MightyOhm.com

Hello!

I recently started to build my own geiger counter. I have already ordered all parts, that are needed, and a fresh SBM-20 waiting to loose its virginity.
I rebuild the circuit from mightyohm 1:1 (resistors diodes,...) but I wasnt able to buy the FJN3303F high voltage switching transistor.
Instead I am using a TS13003BCT (http://pdf1.alldatasheet.com/datasheet- ... 03BCT.html)
According to its ratings it should theoretically do fine as well.
But my problem is, that I cant create ANY oscillation on the 555 timer. I was using an oscilloscope to measure the 555 (10x Probe). On the output, where the tube goes I can just measure about 1,6V, and the rest is all dropped over the inductor. (also single wind inductor as recommended) (http://www.farnell.com/datasheets/50809.pdf).
The very bad thing is, as there is no switching at all occuring, the inductor gets HOT, as its dropping half the voltage. (inductor series resistance is approx 22 ohms)
I am pretty sure, that I did something worng with my 555 chip - I have 4 replacements here, but throwing them in one after one wouldnt be a good idea.

Here are my measurements:
Supply: 5V
CV pin is 3V
Ground connected properly
OUT pin 0V
(hence TRIG is also 0V)
DISCH is about 1,2V

I have a 1nF cap and a 220k resistor, as claimed in the shematic, to set the frequency.

I have made a picture of my breadboard setup. (see attachment)
In the picture the resistor from OUT to the High-Voltage transistors base, was just pulled out at this moment.
I tried 1k 330Ohms and 3,6k resistors, but it did not change anything.
I have checked all connections about 5 times with a multimater, and did check the supply voltages with the oscilloscope (and go throgh the READ BEFORE POST thread)

I hope you experts can save my day.
grz Dan

Are you using a CMOS 555 (like TLC555)?
Have you double-checked all your connections, especially making sure that you have the transistors wired properly?

See this post for voltages to check:
viewtopic.php?f=15&t=3358&start=10&hilit=voltages#p5489

Something is wrong with your circuit, as you shouldn't see current through the inductor with OUT at 0V. Also, if OUT is at 0V, DISCHG should be low as well. I'm guessing you have Q1 in backwards or the transistor is damaged.

woha! Thank you a lot.
I had the wrong datasheet of the high voltage transistor!
It seems to be usual that the base is not in the middle of the TO-92 package with the high voltage transistors - did not know that.
But now it works like a charm! I measure 214V with my multimeter, and I am counting about 32 pulses per minute on my oscillisocope!
I think that should be normal background radiation. Placing the bag of my vaccum cleaner on it, did not change the CPM.
Now i am going to build a nice LED digit display with an PIC

Thx - grz max

Awesome, have fun!

Hello DanSto

I am a beginner.

Can you post a schematic circuit diagram with a PIC microcontroller and source code.

I'm sorry for bad English