Audible continuity tester...

[haklesup]

I think this is it.. I had to use a DMM beeper to ohm it out (ironic). I didn't look at the backside of the PCB to verify. Also I drew the transistor backward. not that it matters because I haven't checked to see if its NPN or PNP or that the leads are as expected. I would expect the collector to be connected to the + side of the battery which would make it an NPN but I could have it all backward.

Enjoy!

[Externet]

Isn't that a beauty of simplicity?

Now the task is to find if the TO-92 device is a UJT, JFET, mosfet, bipolar transistor, or what.
Its part number does not show any specifications.

Good job, Haklesup.
Regards,
Miguel

[ecerfoglio]

to find if the TO-92 device is a UJT, JFET, mosfet, bipolar transistor, or what

I would say that it's only a "garden variety" NPN bipolar transistor (ie BC548 or PN2222 or whatever you vave at hand)

Colector to +V (bottom), emiter to the transformer's "primary" (top), base to the secondary (right)

The transformer's "primary" (left side) would be the lowest impedance (or resistance) of the two windings.

When you close the circuit there is some current flow from the battery >> the resistor >> transformer's "secondary" >> transistor's base >> Base-Emmiter junction >> transformer's "primary" >> speaker >> test leads >> battery.

With this base current the transistor conducts, so the current increases in the transformer's "primary" and that induces a voltage in the secondary that turns the base off (**) and the circuit oscilates.

(**) Of course, you will have to experiment with the transformer's polarity, but there are only two ways to connect it

[Externet]

Highly doubt it.
Cannot see how a plain transistor would oscillate there.
Edited: added----> Are you saying the base is biasing the transistor for saturation at the moment the probes contact, and the secondary reverse biases to shut it off ? That may work. <----

Edited: Deleted some text.

A JFET seems more probable, conducting when probes touch and turned off by the transformer feedback. When the turn-off pulse decays, conducts again and the next secondary pulse would turn it off again.

A UJT has probabilities there too, curves show it conducts at zero voltage, peaks current at a small voltage and decreases conduction when voltage increases further.

Miguel

[Chris Smith]

Cut through the BS, stick with the 555.

No transformer or special transitors needed.

[dyarker]

He got your opinion the first 3 times you posted it! And been told that a 555 is not wanted, and why it's not wanted.

[Chris Smith]

And what?
Your going to show him how to re invent the wheel?

Spin your wheels some where else.

I realize that common sense drives you batty, just don’t pass it around, its not even intelligent.

[haklesup]

While it is the fool who reinvents the wheel, it is the Scholar who seeks to study its every detail so that he may use that wheel if the opportunity arises.

This falls in the category of cleaver little analog designs that are no longer favored due to newer options. It may pop up in a circuits encyclopedia.

I tried to simulate this (Multisim) but I'm unsure how to model a speaker (there is none in the library) plus the transformer was a total guess. First try crashed the simulator, Ill try more later when I have time.

[rshayes]

The circuit is an example of a blocking oscillator. It isn't a particularly new or unique type of circuit. Similar circuits were used with vacuum tubes in World War II radar equipment. It was simpler to use a small pulse transformer as a phase inverter instead of another vacuum tube section. Rise times were also usually better due to the high pulse currents which were possible with positive grid drive.

The transformer is arranged for positive feedback. In other words a negative signal on the collector should produce a positive drive signal to the base. The collector will saturate at a low voltage causing most of the supply voltage to apppear across the transformer primary. At first, the current is limited by the transformer inductance. Eventually the primary current increases until either the transformer saturates, or the base current, limited by the series resistor, is not enough to support a further increase in collector current. This stops the increase in collector current and reduces the base drive. This change causes a positive change in collector voltage and gives a negative base driveturning the transistor off. The collector voltage will spike up to a high value until the transformer current drops to zero and then the cycle repeats. Often, an RC circuit is added in the grid or base circuit to define the off period of the oscillator.

Results are unpredictable, but this type of circuit can work with low gain transistors, such as the leaky, low gain transistors that Radio Shack sold in packages of eight in the late 1950s. The small audio transistors used for interstage coupling in transistor radios worked fine in these circuits.

