[HH] cheapest/simplest way to control a relay from a PC

[Tom Metro]

Jack Coats wrote:
> http://www.fullcompass.com/product/366255.html?utm_source=googleps&utm_medium=shopping&utm_campaign=googleps&gclid=CKSL0re7xLMCFao7MgodIWcA7Q
> -- Interesting device for controlling and monitoring 2 audio lines in
> and out.  USB controlled and powered for $30

This is a USB sound card, much like hundreds of others, although it
claims to use 48-bit converters, which is a bit different.

In any case, not particularly relevant to the question at hand.


> http://procontrol.com/products/pro-relay-4/ -- professional looking, 4
> relay, ethernet or IR interface, POE or 1A 12V external power, no
> price on page at first glance

Looks like it is aimed at high-end home automation and home theater
integrators. Probably not priced to be hacker friendly.


> http://www.sigma-shop.com/product/7/usb-relay-controller-one-channel-box.html?&CurrencyID=1
> -- 1 relay, USB controlled, nicely packaged

Agreed, nicely packaged. USB-B connector on one side. 3 screw terminals
for form-C relay contacts on the other. No relay specs provided.
Emulates a serial port. $30.


> http://numato.com/2-channel-usb-relay-module -- 2 relay, USB
> controlled, board with exposed components assembled

No case, but you get two relays here. The down side to this one is that
they require you attach a 12V supply (not included) to power the relays.
(Discussion thread below the product info indicates the vendor is
working on a version that is fully USB powered.)

The product description says the relays can handle 10 A, but the manual
cautions that for anything over 2 A they recommend you thicken up the
PCB traces to the relay contacts with solder. They might have well just
used low-current reed relays, and kept it USB powered.

Nowhere do they mention what the voltage rating is on the relay. While
most relays will handle 120 V, some don't have adequate contact spacing,
so you can't just assume. I have a feeling they left off this spec to
avoid any liability for inadequate high voltage isolation and lack of UL
approval. (All of this applies to the other relay product above, as well.)

This device also emulates a serial port. The command language they
designed seems rather clumsy, but adequate.

The manual says, "On board ICSP header makes it easy to be used as a
development system as well." What's an ICSP header? (Aside from being a
6-pin header?) Ah...In-circuit serial programming:
http://en.wikipedia.org/wiki/In_Circuit_Serial_Programming_%28ICSP%29

Not sure how that applies to this device. There's no indication that you
can reprogram the micro on the board. Are they using it like an I2C
interface? Based on the Wikipedia page, it looks like it is only used
for programming, so I guess the intent is that hackers can customize the
firmware.


Thanks for digging these up. I have a bookmark folder full of serial and
USB controlled relay products. Others include:

http://www.j-works.com/jsb315.php
http://canakit.amazonwebstore.com/CanaKit-4Port-USB-Relay-Controller-with/M/B004JWW1GQ.htm

They're both more expensive than the ones you found.


> Now $30 isn't cheap (under $2 is in my book), but these options are
> not bad for $30 equipment.

Agreed.


> Tell us what you choose, and how it works for you in your application.

I'm still in the mood to build something, so I'm pursuing that avenue.
But I've saved the above for future reference.

I found one of my solderless breadboards that I haven't used in a long
time, and the parts with it - a DB25 with some 22 AWG solid wire leads
soldered on, and some optocouplers - suggests my last project with it
was another parallel port interfacing project.


> I use an earlier version of http://www.digital-loggers.com/lpc.html
> that cost about $130.

Yes, you wrote about that on the list before as part of the PC watchdog
thread. I never did get the Trip Lite UPS with built-in watchdog reverse
engineered. The device I was planning to use it on worked with adequate
reliability on a simple timer that forced a daily power cycle, so I
didn't have adequate motivate to spend the time on it. Maybe someday, as
it would be nice to have a $40 UPS that includes a watchdog that's
supported by Linux.

 -Tom