Solid State Relay Details


Senior Member
I would like to develop a power control system for an unattended house monitor using picaxe and simplelan where it is possible that the power will be out for a long time--days or weeks--while the cable providing internet access may be available. I plan to use a UPS system (550VA/330W) to keep the picaxe, cable modem, and router running, but the cable modem and router draw about 13 watts and could draw down the UPS if left on all the time. In the event the UPS signals that power has been withdrawn, I would like to use the picaxe to shut off power to the cable modem and router except for a brief period once a day when an update email would be sent via the simplelan.

I would like to use the Sharp S101S02 Solid State Relay ($3.95 from Jameco), switched on with a BC547 transistor, to control the power to the cable modem and router (always on when there is power to the UPS, and only off when the power has been off for, say, 5 minutes). It is possible that there would need to be two SSRs, with the cable modem turned on first, and then the router a minute or two later.

Is this a reasonable way to do this? Is there a better way? How should the SSR be packaged for greatest safety?

    |                                    |
    o--9V DC wall wart--regulated 5V--picaxe----------------------.
    |                                     |                       |
    |                                   BC547C transistor         |
    |                                     |                       |
    o------------------------------------SSR----o---Cable modem   |
                                                |     =           |


New Member
UPS systems are terribly inefficient at driving low loads...

You might be better off using a linear regulator and 12V gell cell to supply power to the cable modem and router.

With any luck, both devices use the same voltage power adaptor, hence you only need one power rail to control through a standard relay. With any more luck, they might be 12V each, but I am betting they are 7.5V.


I'd go along with Haxby's suggestion.
The UPS could well draw 1/2A totally off-load. Running direct from the 12v would make switching on/off much easier as well. Could just be a simple FET.
I have a couple of similar scenarios where I've opted for latching relays.
If your devices require 7.5v, it might be worth while investigating the use of a switching regulator to go from 12v to 7.5v but even a linear regulator at those voltages will almost certainly give better efficiency than an off-the-shelf UPS.


Senior Member
in addition if you use a latching relay it only takes a pulse to change and hold the new state... and of course a switching down converter helps a lot as well as a photovoltaic panel to top off the battery... I think I've seen some very low power Internet connect modules too but I don't recall where. The October '07 Home Power magazine has a bunch of companies listed who monitor solar systems to the Internet... and there's wind power and if you have flowing water anywhere [hoepfully intentionally!] you can extract power from that...


Senior Member
Thanks to Haxby and BeanieBots for your suggestions--that sounds like it would give me a much longer runtime.

premelec--the SimpleLan internet connect module requires only 130ma max for less than a minute a day (for this use), so I'm not worried about its draw. I don't think it can so easily be connected directly to the cable modem, since I had to set up the router to tell the cable modem that its mac address was the same as my laptop's--but it would be nice to be able to take the router out of the picture--perhaps a SimpleLan firmware update would do it, if such a feature could be added. Solar topoff would be possible, but I think it would be possible to get a couple of weeks out of the arrangement suggested by BB & Hx--if the mains power would be off for much longer than that, I think the surrounding community would be in a heap of trouble.


Senior Member
Cant you get the ISP to accept the simplelan MAC address ?
For now I dont think you can fake another one with simplelan.

Fo safety you can use those USB save power power plugs, it take 5V on the usb connector (cut it off and theres 2 wires) and then switches the 4-6outputs on.

About 20USD here anyway, and no homemade mains usage, although it draws another 25mA per USB box for the relay.

The UPS will run itself dry in less than a day, but maybe you can power the router and modem directly, guess they all use 5-12V, so a car batt could last for a while ?
You could then always power them that way, and make a circuit that will switch between mains and battery when mains is lost (a relay plus a few components could work, when mains is lost relay falls to NC).


Senior Member
mora99--I doubt the ISP will be flexible about allowing the mac address of the laptop which configured the cable modem to be changed--they only "support" a direct connection to a single PC, but "allow" connection to a router if it looks like they're still connected to the PC.

Can you explain about the "USB save power power plugs"--I'm not familiar with them?

I'm no longer at the location in question, but my cable modem and router here both have 12-volt wall warts--1 amp and 750ma. Starting up those once a day or so off of a 12-volt battery under picaxe control in the event of mains failure would seem to be the solution, with a UPS to handle the brief glitches in mains power.


Senior Member
mora99--I doubt the ISP will be flexible about allowing the mac address of the laptop which configured the cable modem to be changed
That might depend on the ISP / modem, so could be worth asking. My ISP allows swapping to PC's with different MAC's but used not to.


Senior Member
Hmm, couldnt find much on google on the usb strip, maybe its not used so many places (yet)?

Heres a picture
Basically its a (5V) relay with the protection diodes, combined with a regular power strip.
So when the computer is on (5V on the usb cable) the power strip is on, and when the computer is off, the power strip is off.

Used to put all your printers, mise, speaker, and so on, so they dont draw standby current.


Senior Member
USB-relay power strip looks useful. I'd need one for North American power, but can't find anything like this.