Sonorous tester of short circuits. please, urgent!

[gothmog_es]

I found the circuit at a site web (i wont do it never again): megatronica.cjb.net (in spanish).

In your message, i got lost at any time :? Till i know, transistors amplify stream from IC1A, which is connected like astable multivibrator. If the ends are opened circuit, signal at exits IC1b & IC1Awill be the same, so the speaker wont sound. If the ends are shorted circuit, signal at exits will be different and the oscillations of IC1A should arrive at the speaker, taking place the sound.

 

[bogdanfirst]

ok also if you were to short the leads you cant get a continous indication because you have the capacitor, c2....immm

 

[Gene]

I know you said...

bogdanfirst, i cant change the circuit now, because is for a project. Anyway, thanks for your help

and that's too bad. The circuit is a pile of confusing nonsense - at least to me. For the time spent trying to make it into a working short-circuit tester, you could have built 5 that actually worked from some of the circuits that have been recommended on this thread. If you do decide to start over with a new circuit, perhaps your instructor will be understanding. I would bet that every member of this group has tried to build published circuits in the past that just did not work - I know I have. Good luck in which ever path you decide to follow.

 

[bogdanfirst]

yes thats what i can say too...lots of circuits are not checked and there is lots of confusing stuff ok the 'net. you should check it with a sim program or something

 

[mechie]

Continuity Tester

Maybe I'm a bit late joining in (sorry) but ...
I reckon the astable around IC1A and the transistor driver stage is OK,
What worries me is this +/- 12v oscillator is expected to drive into R4 and R5 (101 Ohms with leads open circuit - ONE OHM with them shorted :shock: ) No wonder they fry :!:

How true that the 'net catches as much as it helps :cry:

If we can rescue this with a bit of redesign ... :?:

Gothmog (if you are still here) what resistance do you want the circuit to sound at ?
Do you want to detect only absolute short circuits or should it sound for a few Ohms?

If the circuit is altered a bit it could work - your tutor should allow for this as almost every design I have seen 'evolves' rather than being perfect from the start ...

 

[gothmog_es]

What worries me is this +/- 12v oscillator is expected to drive into R4 and R5 (101 Ohms with leads open circuit - ONE OHM with them shorted ) No wonder they fry

Well, i changed R4 & R5 values but i didnt get good results.

i think the speaker should sound with resistor's value around miliohms but precission isnt very important at this time.

thanks

 

[gothmog_es]

you should check it with a sim program or something

i thought it for a while simulate it on pspice but i havent ic**'s or transistor's models :cry:

anyway thank you very much for helping to me

 

[mechie]

Circuit Tester

Just finished scribbling this ...
I have NOT tested it, it uses the same oscillator as your design (IC1a, C1, R1, R2). The feedback is taken from the IC's output rather than the transistors so I can control the transistor stage's output (volume).
Component numbering containing a lower case 'a' denotes a new component (Ra2 for example).

A totally new input stage is created around IC1b.
Ra2 provides a bias into the test probe pair. If the probes are open circuit there will be a higher voltage on IC1b inverting input than that on the non-inverting. Ra3, Ra4, Da1 and Da2 provide protection to the op-amp input, Ca3 smooths the input.
If the probes are shorted together the output of IC1b will go positive. The diode Da3 will be reverse biassed and the oscillator, IC1a, will start. If the probes are open-circuited IC1b will go low and forward-bias Da3 which will 'clamp' the oscillator and stop it.

Any use to you ?

 

[Gene]

mechie - How considerate of you to provide another circuit. Hopefully, yours will do the trick and goth will use it to complete his project. For some unknown reason, all previous attempts to convince him to alter his circuit have fallen on deaf ears.

 

[gothmog_es]

mechie, thanks for your work, i will answer for changing the project but i think is too different

by the way, which component is VR1? i dont recognize that symbol.

 

[kinjalgp]

VR1 is variable resistor or potentiometer. You must have seen it as volume control knob on cassette player/radio.

 

[bogdanfirst]

how about the values of the components?

 

[gothmog_es]

Oh, i only saw potentiometer symbol without ground. Thanks kinjalgp.

bogdanfirst, excellent question. Values will be essential if i can change the project!

 

[mechie]

Circuit Values

Sorry for not answering sooner but things here are hectic...
I would suggest the following values for the circuit, I am not a great believer in being choosy with components, I prefer to see what is in my bits box :wink:

I never considered component values earlier as it would have been a waste of time if no-one wanted them...

Is somebody planning to try this circuit out :?:
It's still untested but should work "as-is", I will test it if there is interest in the results :?

R1, R2 = 100k
Ra2 = 1k0
Ra3, Ra4, Ra5, Ra6, Ra7, Ra8 = 10k
Ra9 = Prbably not required - 1k0 or short out
VR1 = 22k or 47k LOG, not fussy
C1 = 22n
C2 = 100u, 25v
T1 = 2N3904
T2 = 2N3906
IC1 = LM148 or any two op-amps
Speaker = any small transistor radio speaker, maybe 8 Ohms or so.