Geiger Counter Problem

Support forum for the mightyohm.com Geiger Counter.
http://mightyohm.com/geiger
Chronosock
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Re: Geiger Counter Problem

Postby Chronosock » Fri Oct 12, 2012 10:12 pm

Pin 4 remains at battery voltage before and after shorting pins 3 and 8.

TP2 is extremely erratic both with and without the short. I've seen it climb steadily in voltage starting from 0v and more than 100v. I also saw it climb extremely fast from 0v to battery voltage, then reset and climb again. The short didn't seem to change the slow climbing behavior at all.

Would it help at all if I recorded a video of me taking the voltages? I'm sorry that I'm not more electronically inclined, wish I could help more than this.

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mightyohm
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Re: Geiger Counter Problem

Postby mightyohm » Sun Oct 14, 2012 4:32 pm

The fact that pin 4 doesn't change tells me that Q3 and/or Q4 are not switching.

I'm not sure what is going on with your TP2 readings. With U1 removed, you shouldn't see any higher than 3V on TP2. It might jump higher briefly as you are connecting and disconnecting the short across U1 pins 3 and 8, but once you make the connection and take your hands away, the reading should be stable. I recommend using a short piece of thin wire (like a cut-off resistor lead) to make the connection, so that the fingers of the socket will hold it firmly in place. The reading on TP2 should not jump around.

Make sure your meter probes are pressed firmly against TP2 and the ground terminal on the PULSE connector. If you have any small alligator clips, you can use them to make the connections to the meter as well.

It's still possible that you have a bad solder joint somewhere. I would recommend touching up all of the connections to Q1, Q3, and the surrounding components (L1, R8, VR1, R5, U1). Heat up each joint and add a small amount of solder to reflow the connection. Then run the test in my last post again, and see if the voltage on pin 4 changes when you short pins 3 and 8. Make sure the solder completely surrounds the leads for each component.

When you repeat the test, measure the voltage on the left leg of R8 (the side closest to Q3) and record your readings both with and without pin 1-3 shorted. That will help us narrow down whether it's Q1 or Q3 that isn't working (or both).

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mightyohm
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Re: Geiger Counter Problem

Postby mightyohm » Sun Nov 04, 2012 4:09 pm

Any updates? Let me know if there is anything else I can do to help.

Chronosock
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Re: Geiger Counter Problem

Postby Chronosock » Wed Nov 07, 2012 1:05 pm

Yea sorry, been swamped with physics and calculus homework, should be updates sometime this weekend.

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mightyohm
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Re: Geiger Counter Problem

Postby mightyohm » Fri Nov 09, 2012 1:12 am

OK, sounds good.

chadlawson
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Re: Geiger Counter Problem

Postby chadlawson » Wed Feb 08, 2017 1:47 pm

Hi Mightyohm!

I'm hoping you can help me as well. I received and assembled my kit via Adafruit last week, but haven't been able to get it to work. I started my own troubleshooting, but got stuck. Luckily I found this post with anticipated voltages.

Using my Exetech EX330 I came up with the following readings for the pins you listed in this thread:

TP1: 3.0
TP2: 2.785 (I checked the scale a lot, since this should be MUCH higher)
U1: 0
U2: 0
U3: 0
U4: 0.522
U5: 2.0
U6: 0
U7: 0
U8: 3.0

Do you have any idea what this means?

Thanks in advance,

Chad

[quote="mightyohm"]I tested some voltages of the high voltage supply circuit with two digital multimeters: my Fluke 87V with 10 megaohm input impedance, and a cheap Cen-Tech meter with 1 megaohm input impedance (this is typical for cheap digital meters). The input impedance of the meter will affect the high voltage measurement.

All measurements are in volts (V) and with fresh batteries.

Fluke 87V
Using the Fluke 87V, with the ground lead connected to pin 3 of J6 (the PULSE header). Pin 3 is the pin all the way in the upper right corner of the board, closest to the mounting hole for the case.

Battery pack + terminal: 2.9 (should be around 2.9-3.1V)
Battery pack - terminal: 0.0

If you measure between pins 1 and 3 of J6, you should also measure the battery voltage (2.9-3.1v).

More voltages, ground lead of the meter still connected to pin 3 of J6.

TP1: 2.9
TP2: 214V (note how this is lower than 400V, this is because the meter is loading down the voltage, since I did not add a 1 gigaohm resistor in series with the meter)

U1 pin
1: 0.0 (ground)
2: 1.5
3: 0.95
4: 2.8
5: 1.9
6: 0.0 (ground)
7: 0.5
8: 2.9 (same as battery voltage)

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mightyohm
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Re: Geiger Counter Problem

Postby mightyohm » Wed Feb 08, 2017 2:09 pm

Chad, the HV supply is not switching, based on your voltage measurements. Have you checked all solder joints? Is the 555 inserted correctly?

Can you send a photo of the top and bottom of the PCB?

chadlawson
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Re: Geiger Counter Problem

Postby chadlawson » Thu Feb 09, 2017 1:59 pm

I'm pretty confident on the solder joints (to the best of my ability to test) as well as the orientation of the 555, though I could be wrong.

As requested, the top and bottom are attached.

IMG_5344.JPG

IMG_9651.JPG

chadlawson
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Re: Geiger Counter Problem

Postby chadlawson » Thu Feb 09, 2017 2:00 pm

I'm going to feel pretty stupid when you find the problem in an instant. But at least I'll have an answer. :-)

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mightyohm
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Re: Geiger Counter Problem

Postby mightyohm » Thu Feb 09, 2017 10:58 pm

Your kit looks great!

A few of your solder joints could use touchup, however I think this is unlikely to solve the problem. Nevertheless it's worth touching them up to be sure.

Some pins of the 555 are not fully covered in soldered. Using a clean iron, heat up each pin and add a bit more solder. Then wait 2 seconds before removing the iron, to give the new solder time to flow around the lead.

Here they are:
2017-02-09_22-43-39.png


It's hard to tell, but some of the other pins look like they might not be smooth. It's hard to tell if there's a divot or a shadow. If the solder joint is not shiny and smooth, you can usually fix it by adding a tiny amount of solder and reflowing the joint.

These joints could use some touchup as well.
2017-02-09_22-51-02.png


There might be others, but it's hard to tell in the photo. Take a close look.

There's a guide to soldering here that might help: http://mightyohm.com/blog/2011/04/soldering-is-easy-comic-book/

Back to troubleshooting. Looking at your voltages, the issue seems to be that pin 4 of the 555 is low (close to 0V) which is holding the chip in reset. Pin 4 should only be low if Q3 is turned on, which normally only happens when Q1 is on and the output (pin 3) of the 555 is high (close to 3V). However, in your measurements, pin 3 is also low. One possibility is that Q3 is misbehaving. This can happen if:

  • Maybe Q1 and Q3 are swapped. Check the numbers on each transistor. Q1 should be marked J3303. Q3 should be marked 2N 3904.
  • Q3 might be bad. I've seen this happen before, but usually the kit works for a while and then something or someone shorts high voltage to Q3 and it fails.

Check the voltage from the base (center lead) of Q3 to ground. It should be low (close to 0V). If it's high, that points to a potential issue with Q1.

Let me know what you find. And touch up those solder joints!


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