Black Box

Discussion in 'The Projects Forum' started by PRS, Jan 16, 2009.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    The other day I was trying to design a circuit using op amps and discreet transistors with a faulty breadboard and it dawned on me that I had no idea whether the fault was with the breadboard or with the device. You see, ten years ago I had a very active electronics hobby and I kept all the parts though I walked away from the hobby. Evidently I kept good and bad parts and got them mixed up. And, as I said, I have a tired old breadboard.

    Well, the least I can do is check all my parts! Especially transistors, op amps, diodes, leds, and the value of unmarked capacitors and inductors. So I have designed a component tester based on known designs that I keep putting together on my breadboard in order to test various components and I'm putting it all together in a little black plastic box about 1/4 the size of a shoebox that I bought from Mouser years ago.

    On top of the box I screwed on a rarely used 2 inch by 3 inch breadboard. I used machine screws and nuts for connectors to the function generator, oscilloscope, frequency counter and DMM probes. Also there are 3 banana plug binding posts for plus and minus source voltage plus ground.

    Also a SPDT switch is mounted on top of the black box to switch between plus and minus source voltage for the sake of testing PNPs and P channel FETs on the one hand and NPNs and N channel FETs on the other hand.

    Also mounted on top is an LED to be used in conjunction with a diode tester.

    The black box can check 741 OP Amps, any BJT, any JFET, diode, led and also enable the calculation of capacitance and inductance.

    The transistor checkers are made of common-emitter/common-source circuits to that, not only can the device's functionality can be tested, but its gain compared to other transistors in order to find matched pairs.

    The op amp checker is just an inverting amp with 10v/v gain.

    The diode checker just turns on an LED mounted on top of the box to show it's good. A series resistor was all that was needed here.

    The LED tester just runs the current through a resistor and glows if it's good.

    I'll explain the capacitor/inductor circuit if anyone is interested. A very simple contraption, easily made.

    Inside the box are 13 resistors and 7 capacitors, that's all. Wires run up to the breadboard and I'll write testing instructions down since I know that if I don't I'll forget how to use the darn thing and it will be useless.

    I'm sure many posters in here can make their own component tester, but I thought I'd share these ideas in order to get you to thinking about it. There is nothing more frustrating than designing using faulty components! Good luck! ;)