Audible tones continuity tester...

[Externet]

Hi all.
How would you build a continuity tester that emits a clean sinewave tone at say ~440 Hz when probes sense ~0.1 ohm; and tone frequency increases to say 1000 Hz when resistance is in the neighborhood of 5 ohms, and progressively about 10 KHz for 100 ohm ?
Using a plain 8 ohm 2 inch speaker, and a single 1.5 V plain cell ?

In other words, sort of a current controlled oscillator ; the smaller the current, the higher the frequency. Not a voltage controlled oscillator VCO.

Probe1-------------------1.5V-----------------speaker-------------------¿circuit?-----------------set resistor--------------Probe2

The set resistor is adjusted so the speaker beeps at 440 Hz when probes touch each other. When probes are not in use, there is no current; it is off.

 

[4pyros]

A 555 timer would do that but you are going to need more than 1.5 volts, how about 9 volts?

 

[jpanhalt]

I do not think you want to subject the circuit/component under test to the current needed for a typical 555 oscillator with such low resistances. For example, if one of the resistors in the timing portion is 0.1Ω, the other resistor would also have to be small. I picked 10Ω (99% duty cycle). 440 Hz would require a capacitor something on the order of 400 µF (NB: values estimated using an online calculator). I do not believe that would be a practical circuit.

A more practical approach may be to look at a particular ohmmeter downstream of the actual sensing circuit, say at the microcontroller level, and create your sound output there.

John

 

[alec_t]

How important to you is a tone range of 20:1 for the resistance range of 1000:1 ?

 

[Externet]

Thanks, gentlemen.

Alec_t : Precision is not that much important; the desired effect is being able to discern by ear if a probed circuit path has a poor solder joint; or there is a short circuited trace near the probing point or far from it somewhere else on the board. That is its behavior : its tone varies in frequency by just minute fractions of ohm, specially in the low ohms range.

I do have such jewel of an instrument, about a pack of cigarettes, and it has proven very helpful and valuable for 20 years. But intrigued of its simplicity and clever construction. Reverse engineered to a certain extent as a transistor is unmarked/erased. The tone, if not a nice sinusoidal as originally stated, it is a smooth pleasant one.
Publishing its schematic is not the point here; it is the intrigue of principle of operation am after and how would you come up with such.
The force used to pierce the probes into solder joints does change frequency. Is that sensitive !
Of course, by the nature of circuit, the battery is not consumed at all when not probing, there is no power switch and the battery lasts as much as its shelf life ~10 years.
Using it on energized circuits could destroy it.

The "¿circuit?" as shown in the original post is strictly in series with the rest of components, as drawn. Its two end nodes inserted in the current path. And works great. Either a brilliant design or a finding by chance. Been reading about relaxation oscillators and it may be related to that family, but I do not want to put clues or bias your minds.

A 555 is not the way to go powered with 1.5V.

 

[4pyros]

A 555 is not the way to go powered with 1.5V.

You well be hard pressed to drive a speaker with 1.5 volts at all.

 

[Externet]

1.5V peak to peak as audio into 8 ohms, gets near a quarter watt. That could be too loud for a beeper.

There is similar more complex ones, appear to be based on multivibrators, but on 9 Volt:
https://www.bakatronics.com/shop/item.aspx?itemid=641
https://www.rhoent.com/beep.htm

 

[4pyros]

You will not get 1.5V peak to peak. There is always a forward voltage drop on every transister.
Both your examples use 9 volts.

 

[4pyros]

1.5V peak to peak into 8 ohms, gets near a quarter watt.

What is the RMS value of 1.5 volts peak to peak -.7 volts for a transister give you?

 

[4pyros]

Ok so there are some 1.5 volt amps out there but I cannot imagine they are very loud.

 

[dr pepper]

You can use a joule thief to get 9v from a 1.5v battery.
I have a C beeper copied from a website, its very usefull, but it operates the other way a short stops the tone.

 

[jpanhalt]

It sounds like you are headed in the direction of using a 555. I am not clear exactly how you are going to incorporate the device being tested (i.e., a resistor in the range of 0.1Ω to 5 Ω.) in a 555 circuit to give you the various tones. Can you please place a schematic?

John

 

[dr pepper]

Deleted irrelavent.

 

[large_ghostman]

sounds like you want a toneohm, i have one here it dosnt work because its the older version and has the wrong probe, but if you want i will take the back off and take a pic of the insides, or google service manual for the tone ohm

 

[4pyros]

Like this;

Uses of your circuit
1. As a Morse practice oscillator. Simply connect the two test leads to your
key and, each time the key is pressed, you should hear a note from the
speaker. The frequency of the note may be altered by putting a resistor in
series with the key. To do this, remove one test lead from the key and
select a resistor; connect one end of the resistor to the free test lead and
the other to the empty terminal on your key. Selecting the value of
resistor that you need will be a useful experiment in itself.
2. As a continuity tester. You can check fuses and lamp bulbs by connecting
them across your test leads. If the speaker remains silent, the fuse or bulb
has blown.
3. To indicate changes of resistance. Hold the ends of the test leads in each
hand; you should hear a low note, because of the high resistance of your
body. Squeeze the ends harder, and the frequency of the note should rise,
because you are now making better contact. Repeat this with damp
hands and the frequencies will be higher still.
4. As a thermometer. Connect the test leads to a thermistor (a device whose
resistance changes with temperature) and warm it with a hair-dryer, or
even in your hands, and you will hear the pitch changing with the
temperature of the thermistor. A suitable ‘bead’ thermistor is available
from Maplin (order code FX21).
5. As a diode tester. Use any diode, and connect the negative test lead to the
end of the diode marked with the ring. This is the cathode of the diode.
The other end, the anode, should be connected to the positive test lead,
and a note should be heard from the speaker. This does not necessarily
mean that the diode is working – yet. Reverse the connections and
nothing should be heard. If this is the case, the diode is working.
6. As a light meter. Use a photoconductive cell (a device whose resistance
changes with light intensity) connected between the test leads. A note
should be heard. Shading the device with your hand will increase its
resistance and the note should decrease in frequency. A suitable device is
the ORP12 cell from Maplin (order code HB10).
There are many more applications. Do not connect the test leads to other
circuits that are switched on. Your circuit, or the circuit you are connecting

Ripped from;

 

[4pyros]

sounds like you want a toneohm, i have one here it dosnt work because its the older version and has the wrong probe, but if you want i will take the back off and take a pic of the insides, or google service manual for the tone ohm

Are you talking one of these?

 

[large_ghostman]

yep i have the 550 but some numpty wired the 580 probe on so it dosnt work

 

[4pyros]

yep i have the 550 but some numpty wired the 580 probe on so it dosnt work

Some "numpty"?

 

[large_ghostman]

yeah was like that when i got it in an auction but i dont know how to put it right, it shouldnt have the probe i think it has two normal ones into the one plug

oh a numpty is like a idiot only a bit more stupid

 

[jpanhalt]

4pyros

The circuit you show (post#15) will not give the tone discrimination the OP wanted for a range of 0.1 to 5 Ω. A 5 Ω change in the top resistor is only a 0.1% change in discharge time and roughly only one tenth of that in the charge time. The OP wanted the frequency to more than double over that range.

John