EC-33 Geiger Computer Interface USB
3
Place the Windows distribution CD in the CD-ROM drive, and select CD-ROM
drive when the following dialog appears. If the Windows distribution CD is not
available, it is possible that the driver is in the default location suggested by the
“Specify Location” selection:
Click Next when the following dialog appears:
If all is well, Windows reports back with the following dialog box. If the
“STATUS”
light is still red, try disconnecting, then reconnecting the interface. If
the LED does not turn yellow within a few seconds, see Troubleshooting.
6.
With Windows 98/ME, this procedure may need to
be repeated for every USB port on the computer. It
would be worthwhile repeating step 5 for each port
now.
7.
The
“COUNTS”
LED should flash whenever the GM
tube detects an ionizing particle. If the rear panel
switch is on, there will be a short audible beep for
every detected event as well. This verifies operation
of the tube and Interface.
Installation of the EC-33 Geiger Computer Interface is now complete. See the
software manual, (Windows or Macintosh), for details on software installation
and operation.
Hardware Description
The EC-33 connects to a computer via its USB port. It is a low speed device,
compliant with Version 1.1 of the USB specification. It will operate with any
USB port that meets version 1.0 or newer of the USB specification as defined by
the USB Implementers Forum. Power is drawn directly from the USB port, with
a maximum of 250 mA.
The Interface contains a regulated switching power supply to generate the high
voltage required for Geiger-Müeller tubes. Two fixed voltage presets provide
outputs of 500 Vdc and 900 Vdc, adequate for most GM tubes. A third setting
allows continuous adjustment of voltage from 0-1200 Vdc via a front panel
control.
Counts generated by the Geiger tube are generally on the order of tens of
microseconds. The pulse widths are stretched to 20 ms, so that the counts LED
is visible to the human eye. This stretched pulse controls the audible
transducer as well.
Counts from the tube are collected by the on-board microcontroller during 20
ms intervals, then sent back to the computer. The microcontroller clock is
synchronized with the USB timing, which in turn is controlled by a crystal inside
the host computer.
An 8-bit integer received by the computer represents the counts collected during
the measurement interval. Note that the limit for an 8 bit integer is 255. This
limit is more than sufficient, as the counting rate required to exceed this
