February 2017 11
When we do this as a matrix, we end up reverse
biasing LEDs as well. Therefore, they must have a reverse
voltage that is at least 5V. It’s surprisingly hard to find specs
on the reverse voltage for modern LEDs. There are hand
waving mentions that it’s at least 5V, but probably higher.
Datasheets do not list this characteristic. In this case, all I
can do is hand wave too and say, it’s probably just fine at
As for steady operation with the LED on, that should
be no problem with 20 mA being supplied. Most LEDs
can take even more current than that, but it depends on
the individual LED. If you think that current is too high,
you can, of course, add a separate current-limiting resistor.
The resistor value will depend on the desired current, the
driver’s capabilities, and the forward voltage.
Text Messaging When in Need
QI am disabled with MS and spinal stenosis, and I fall a lot. I bought a remote control FOB with a receiver that will give me a real contact closure. My goal is to find/construct a device that can
text messages to my neighbors when I need help.
AMark, I’m really sorry to hear about your condition and falling. That must be very difficult. Thinking about a system like this, I think it’s a nontrivial project involving
something like an Internet-connected Raspberry Pi and
some kind of client that will send the text message over a
TCP connection to a service provider.
While this would make a great construction article, it’s
probably beyond the scope of Q&A. Generally, I try to give
solutions to specific questions and provide some of the
theory behind what I’m doing. Writing up the design on an
entire project is what other writers for Nuts & Volts are so
good at. (And one of those authors could be any one of
you who read this column.)
That aside, however, I’m hesitant to advise you to do
this on your own. This is almost a medical device, and
reliability is a key aspect of medical devices. I would hate
to think of you trying to use such a homemade device
and having it fail to work properly just when you needed
it most. As it turns out, there are many services out there
that will do this for you. At one point, I had such a device
for my mother before she moved into assisted living. They
work very well and have reasonable range. NV