Geiger counter Circuit, does this look like it'll work?

I want to measure beta and gamma radiation levels remotely, so I want to make a simple Geiger counter that outputs a signal the arduino can read and then log in an SD card. First I step up the voltage to 400v using a re-purposed disposable camera circuit, then apply the positive voltage to the anode of the tube. When a particle ionizes the gas inside the tube, a very small, very short tick of current flows. About 2 micro amps for about 2 nano seconds, from what i hear. I wont have the ability to measure that. Then the little bit of current switches a transistor, which then uses the trigger function of a 555 to make the pulse a bit longer and at a much lower voltage so that the arduino can see it on an input, and then count it and store the data. Does that sound like it would work? Can you tell me what things i got wrong in my schematic?

Here’s my schematic:

D1 is backwards and R6 is too small for most Geiger tubes. You don't need the 555 to produce a signal that the Arduino can read. Most tubes require between 500 and 900 V to operate. There are probably other problems.

jremington: D1 is backwards and R6 is too small for most Geiger tubes. You don't need the 555 to produce a signal that the Arduino can read. Most tubes require between 500 and 900 V to operate. There are probably other problems.

I know the operating voltages of my tubes are 380-450v. I suspected that the diode was backwards when i was looking at it, but the other schematic i saw had it like that so I figured there was a reason for it. If it's a mistake i'll fix that. I just looked at the recommended resistor value and it's 5.1 meg, so ill fix R6. Do i really not need the 555? I figured, the current is already down as low as 2 micro amps and the pulse is so short that i would need it. Should I plug the back end strait into the analog input and pull down to ground? Or should i keep everything until after the 2N2222 and switch pin 2 on the 555 to an input on the arduino?

Here is a simple Geiger tube power supply/interface circuit that I’ve been using for years (and has been copied many times) http://forum.pololu.com/viewtopic.php?f=6&t=1453&p=39331

Did you know that it is not easy to measure the high voltage ? As soon as you connect a voltage meter (with 10M impedance) to the high voltage, that voltage will drop. After removing the voltage meter, the voltage goes up again, and you will never know that higher voltage.

The circuit mentioned by jremington is okay. That is enough to trigger an interrupt. However, the 555 to generate the pulses is not needed, the Arduino can generate frequencies for the coil or transformer as well.

Yes, you need a 100 Megohm input impedance voltmeter to measure the high voltage.

I made my own "multiplier probe" by putting a string of 9 x 10 Meg resistors inside a tube. This creates a 10:1 voltage divider, together with the 10 Meg input resistance of the multimeter.

100M might still lower the voltage with 50V or so. The best way is perhaps using a fixed voltage divider in the circuit (and use that to measure the voltage with the Arduino).

The voltage drop depends on the output impedance of the power supply, which for the 555 circuit I used, is not so high that 100 Megs constitutes a significant load. This is rather easy to determine.

The best way is perhaps using a fixed voltage divider in the circuit (and use that to measure the voltage with the Arduino).

What voltage divider would you propose, given the impedance limitations of the Arduino ADC input?

Perhaps 50M and 47k, using the internal reference of 1.1V and a capacitor (1nF to 10nF) parallel to the 47k.
The range will be 1000V, and the accuracy a few volts. That is more than enough accuracy to get the best voltage for a tube.

Perhaps 50M and 47k, using the internal reference of 1.1V and a capacitor (1nF to 10nF) parallel to the 47k.

Why do you think that the 50 Meg divider would be better than a 100 Meg DVM? Obviously, the 50 Meg divider would be twice the load on a high impedance power supply, leading to twice the voltage drop.

The 50M divider would be permanent in the circuit to measure the voltage as it is.

You use a capacitively coupled sensitive charge-summing amplifier I believe for this sort of thing. A high voltage capacitor means you only have to detect changes in voltage across the load resistor (which could be quite large if the resistor is large, say 1M). By having a lower frequency response than 2ns the pulse will be smeared out in time (but reduced in amplitude).

Summing the charge onto another capacitor means you can measure it at your leisure rather than having to be watching just as the pulse comes in, note. Arrange each pulse to produce a step change in voltage on the sampling cap.

Alternatively amplify the pulses and send them into a logic counter chip.

Sensing the current pulses is easier at the cathode as in jremington's circuit, but that's too easy!

I'm sure there are some good discussions of this sort of circuit in "The Art of Electronics", the bible for many circuit designers.

I like the idea of storing the pulses in the capacitor. I found this UCLA paper on what I think is what you're talking about http://www.seas.ucla.edu/brweb/teaching/AIC_Ch12.pdf But what do i do after I read the cap? Should i trigger a mosfet to short it? That seems like the logical method to me. Currently im having issues with measuring voltages at the analog inputs. I'm running the reference at 1.1v since my batteries are less than 5v. But the formula for turning the 8 bit number into a voltage is off by about 0.1v at the top of the battery but about accurate on the bottom half scaling this formula for higher voltages will cause grater errors, so i need a new one. In my circuit i'm attempting to measure at the cathode. Do I need to measure at the anode to store a pulse in the capacitor? The way i see it, i can use the 555 the way it is to produce regular and consistent blocks of voltage to add to the capacitor. Or should i just measure the voltage across a high voltage cap being directly filled by the pulses? I might just go with the logic counter if it gets too complicated with the capacitor voltage measuring

In reply #3 jremington gave that link. That circuit detection is good. The high voltage with two 10nF, a 1M and 4M7, and the 22k and 300pF at the cathode. That will work. I have an extra capacitor of 1nF at the output to GND (in the schematic labeled as INT0) and a diode to protect the base of Q4 for negative voltages. I have a normal BC547 for Q4.

In my sketch, I have a "attachInterrupt (0, isr_geiger, FALLING)"

jremington: Why do you think that the 50 Meg divider would be better than a 100 Meg DVM? Obviously, the 50 Meg divider would be twice the load on a high impedance power supply, leading to twice the voltage drop.

Its quite possible to get an accurate reading from a dmm, you know the impedences involved, just correct with kichoffs laws like we used to do when avos only had 20kilohms per volt