These are some helpful things to watch out for when you decide to make your own test equipment to go with your AVR’s and mini programmable computers.
For a few years people have done the ATX bench power supply conversion. It will come up about 95% of the time when you search up for a home made power supply. It’s not totally bad but there is a major flaw with the conversion. The ATX power supply uses a earth ground reference. What does this mean exactly? Well The DC ground is tied into the chassis ground that the chassis ground uses the AC socket’s earth ground. When it’s all said and done it is looped into the Neutral line to your house. If you were to probe a running circuit with your Oscilloscope it will go two ways, blowing up the circuit or blowing up your scope.
To over come this issue there is a easy solution. Since you would have to take the power supply apart begin with you’ll need to to a continuity test on the mounting points of the power supply and the chassis ground. There should be only one or two points to hunt down. To isolate this you need to use a plastic standoffs in place of the old metal standoffs. It’s kind of tricky to do this if you plan to reuse the original ATX case. After you have successfully bypassed the earth ground reference then you will be alright. However I wouldn’t plan to run the supply at it’s full amperage rating. Also be sure to not change out the three wire power line to a two wire power line. Last thing you want is that power supply’s case to get charged up.
USB power devices are also overseen by this. If you’re using a Desktop computer to run a USB device such as a Arduino and you decide to probe around with a scope then you’ll blow something up. You have two options for this. Use a Laptop or one of those offical Apple power adapters to power the device. Also be careful with a Laptop. The charger could be earth ground referenced. A simple continuity test will reveal that. A simple fix for that if you can’t just use the battery is to use a isolation transformer.
Another trending DIY test device is the Sound Card Oscilloscope. The theory is sound. Only works with low voltages and a few KHz resolution. However what a lot of people forget is a computer’s audio input works with AC signals only. In other words it is AC coupled and will not read a DC signal. In my opinion that really sucks and isn’t really worth building. However you can slap together a Octopus circuit and use it as a component tester. While on the subject avoid the cheap PC scopes. They are either AC coupled only or DC coupled only. Just buy a analog scope for like fifty bucks and save up and buy a real DSO then resale or give away the analog scope to a fellow newbie.
Cheap multi meters are 50 – 50. You want to spend around 20 bucks for one. We’re not working for NASA let alone scientific reasons so a super high resolution meter isn’t needed. We’re just having fun building one off devices. You just need a meter that confirms you have the proper voltage going in the circuit or if two traces are connected. It doesn’t really mater if the resistor you want to use is exactly 100ohms since there isn’t really a way to make a perfect resistor since they’re 1% tolerance or 5% tolerance if you use carbon resistors. However avoid using the transistor test function, they will kill the transistor or give you a incorrect reading. Make sure the meter can handle CAT II or even better CAT III. If you’re just going to use it for low DC power circuits a CAT rating isn’t really needed. Now the more you progress by all means buy a Fluke, Gossen or Keithley. If anything you can score a nice old bench digital meter for a few bucks on eBay.
Avoid by all costs on most of the Chinese marketed products such as Oscilloscopes, Function generators, Multi-meters and Soldering Irons. Since China is a foreign non-extradition country we can’t really do anything about crappy made, counterfeit, non-safe products. It’s not the typical stuff they make then sold here in stores since there is usually a company locally to be held accountable but stuff they make will sit over there until we order something then they ship it over. This way they can’t get busted when you call shenanigans, they will tell us to #&*$ off. However there is a slight bit of hope, it requires a ton of research. There are legit companies in china that made some pretty good stuff but the market gets flooded with counterfeits. Sometimes dealers end up with counterfeits that they think is legit but it’s not. This is where the Open Hardware scene starts to hurt. Kinda like the FTDI driver scandal and the fake o-scope kit I got.
In all have fun and stay safe.