You can find them on EBay for $15 on up. They are a simple microcontroller based device tester that applies voltages to a device through one of two precision resistors and then watches for what happens. Karl-Heinz Kubbeler describes the device and how it works in TransistorTester with AVR microcontroller and a little more Version 1.04k (PDF).
“Every hobbyist knows the following problem: You disassemle a Transistor out of a printed board or you get one out of a collection box. If you nd out the identi cation number and you already have a data sheet or you can get the documents about this part, everything is well. But if you don’t nd any documents, you have no idea, what kind of part this can be. With conventional approach of measurement it is dicult and time-consuming to nd out the type of the part and parameters. It could be a NPN, PNP, N- or P-Channel-Mosfet etc. It was the idea of Markus F. to hand over the work to a AVR microcontroller.”
There is a discussion on Adafruit with links to the schematic and Arduino program. (see Ardutester – Arduino Component Tester)
The hardware is rather straightforward: a 2 line by 16 LCD and a microcontroller with a bit of glue to provide power from a 9v battery. The test leads can be via ZIF socket, PC pads, or pin header. Pusha’ da’ button, wait a couple of seconds, then read the display to see the result.
The test voltage is low enough that polarized components like electrolytic capacitors should not be harmed. The design is such that the lower test pin numbers are more negative if you want to minimize such risks. The low test voltage means that zener diodes, for instance, with breakdown voltages greater than 4 volts or so won’t provide good information
One interesting item in the schematic is the power management. The push button provides power to the MCU via a transistor switch. The first thing the MCU does is to set a pin to keep the switch closed. After an appropriate time-out, the MCU pin is reset which turns off the transistor switch and powers down the device. That saves battery,
The tester does work to help sort parts. Sometimes color codes can be confusing or hard to read so an actual measure is needed. With capacitors, you get an ESR indication if they have a value larger than 250pf or so. With the measure provided and other information, you can get a good idea if the part is worth saving and what bin to put it in to help finding the right part later when you need it. It is a very good screening tool for parts salvagers.
One place to look for parts and pieces is your local thrift store. You can often find interesting electronics the store doesn’t know what to do with. There is usually also a good collection of digital clocks that can provide project boxes and LED displays. A recent thrift store visit uncovered a 4 line music on hold device with 4 PIC microcontrollers and isolation transformers plus other through-hole parts. The case will be ideal for any ‘indicator on front and plugs on back’ type project — $3 for that and a clock. With that sort of find and a few defunct motherboards, you can put together a parts bin that will be very handy in filling parts needs for future projects.
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