Hi Tom, jackrae and Paul,
Thanks very much for the replies! (Edit: Ah, good idea CrossRoads, the system just said your comment came in. I've also added an attachment.)
Tom: I uploaded an awful drawing of the setup to Imgur, which I hope doesn't just make things worse; I also included a picture of the input port to the device, and the USB-to-TTL cable I was thinking of trying as an alternative to the Arduino. USB to BNC question - Album on Imgur
jackrae: The medical device itself (which gives electric shocks on purpose, for background) is all isolated. The concern is more the connection that would let us send a signal from the desktop's USB port, through "some kind of adapter" (which itself might not be safe, even if the shock device is), to the device's TTL trigger input. I think the current coming out of the electrodes connected to the subjects fingers is acceptably secure. Fingers of the same hand, to confirm, no current crossing the chest / heart!
Paul: this is for a psychological fear conditioning experiment. Stimulus-presentation software (Neurobs Presentation) is used to run a task on the desktop computer, subjects sit at that and use the keyboard to respond. That software also has to initiate the trigger to the shock-device so subjects get their shock at the right moments.
The trigger the device needs is a pulse from +3 V to +15 V for at least 5 microseconds, and it responds to the upwards flank. The device is set up to give the desired shock when it detects the trigger.
So if I were to use an Arduino, all it would have to do on each loop would be to read the serial input, check whether some code has arrived, and if so set a pin to HIGH for at least say 15 microseconds. That pin would be connected to the signal-wire of a BNC cable, and the GND pin would be connected to its ground. (I'm afraid I don't have the code for that yet, but I'm fairly confident about that part. I could try to program it already though if it would help. I know I can send codes to the USB port using the stimulus presentation software, I've done that before.)
I also ordered the FTDI USB-to-TTL adapter in any case, but that would still need to be connected to a cut-off BNC cable via jumper cables, which I'm sure will evoke concern. I guess all I can do there is wrap their connection in electrical tape? With that solution it's less a specific Arduino question and more a general "what could conceivably go wrong en route from the USB to TTL connections" question...
Thanks again!
Best,
Tim
USB_to_TTL_question.zip (559 KB)