Way back in 1980, my dad didn't have a program for drawing circuit diagrams.  Heck, we didn't get a home computer (a used TRS-80 Model 1, level II) until a few years later - and it couldn't have been used to draw a schematic diagram.

My dad's hand drawn diagrams disappeared decades ago. 

I've recreated them from what I remember and by referring to the "Engineer's Notebook" from Forrest M. Mims.  That's the book my dad used when making the diode tester, since we didn't have datasheets or any other kind of documentation.

My copy of the "Engineer's Notebook" is the later one with the blue cover from 1980 rather than the first printing from 1979 with the yellow cover.

I used KiCAD to draw the circuit as best I can remember it.

Here's the schematic:

The idea is simple:

1. On the left is a battery that powers the circuit.

2. There's an LM3909 as an oscillator driving the clock of a JK flip-flop.

3. The JK flip-flop is set to toggle its outputs by connecting the JK inputs together.

4. The Q and /Q outputs swap state on every clock pulse.

5. Viewed between Q and /Q, you have an AC square wave.

6. There are two LEDs (D2 and D3) in anti-parallel in series with the device under test.

7. The DUT will remove one half of the AC cycle.  If it is connected in the same direction as the green LED, then only the green LED will light.  Same with the red LED.  If the DUT is open, then neither LED lights.  If the DUT is shorted then both LEDs will light

8. LED D1 is part of the oscillator circuit, but also indicates power on so that you can tell the difference between open DUT and dead battery.

So far, so good.

It really does work.  My dad built several of them for work.  I built a couple of them myself for an electronics course in highschool and later for other projects.

It is how it does it that is really messed up.

Look it over and see how many ways the ICs and LEDs are being abused.

I'll put a list of all of the abuses in a later log.  See if you can find them all.