Questions

[atomic.dave]

Can you tell me what the board dimension is? I want to order this kit, and I want to figure out the case for it too at the same time. Also, does this have LCD display capabilities?

[mightyohm]

The PCB is 4.75 x 2.75 inches.

There will be a case for sale with the kit in the very near future. I'm putting the finishing touches on it now. More details soon...

There are no provisions for an LCD display built into the kit. To read data from the counter, you'll need a computer and a 3.3V USB-Serial cable.

[atomic.dave]

Oh, cool about the case. I look forward to seeing it. How soon are we talking?

Will this work to connect to my mac and arduino?
Sparkfun FTDI Cable 5V
sku: DEV-09718

Or is this better to use?
Sparkfun FTDI Basic Breakout - 5V
sku: DEV-10008

Ok, so the serial jumpers are for logging, the ICSP is for programming the chip, but what would you do with the pulse output? maybe another piezo? or speaker?

[mightyohm]

The case will be released as soon as I can take pictures and write up assembly instructions - hopefully by Monday.

You'll need a 3.3V USB-Serial cable, like the FTDI TTL-232R:
http://www.adafruit.com/products/70

The pulse output can be used to trigger all kinds of other circuits - you could use it to connect the counter to an Arduino or an external LED or beeper.

[atomic.dave]

Great thanks.

Two last questions:

Do you think this display will work with your kit?
http://www.imagesco.com/kits/dmad.html

Will an LND7317 alpha pancake tube work with this kit?

[mightyohm]

atomic.dave -

That display might work. They don't provide any information on their website about whether their "TTL pulse" is 3.3V or 5V (I'm going to assume 5V) or how long the pulse is. Even if the display expects a 5V pulse it might trigger off 3V (as long as the batteries in the counter are alkaline and brand new). It really depends on their circuit, and since they don't provide schematics or specs I can't really say. It would be worth a shot (I don't think you'll blow anything up trying).

The LND7317 is a 500V tube, which is about 100V higher than the nominal voltage of the SBM-20. The high voltage supply will probably operate fine up there, but I don't have an LND7317 to test with so I can't be sure. Try it out and report back!

[will1384]

I found two pages that might be useful.

Universal Expansion Module schematics
http://www.imagesco.com/articles/dmad/d ... r-pg5.html

DMAD Adaptor page
http://www.imagesco.com/articles/dmad/d ... r-pg2.html

This page says it uses a 3.5 mm plug, and +5V TTL pulse

[malte]

Hi,

These questions are for me just to understand and learn.

1)

Listen for clicks and watch the LED. SLOWLY rotate VR1 clockwise until you hear clicks. Once you start to hear clicks, rotate VR1 about 45 degrees more and stop.

Where do the 45 degrees come from?


2)

Wouldn't it be more energy efficient to use a resonator with lower frequency (like for example 1 MHz) instead of the 8 MHz provided? Was the choice made with hackability in mind, so that even logistically demanding expansions could be done

[mightyohm]

Hi,

These questions are for me just to understand and learn.

1)

Listen for clicks and watch the LED. SLOWLY rotate VR1 clockwise until you hear clicks. Once you start to hear clicks, rotate VR1 about 45 degrees more and stop.

Where do the 45 degrees come from?

The idea is to bias the geiger tube in the "plateau" region of operation. This region is very broad on the SBM-20, so the exact position on the dial is not critical. The desired voltage is around 400V. With fresh batteries this corresponds to VR1 being adjusted somewhere between 1/4 and 1/2 way up on the dial.

2)

Wouldn't it be more energy efficient to use a resonator with lower frequency (like for example 1 MHz) instead of the 8 MHz provided? Was the choice made with hackability in mind, so that even logistically demanding expansions could be done

Yes, a lower clock frequency would be more efficient. However, 1MHz will cause problems with the serial interface, see this page:

http://www.wormfood.net/avrbaudcalc.php

I picked 8MHz because the resonator was readily available and allows operation of the UART at 9600 baud. If the clock frequency is too low, there is a possibility that the AVR will "miss" some of the geiger pulses, but I have not investigated this.