Vintage Continuity Checker With a Twist

In November 1974, more than 50 years ago now (!), the circuit of a DIY continuity checker was published in the German edition of my (then) favourite electronics magazine, ‘Elektor’. I built it immediately and considered it an extremely simple, but very useful tool, because it can identify both low- and high-impedance circuits. It was lost, unfortunately, to leaking batteries. Soon after that I built a next one. Then, a few years ago, I had started building several of my test tools in a cheap and easy way that, in addition, rather looks fun and a little special. I call my method the 'solid state front panel', and I decided to make one more continuity checker in the same style. This somewhat special look is what I try to describe here for you.

For simple circuits I often use small, plastic project boxes made by Teko S.p.a., an Italian company that was founded in 1957 and is still going strong. Their P/1 to P/4 range is cheap, and they come with top panels made from aluminium. My idea was to replace the aluminium panel by a matching piece of matrix board, usually a scrap piece from my junk box, and build the circuit directly on this board. The electronic components are then visible from the outside, rather than being buried inside the box. Within the box, there remains sufficient space for batteries, if required. I’m sure that my ‘solid-state front panel’ method offers a maximum of simplicity, because no additional PCB has to be mechanically attached to the box, and no additional wiring must be made between PCB and front panel, since everything is located on the front panel anyway. What I try to achieve, in addition, is a component layout that pleases the eye, for instance with the help of symmetry or the colours of the components used. This, however, also depends on the circuit to be built and is sometimes more hit, sometimes more miss...

A group photo of my little tester family is shown above – in the front row, from left to right: Continuity checker V1.5; continuity checker V2.0; USB supply voltage checker; LED tester; NE555 tester. In the back row, from left to right: Single/dual/quad op-amp tester; power semiconductor tester; signal tracer. The second continuity checker from front left is the one I’m bragging about here.

The circuit itself is extremely simple, and it might therefor be a nice project for a beginner in electronics tinkering. Because what does a newbie need when diving into her/his new hobby, next to a soldering iron, an adjustable DC power supply and a multi-meter? Right you are – a continuity checker!

The original article in German language is attached below in PDF format. Diagram #3 is repeated with a small modification by myself, for use with a toggle switch instead of a rotary switch. More about that a little later. The English language edition of Elektor wasn’t launched until 1975, and I don’t know whether this circuit was ever published in English, too. I wouldn't have access to it anyway. Therefor, DeepL and yours truly provided a translation for English-speaking readers. The PDF file in English only contains the modified #3 diagram.

The original diagram requires mentally combining the diagrams #2 and #3. In my own, updated circuit diagram that is provide a little later, I did that already, and I think the diagram is a little easier to be 'read' and understood this way.

The V1.5 continuity checker, btw and fyi, doesn’t feature my ‘solid-state front panel’ design, but – thanks to the simplicity of the circuit – its internal wiring was done 'free-air' and with ‘flying’ components, as shown in the 3rd picture above. However, troubleshooting within such a setup is very difficult, so I cannot recommend it.

Materials

1. 1 Teko P/1 project box – the smallest one from the Teko P/… range
2. 1 piece of matrix board, 80.5 × 51.6 mm, replaces the P/1’s aluminium top panel
3. LS – 8 Ω loudspeaker, max. 40 mm dia.
4. SW1 – miniature, DPDT toggle switch with a center-off position (normally labelled ON-OFF-ON), or rotary switch with 2 decks and 3 positions, plus a matching rotary knob (this will, however, not fit in the P/1 project box)
5. SW2 – miniature, SPDT toggle switch (normally labelled ON---ON) (optional)
6. J1, J2 – panel-mount sockets for 4 mm ‘banana’ plugs
7. Q1, Q2, Q3 – NPN transistors, such as BC108, BC548, 2N2222 etc.
8. R1, R4 – 2.2 kΩ (= 2’200 Ω, sometimes simply written as 2k2)
9. R2, R3 – 56 kΩ (= 56’000 Ω, sometimes written as 56k)
10. R5 – 10 Ω (sometimes written as 10R or simply 10)
11. R6 – 220 Ω (sometimes written as 220R or simply 220) (optional)
12. C1, C2 – 10 nF (sometimes written as 10n or 10’000 pF, or 103)
13. BT – 2 × AAA or AA batteries with matching battery holder
14. Some bare wire, approx. 0.6 mm dia.
15. Some thin, insulated, stranded wire
16. Solder; easiest to solder with is the old-fashioned, but no more widely used, tin/lead alloy (63 % Sn, 37 % Pb) that unfortunately fell from grace due to its lead content; but it may still be used for DIY and medical applications. The current, lead-free solders cannot be processed that easily; they require a higher soldering temperature and usually some more flux.
17. Hot glue

