S-Parameter Test Set
for the SDR-Kits VNWA, page 3

August 2016:

Programming the ATTiny45 got off to a faltering start. I’ve programmed PICs in the past but this little 8-pin chip posed a real problem. In theory a few wires from a PC Parallel port would work … but didn’t. I asked around if anyone could provide me with a pre-programmed chip. Then Stephen, G0XAR suggested I use an Atmel USBASP programmer to do the job. This in conjunction with a tiny board with an 8-pin socket and header proved to be a very cost effective solution. See below.

It looks like the most popular application for programming these devices is AVRDude. This is a command-line driven interface and so may not initially appear very user-friendly. However after looking at the GUI-based options, I opted to go for the steam-driven, Command-Line approach; and with a bit of help from this website I was able to program the ATTiny45 in a matter of minutes. The only issue that I encountered was due to the fact that the chip was not running an external crystal clock and was thus running much slower that the programmer. This issue is covered on the tutorial website.

Once programmed, the chip was soldered onto my USB-Switch PCB. See below.

A tiny DC-DC converter sourced on Ebay is used to provide the +28V for the transfer relay. Unlike Peter, DC2PD, I wasn’t able to  use the 5V from the PC’s USB port … even with a large reservoir capacitor … for two reasons. Firstly, my transfer relay draws 650mA when asserted. This is far to high for any USB port to deliver. Secondly, because the initial state of the relay is essentially unknown, it is possible that the controller will attempt to assert the relay the moment the software is run … in which case, any reservoir capacitor will fail to charge. So I opted to provide an external power source.

Note that the relay is actually too big for the box that I used! The control connector pokes out through a hole at the rear; hence the control lines from the PCB exit the box through a grommet and mini header sockets are used to interface directly with the pins in the Amphenol connector … seriously less expensive than buying the mating line-socket!!