harvest components from such. Anything valuable I should be looking for?
You'll be paying youself only pennies per hour. I wouldn't bother with basic and cheap stuff like resistors and capacitors. ICs are often hard to identify, and old ones have little value today.
I would look for things like power transformers, speakers, heat sinks, relays, solenoids, motors, and large mechanical parts. Those cost more and are often hard to find. Sometimes just the box and chassis are the more valuable parts.
The most valuable thing you can get from discarded electronics is understanding. Try fixing them, or at least trace out the circuit until you can discover what is wrong.
It used to be very hard to get schematics, but now we have the Internet. Sites like http://eserviceinfo.com and http://ko4bb.com/ have service manuals for a variety of equipment. Even if you can't find the manufacturer's service manual, you can often find datasheets for the important components. Many designs don't stray too far from the datasheet example circuits.
Of course, you can learn about circuits from just reading datasheets and textbooks, but having the physical circuit in front of you gives another level of understanding.
These days, there isn't much to get from consumer products. Look for reusable sub-assemblies rather than discrete components. For instance, laser printers yield nice high voltage supplies. Printers in general can be good for a motor or two, though good luck finding any specs on them. Sometimes you can get a display suitable for re-purposing from a dead laptop, but again, specs are the problem. Figuring out how to use something from just looking at the end of a ribbon cable and a power connector can be daunting.
Of course, if you're pulling apart old electronics, that's a different story. If it has vacuum tubes in it, while you may not personally have a use for them, there are people out there who would be happy to have even used tubes. The sockets for same are getting harder to come by as well, as are step-up transformers that supplied the voltages needed. If you encounter nixie tubes, just google them before you consign them to the trash can, you'd be surprised.
The best thing I've pulled apart in the last 10 years was actually a treadmill. It yielded a whole bunch of nice 7-segment displays, a power supply for the logic board, a great big DC motor, and a speed controller for same. Also, microwave ovens sometimes give up beastly large transformers - but be exceedingly careful with those, they can easily kill a person.
I recommend harvesting components 1) in the case of custom chips that can be used to repair other devices of the same type, OR 2) if you are in a situation where you can't quickly get any parts for repair work.
Number 1 only really applies if the device is something that someone will, someday, want to repair. For instance, older arcade machines are still in demand by collectors, and we repair our equipment (because you sure can't buy it new anymore!). When the manufacturer used custom chips, then the only good solution for collectors is to save old, dead boards and strip them of custom parts to keep other boards running. I'm sure there are other classes of equipment where this would make sense.
Number 2 applies if you are located somewhere that doesn't have local suppliers of parts, and slow or excessively expensive deliveries. Islands or small towns in far northern latitudes can have stupidly high shipping costs, and low local populations that will prevent the operation of a local supplier. In this case, throw out NOTHING. You keep the broken stuff and when you can't get a part, you go scavenging in your junk pile for what you need.
I like salvaging parts. It is really helpful if you are a student (or just anyone) strapped for cash.
Sometimes you also find real gems. I always keep boards with electronic components on them in a box, and enjoy looking through them to see what I can use. They teach you a lot about how circuits work as well.
Very few people have also considered the environmental benefit of recycling old components.
An essential tool for me when tearing apart electronics is a desoldering pump. If you don't have one, seriously consider getting one.
Personally, I like salvaging old parts. If you are just starting out with electronics, I would consider it a good learning activity. You might not get anything very useful, but you will probably learn something.
Take a look at this book: How to Diagnose and Fix Everything Electronic. You might find it interesting.
Off-hand I'd say it's not really worth it. I'm not sure you have the time/patience/energy to test all components you retrieve. Testing is necessary not only because they might be damaged in the first place, but also because they might be damaged while soldering them off (i.e. too much heat).
In any case there might be some exceptions, like more expensive components (nixie tubes are a good example - I think there is only one place in the world where they are manufactured) and/or ones that are easy to remove like transformers.
IC labels are often made unreadable by the manufacturer (scratched off, painted over) in order to prevent you from identifying them and possibly copying their designs (yep, paranoid...). Again, I feel most of them are not really made to be soldered off (especially so for BGAs or their brethren), but sometimes they are just plugged into a socket, in which case you can try gently prying them out (careful not to break off a leg).
Maybe some 'interesting' type of jack may also be useful, if you can open it without damaging it. (Most of the newer ones are not made that way - once you pop them into place, you can only remove them by braking the casing.
As I said this is a really painstaking process. Unless you are very short on money or supplies, I wouldn't really recommend it as general practice (I heard Russian hobbyists are diligently salvaging...). On the other hand, this might be something you could enjoy and you might contribute to recycling electronics.
:) This is an unsolved problem, and even retrieving the gold from the traces and other components seems to yield so little that nobody can make a profit out of it.
About the worth, I'd have to say that it depends mostly on the ability to quickly survey the device and on the condition of electronics market in the area.