Tools

1. Component bending gauge (optional but really handy)
2. Soldering frame (optional but really handy)
3. Small wire cutter
4. Soldering iron with a fine tip. I am using – and recommending – my faithful Weller Magnastat soldering station that has served me well for more than fifty years now – not exactly cheap but worth every penny; see here: https://www.instructables.com/Revival-and-Upgrade-of-a-Vintage-Weller-Soldering-/
5. No. 2 Phillips screwdriver
6. Tools for cutting/filing/sanding the matrix board cut-off to size (80.5 × 51.6 mm)
7. Table drill with machine vice (or cordless power drill and vice)
8. Drill bits: 3.5 mm for the four mounting holes, plus some larger ones, depending on the diameters of your banana sockets and switches – usually around 10 to 11 mm and about 6.5 mm
9. Hot glue gun

The circuit itself is very simple. It basically is an ‘astable multivibrator’ generating a square-wave output signal, with an additional, low-power output stage for driving a small loudspeaker, and since it is well described in the original PDF articles already, I don’t have to lose any words about it here.

In the original diagram (and in my V1.0 that no more exists), there is/was a rotary switch for both switching on and selecting high-Z or low-Z operation. In versions 1.5 and 2.0 I didn’t have sufficient space for a rotary switch, and I selected a miniature DPDT toggle switch. ‘DPDT’ means ‘double-pole, double-throw’, which is, in Wikipedia’s words, ‘a dual, break-before-make changeover switch operated by a single mechanism’; in my case, an additional centre-detent OFF position is required. Usually these switches are labelled 'ON-OFF-ON'. As already mentioned, I had to modify the circuit diagram for the centre-OFF position of the switch – which was not too difficult, fortunately. Some more inside(r) information on the different toggle switch versions is given in the last step.

I often tinker – and use my continuity tester – late at night. Then its normal buzz is much too loud, and I use an additional, single-pole toggle switch (SPDT, ‘ON---ON’) that inserts a resistor in series with the speaker.

In version 1.5 this ‘volume’ switch is not provided. The somewhat larger speaker behind the top panel radiates through a hole of about 6 mm dia., and as you can see in the 'family photograph', I reduced its volume by sticking a piece of adhesive tape across this hole – a not exactly professional, quick-and-dirty solution that works fine nevertheless.

For my version 2.0 I wanted to use the smallest Teko box, so the loudspeaker must be no larger than 40 mm in diameter. When using matrix board, the speaker can simply be attached to it from the rear using your hot glue gun, and it can ‘breathe’ perfectly through all the small holes.

After cutting/filing/sanding your matrix board piece to size, drill holes for the mounting screws, the two banana sockets and the two toggle switches.

Place the components according to the ‘component side’ drawing, starting with the ones having the lowest profile – the single wire bridge and the resistors; then the capacitors and the transistors. Most components can be interconnected on the opposite (‘solder’) side using their own leads; if not, you can use some bare wire of about 0.6 mm diameter for the connections. To do so, refer to my ‘solder side’ drawing.

These drawings, by the way, were done in Abacom’s ‘LochMaster’ windows software – a not-too-sophisticated but cheap and easy-to-learn tool that comes in handy for, you guessed it, matrix board layouts of simple circuits – see here: https://www.electronic-software-shop.com/elektronik-software/.

In the solder side drawing, the ‘standard’ wiring is shown in blue; the battery connection points are in black (–) and red (+). The remaining, differently coloured wires show the connections between the circuit itself and the switches. These wires are crossing over at several places, it is important to use insulated wires for these links.

You might notice two additional resistors located close to the toggle switches in the photographs of my tester. They are unnecessary for the correct function, and they are not included in the materials list, the circuit diagram, and the layout drawings.

For the two AA alkaline batteries I used a recycled battery holder from a small LED lamp I once got for free; there was, in fact, room for three batteries, so I simply filled up one of the spaces with hot glue. The hot glue gun was convenient, too, for first attaching the speaker in the correct position on the matrix board, and then the battery holder on the speaker’s magnet – not exactly beautiful but functional nevertheless. The battery holder will then be open towards the bottom, and the batteries tend to drop out upon mechanical shock. To keep them in place, I put some not-too-soft sponge rubber of convenient thickness inside the box; its simple but important duty is keeping the batteries in the battery holder as soon as the matrix board cover is fitted and attached with its four screws.

