Many years ago I made up a continuity buzzer circuit using a Germanium transistor as the front end detector, it worked well on MCU's and digital circuits.

It will be nice if I can "chose" only the continuity track on pcb with resistance under certain resistance. For example it will beep only in case the resistance between two probe is less than 10 ohm.

Here is my idea for a low voltage continuity tester.

This circuit is beeping.
Threshold can be adjusted from 1 Ω to 17 Ω by resistor U1.


20 pcs Germanium Ge transistor NPN MP37 ~ 2N445A, 103NU70 USSR NOS
JCD 1Pcs New Ultra-thin speaker 32 ohms 0.25 watt 0.5W 32R speaker Diameter 23mm 28mm 29mm 36mm 50mm
NOTE:
Enhanced circuit see in post #22.

 

Сonnect parallel to probes germanium transistor.

In this circuit voltage between probes is lowered.

 

Is Q2 emitter in the correct place?

 

@Danko in the second solution (post 22) everything remain same as first solutions (post 21) except the voltage correct?
Since germanium transistor is low gain and are not more in production, the ones I can find maybe are made somewhere ( Russia, Europe, Usa etc.) 30 year ago it is sure they are going to work well? T here's no other "modern" transistor who can make this job?

 

@Danko in the second solution (post 22) everything remain same as first solutions (post 21) except the voltage correct?

Yes, range of threshold is same (1Ω - 17Ω).

Since germanium transistor is low gain and are not more in production, the ones I can find maybe are made somewhere ( Russia, Europe, Usa etc.) 30 year ago it is sure they are going to work well?

After 50 years of storing i did not notice any changes in transistors behavior.

T here's no other "modern" transistor who can make this job?

I am not informed about modern transistors which have similar input characteristics.

Is Q2 emitter in the correct place?

Yes, it is. Q2 used in reverse-active mode to decrease β.

 

Today my sister went to the local electronic shop and the seller tell to her that hi have only LM3909. I have to buy the germanium transistor on line. Maybe in the weekend I will try to do the circuit with lm3909...

 

Maybe in the weekend I will try to do the circuit with lm3909...

 

View attachment 288816

Nice! ... why does Q2 look like that?

 

hola cmartinez
Double Collector transistor.
E
Clip:
it's a double Collector pnp (the arrow represents the emitter), and can also be interpreted as a current mirror, since it appears to be 2 pnp with shared Emitter and Base.
You can simulate it with 2 transistors, in a mirror configuration.


https://www.google.com/search?client=firefox-b-d&sxsrf=AJOqlzVPCbHEpcMzO7IrpNedR769UwfZug:1677860116914&q=double+collector+transistor+symbol&tbm=isch&source=univ&fir=NE06DiImSihnTM%2C756jPAX3M_V7oM%2C_%3BjI69Iua259cxzM%2C756jPAX3M_V7oM%2C_%3BrOOufBNFm5iLQM%2CHdgHGMSK-Jwn1M%2C_%3BF0HnQg7tAPD_HM%2CO3ddecQT4KxDsM%2C_%3Bw__nBgDhizuuIM%2CTSGQ34GS3M2BoM%2C_%3BTprckIe-lTSBgM%2C756jPAX3M_V7oM%2C_%3BZoziAeoEnirxoM%2C-Byo47Gep4RH5M%2C_%3ByZ16bjw_BSOL7M%2CW5Npyw7s_MTBcM%2C_%3BneYvr0ixyS8y1M%2CGluV-bmjQMl3yM%2C_%3BpfyW1fbpGfXdNM%2ChLoBBajy1PL4vM%2C_&usg=AI4_-kS3I1dKHphEzsfmja68H8kzLwzWNw&sa=X&ved=2ahUKEwjylIGClMD9AhWTj1wKHQkRDMYQjJkEegQIChAC&biw=1086&bih=635&dpr=1.4

 

View attachment 288816

Someone designed an equivalent to the LM3909 using discrete transistors http://pigeonsnest.co.uk/stuff/picprog/single-cell-led-flasher-two-year-battery-life.html

 

Maybe in the weekend I will try to do the circuit with lm3909...

This is mine. I made an improvised coin cell holder. Let me know if you want more photos of the construction.

 

BTW: I have also been in the electronics industry for a long time, some 72 years, now aged 90 years.

Not many people could say "I've been doing this longer than you've been alive" to a 70-something retired professional.

"There's always a bigger fish" unless you're @ericgibbs, and then I'm not actually sure there is a bigger one.

 

This circuit is beeping.
Threshold can be adjusted from 1 Ω to 17 Ω by resistor U1.

View attachment 288476
20 pcs Germanium Ge transistor NPN MP37 ~ 2N445A, 103NU70 USSR NOS
JCD 1Pcs New Ultra-thin speaker 32 ohms 0.25 watt 0.5W 32R speaker Diameter 23mm 28mm 29mm 36mm 50mm
NOTE:
Enhanced circuit see in post #22.

In this circuit voltage between probes is lowered.
View attachment 288612View attachment 288614

Danko, a question:
Aren’t Q2’s collector and emitter leads swapped?
Or are you using them in an inverted mode transistor configuration?
EDIT: Sorry, I noticed after posting that this had already been answered.

 

Nice! ..

You can play with it:

 

Danko, a question:
Aren’t Q2’s collector and emitter leads swapped?
Or are you using them in an inverted mode transistor configuration?

Q2 is intentionally reversed:


ADDED:

EDIT: Sorry, I noticed after posting that this had already been answered.

I know, you wanted to prevent possible error. It's good!

 

Edited to correct "coil cell" to "coin cell".
I've been using this for about 7 years. It fits in a small project box (2.25 x 1.25 x 1 inches), including 4mm banana sockets. Powered by a single coin cell.

View attachment 288416

today I made it and it's working like a charm. Just the volume is a little bit low.

 

today I made it and it's working like a charm. Just the volume is a little bit low.

Oh cool. Mine is plenty loud enough. Is it in a box, if so maybe it need more holes in the front to let the sound out. Other than that I guess the difference might be the type of speaker. Mine was just a cheap one off Ebay

 

I was thinking to put more then one 1.5v battery in parallel.