[Edd]

.

Sooooo, do the current conditions mean that the production test line is now relegated ever upwards and now using the continuity test function of a much higher dollar Fluke ?

I had my guess-ti-mations of what was inside, with your specifying its use of a 9V battery supply. However, there was some variance after Hacklesup secured a schema …resplendently complete with its special single 3VDC “Câ€

[Externet]

Thanks, Edd, nice schematic.
So you built it and works as intended with a plain 2222, nice!

No, the company moved to another state, and probably took these gadgets and hope they are being used by the new technicians. At my new workplace, they do use the fancy Fluke$ to check continuity, without the convenience of the variable pitch audio.

The model I remember having on my bench was powered by 9 V, and besides having the varying tone, was an in-circuit led tester (enough voltage); very convenient. Production of electronic equipment does encounter reverse installed/factory bad/infant mortality leds often, and this gadget was a superb tool for such.

OK, I will build one, you have given away the whole enchilada in a silver plate with the proven part numbers.

Miguel

[MrAl]

Hi there,

I hate to be the bearer of bad news or maybe bad 'possibilities' but
i would be somewhat careful using this circuit on parts other than
relays and switches or stuff like that that usually isnt damaged by
short high voltage pulses.
The reason i bring this up is because *both* the transformer and the
speaker can act like inductors (because after all they have coils
of wire inside them) and when open-circuited can produce unexpected
high voltages thus frying the circuit under test. Granted this isnt going
to happen with *every* circuit under test (because some will be self-
protected already by mere chance) but there are some that will simply
blow out or at least get over stressed because of the voltage spike
that will appear between probe tips when the probes are disconnected
from the circuit under test. Note also that there will be some bounce
when the probe tips are connected and when disconnected, meaning
there is the potential to zap the circuit under test with several high
voltage spikes just before and just after the 'test'.
At the very *least*, i would connect a silicon diode (such as 1N4148)
across where the probe wires connect to the circuit so it's normally
reverse biased, as well as a zener (with maybe a 5v rating) such
that it conducts if the voltage across the probe tips tries to go too high.
This will protect the circuit under test when the speaker and/or
transformer rings when the probe is disconnected from the circuit under
test. The diode will shunt the negative spikes to 0.7 volts or so, and
the zener will shunt the positive spikes over 5v or so. This should
protect the circuit in most cases unless it's a 3.3v device, in which case
maybe two or three diodes in series to replace the zener, which will
limit the positive spike to around 1.4 to 2.1 volts, which should be safe
for most circuits and devices.
If the tester is only going to be used for switches, relay contacts, or
simply wires this isnt needed, but for any ic's, transistors (bipolar,
mosfet, etc), diodes, LEDs, id be sure to use the protection diodes.

BTW, the cheap headphones usually have speaker elements that have
typically around 20 ohms or more impedance which makes for a nice
replacement for the speaker and means less current through the
circuit under test.

[Bob Scott]

.

Testing the unit by dead shorting the test leads gives about a 400~ tone, while the probing of a 27 ohm resistor shifts the tone on upwards into the ~ 5Khz range……..akin of a mosquito buzzzzzing in a high tizzy, whilst zeroing in on a fat man at the beach.

The transformer required is still stocked thru Radio Shackamus, or, if available , an old xstr radio chassis will bag one, along with the required speaker.

Hi Edd,

A long time ago, the large consumer service department that I worked in needed a doorbell for the back loading doors. Seeing that we didn't have a doorbell in stock, but we did have a quite nicely stocked parts department and at least one of ten technicians interested in doing "something, anything else", I designed and made a circuit quite similar to the one above, except that instead of a transformer I used a loudspeaker with a center-tapped winding. Instead of a 2n2222 I used a higher current 2n3055 power transistor just in case I accidentally shorted anything whilst experimenting.

It too changed frequency, but randomly- from one flat note to the next an octave higher-and back and forth possibly with contact resistance change of the push button. It sounded like a wounded bleating goat. They kept using it for a year until the supervisor, caught at a frustrating moment, couldn't stand the sound anymore.

Regards,
Bob