From my experience the most important thing about salvaging components is to know what you can buy in the store around the corner, what you can buy in a specialized store, what you can order from the Internet and what needs to be imported and to know what types of components to expect in a device and then to only focus on the components which would be difficult to easily obtain and ignore the rest.
The problem with this approach is that what exactly is good to keep and what isn't depends a lot on the local conditions so it's difficult to provide good advice on that and that some experience is required to actually know what to look for in the first place.
Another thing that should be obvious but I'll mention anyway is to go first for the most accessible components. For example printers, scanners and similar office devices often have lots of electrical motors and in some cases they can just need to be unplugged. Most mobile phones nowadays have vibration motors that can be very easily removed from the device, sometimes even with no tools.
Also look for whole modules what can be removed completely like transmitter/receiver PCBs in various wireless devices or power supply PCBs. It is sometimes easier to just include an off-the-shelf PCB inside of a product that uses some well-defined method of communication with the rest of the device than to design one yourself and such modules are interesting for salvage. The problem with more complicated modules is that since the device is dead, there's a chance that the module you're looking at caused the failure, but if it's rare enough and you know to check if it's working, it could be worth the bother. Another problem with this idea is that since there is a major drive to lower the cost of the device, the modules themselves may not be so independent and may expect the rest of the device to do some processing for them. Unfortunately, this can be a bit hard to detect early on.
Another thing which I should mention are custom made components. It takes some experience to find them and sometimes they may not have easily decipherable markings, but they could help a lot with the problem of rarity of come components. Examples of such devices are custom potentiometers which may have switches in interesting positions or may be powered so they can turn themselves or that may have strange resistance distribution (for example antilogarithmic potentiometers seem to be rare now), crystals, resonators and oscillators with strange values, linear voltage regulators with unusual outputs and similar. The main problem with them is as I've said detection. Unless the component is very obvious, there's a chance you won't notice it and even when you do, you can expect problems locating datasheets or facing the challenge of using a component without a datasheet.
I'd also like to write a bit about some components that often aren't worth salvaging. For example it's pretty much impossible to use the "epoxy-blob" packed integrated circuits. Some very specialized components may make sense only for a device of a single type or with same use, such as certain types of LCDs. Basically any LCDs that have any special character on them will often end up useless since it's going to be complicated to drive them. Next, some types of passive components are usually very common, very cheap (say resistors) and may deteriorate with time and use (like electrolytic capacitors, switches, push-buttons, potentiometers). I'll also add microcontrollers to this list. Unless the microcontroller isn't one I'm familiar with and isn't used in some very easy to extract package or has exposed programming connection, I wouldn't touch it. Basically for microcontrollers, there's some setup cost needed for each family and many microcontrollers have options to prevent later flashing or may even use ROM or EPROM, so they can't be reprogrammed.
I'd like to spare a thought for the mighty Vacuum Tube. Not real likely you yourself will be able to use it successfully unless you know what you're doing, but AFAIK given the fascinating prices ppl will pay on old tubes on eBay (give a search, why doncha) you can easily make a buck or 10.
In fact, feel free to PM me with any vacuum tube y'all come across. That goes for everybody.
One often overlooked thing that is worth grabbing are wiring harnesses.
The wired .100 headers typically used for PC front panel buttons and LEDs are great for connecting to eval boards, ISP headers, etc and save making your own.
Some of the smaller, higher density connectors used in other electronics can be very hard to source, but are worth saving for a potential match to a module you are trying to interface, or the console serial port on some consumer-market embedded Linux device, etc.
Fans and heatsinks, if easily removed, can come in handy too.
What I haven't seen mentioned here are your needs.
You shouldn't salvage a part that you know you'll never need and you can't sell.
When people mention vacuum tubes, they usually forget to mention the Socket and the high-wattage resistors.(If you can use them)
High-capacity high-voltage capacitors are useful too, but only if the circuit is not too old, because they have a high failure rate with age.
And here I would also like to add, that if you have the skill repairing it, then selling it as a whole product may be even better, as there is a lot of people looking for Valve amps for guitars, or hifi.
Old stuff like Valves and Audio output transformers can be sold easily on the internet. Sure, you will strip what you can use if it doesn't take too much time. This means that resistors won't be worth it, but high quality polypropylene capacitors will be. Electrolytic caps can be a bad second hand proposition when in SMPS or really old equipment, so most won't be saved. SMD boards have a low % of strippable parts. It may be wise to save some through-hole parts because they may get more expensive in the future and they are good for fast prototyping. Semiconductors have a rather constant failure rate so the stripped ones should be just as reliable as a new one if there isn't any heat or moisture damage. Germanium transistors and diodes are expensive new and should be saved because they don't take up much space. Remember to have a good bin system that works for you before you start desoldering. Also remember that scrap aluminum and copper are worth selling.