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Support needed making a Geiger counter

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Newbie level 4
Feb 24, 2007
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Hi all, i'm new to this forum!

For many reasons (Which i'm not listing here...) i need to make a Geiger counter, but i've nearly no experience in this brand of electronics, so i'm asking to you some support.

I've already bought the Geiger tube: an LND 712
**broken link removed**

basically, i need a circuit to extract a digital signal from the LND712, which will go directly to a PIC16F877a (I can handle the PIC part)

Months ago i've made this project:
**broken link removed**

It's basically made of a supply for the Geiger and some components to prepare the output to the PIC.
The grounds are unlinked by L1.

Then the project was print on a PCB:

**broken link removed**

And now it's waiting for an approval to be mounted!
Thank you in advance for any help!


schematic :
- place for C17 and C16 connection should be connected to ground .
- you forget 50 pF cap before R2 as it is recommended in design.
**broken link removed**
but i can not say whether you need it or its value as dont know tube output signal width and amplitude . I guess you can skip it for now
- Good to protect the U2A pins 1 and 2 inputs against high voltage by clamping diodes (in addition to its internal protection diodes )
- Test HV generator before making PCB

- take out ground around zeners - there is 100 V drop on each of them , and at most you have 300V spaced by clearance from ground .

- pay attntion to potentials at which the elements are ran

- Place C16 close the R2 , may be worth to palce good quality cap something 100n parallel to it .

Yet there are i guess could be other issues , but other members can help you as well , i have a limited time .

Good luck

I've upgraded both the schematics and the PCB:

**broken link removed**

**broken link removed**

-Grounded C16 and C17
-Skipped the cap (For now)
-Regarding the clamping diodes: i'd like to keep the simplest possible circuit between the tube and the PIC, so i'll test without then check if it's really needed.
-I'll test the HV Generator.

-Ground adjusted, do you think it's fine now? (Or i should adjust it on DS1~4 , C7~C10 ?)
-I'll try

Thank you,


Help needed!!

I've built the pcb to test the HV generator, without connecting the geiger, and now i'm getting about 5,6V DC around the secondaries and nothing out of the primary, I'm supplying it with 6.0V DC.

What can i test? It's possible that i've inverted the emitter-collector of transistors? (How do you recognize The emitter from the collector with multimeters?)


What type of transformer you are using to generate the high voltage ?
It's common transformer ?


Do you know what frequency the HV generator is oscillating at? If it's more than about 1KHz you should consider replacing the 1N4007 diodes with ones with faster switching times.
The voltages you measure suggest it isn't oscillating at all. Bear in mind that you should be measuring AC not DC on the transformer.


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