Using a battery compartment harvested from the plastic case of some disposed-of gadget (such as, for instance, an outdated remote control) and inserting that in a cut-out in the bottom of the box might also be thought of. Then its snap-on cover will be accessible from the bottom of the box, and there will be no need to remove the top panel for checking or replacing the batteries. An example battery compartment is shown above. However, in this case there will be two wires from the battery box to the top panel.

The first video clip attached below shows the high-impedance (‘Hi-Z’) operation, testing a 470 kΩ rotary potentiometer (that is, a variable resistor). As you can hear, the pitch rises with rising resistance, and at maximum resistance it still is audible. This also allows a (very) rough estimate of the resistance under test.

Low-Z operation, on the other hand (see the second clip) shows the limits of this mode with a power resistor adjustable from zero to 100 Ω. The tester can also be used in Low-Z mode for a go/nogo check of diodes; simply connect the test leads to the two diode terminals; if you can hear a sound, but no sound after swapping the leads (or vice versa), the diode is ok (that is, it only conducts in one direction, similar to a one-way road for electricity).

Warning: Please remember to always switch your unit OFF (switch in the centre position) when it is not in use, because in High-Z mode there is a constant, small drain on the battery; I forgot this with my very first version, causing the batteries to be drained, and leaking soon after that.

And one more warning: This circuit is not protected against overvoltage at the input. Never, ever use the tester on a circuit that still is powered on, or on a capacitor that still holds a charge!

The original circuit diagram specifies the low-power/small signal transistors as ‘TUN’, without any type designation. It was a special trick of Elektor magazine, meaning ‘Transistor Universal NPN’ (and TUP, ‘Transistor Universal PNP’, as well).

This must be regarded against the background of the worldwide, severe semiconductor shortage that hit both the industry and the tinkerers in the first half of the 1970s. It's difficult to imagine that then, with my tight student's purse, I had to pay more than one US$ for a simple BC108! The magazine supplied a long list of different transistors that all satisfied certain minimum requirements, and any of these could be used if a TUN (or a TUP) was specified.

Un-labelled transistors – in clear: factory rejects – were available in bulk from some retailers for little money, but they had to be checked piece by piece. And if you were lucky, a few of them matched the TUN/TUP specs. There even were two different ‘TUP/TUN Tester’ projects published, first in 1973, and an improved version in 1975 (that I built too), exactly for this purpose.

There exist several different versions of the miniature toggle switches used in this project. The picture above shows four of them. The first two from the left are the more standard versions, both DPDT switches.

The first one is labelled 'ON---ON', meaning that it has just two ON positions, on the left- and on the right-hand side. You have to consider, btw, that the center and the left terminals are connected when the toggle is on the right, and vice versa, as shown in the diagrams above. And if you need a plain on-off switch, you can use one of these but leave the terminals on one side unconnected – as I did with the SPDT 'volume' switch.

The second one features an additional, centre-OFF position, as shown in the second diagram, and it is labelled 'ON-OFF-ON' accordingly.

The third one is a little weird... When I had finished my V2.0 version and powered it up for the first time (of course, after a thorough, visual check for wrong components, faulty connections, or unintended solder bridges), I expected it to work straight away, and flawlessly, at that. Boy, was I wrong! Most annoyingly, I couldn’t find out what was the problem for quite a while. Then I remembered that there exists a very unusual, different version of the toggle switch that almost looks like a DPDT switch. It indeed has a centre detent, but doesn’t switch OFF in the centre position. Such a switch can, with an additional wire bridge between two of its terminals, be used as a SPTT (single pole, triple throw) switch and is used for special applications. These switches are labelled 'ON-ON-ON', which seems not too logical at first sight if one doesn’t know the background. The last diagram above shows the strange behaviour of this switch. And of course I had, by accident, grabbed exactly such a switch from my junk box. After replacing it by a ‘normal’, 'ON-OFF-ON' DPDT switch, my continuity checker finally came to life as expected.

The fourth switch at the far right is more peculiar still, because it features a centre-OFF position, but a momentary-ON position in one direction. This means that it can be switched permanently OFF (toggle in the centre), permanently ON (toggle to one side), but only momentarily ON (toggle to the other side; the lever snaps back to centre-OFF upon release).

To make matters not too easy for us, there are switches that are not labelled at all, such as the fourth switch at the far right. This means that you have to test it manually to be sure it is the one you want (you might also use a continuity checker for that, btw ;-). I have, however, seen such switches labelled – 'ON-OFF-(ON)', indicating the momentary position on one side, or '(ON)-OFF-(ON)' for momentary positions on both sides.

The 2nd picture from step 2 shows the wiring still with the wrong ('ON-ON-ON') switch, the 3rd and 4th pictures were taken after having replaced it by the correct ('ON-OFF-ON') switch.