As a result of a recent operation, I'm under orders from my doctor to keep my arm low, ie not to raise it above shoulder level.

It sounds like an easy request to remember - you're very aware of it when you do things like putting on a shirt - but there are other times when it slips your mind, such as for example when you feel like stretching your arms when you yawn!

So here's my solution to stop unwitting accidents...

Step 1: What You'll Need

Anti-dozing driver alarm aka nap alert (under $3)
Velcro (pennies)

The device we're going to use is a driver nap alert which is designed to fit over the ear and buzz if you nod your head forwards too much.

Step 2: Attach to Shoulder

Simply velcro the nap alarm to your arm so that it triggers when you raise your arm to the maximum allowed height. Note the orientation from the attached photograph - it's important that the alarm is facing the correct direction to trigger as it is rotated upwards. (The picture depicts the left arm - the one that would be affected if you've just had a pacemaker fitted) - you'll need to velcro the other side if you're wearing this on the right arm for some reason.)

Attach a square of velcro against the larger part of the device, and a thin strip to the ear piece so that it is attached to your clothing at two places, otherwise it is liable to flop over...

Note that this is just a 1-axis tilt switch, not a 2-axis accelerometer, so it might not alert if you extend your arm sideways, only if you raise it in front of you. But having been using this for a day or two now, it does actually work pretty well. The only time it has alerted by accident was when I was reclining in a Barcalounger, and it was easy enough to just switch off during that time.

The device is indeed buzzing as this picture was being taken.