Serial to Relay need help please

[sgear1]

Hi

I hope someone can help me with this

I am tray to make Screen saver and when the Screen saver start I send command to the Serial port
and open the Relay and then the phone don't ring when I am not in work or not sitting with my desk


this is plan how I want to make this
I thing I will use 5V adapter from mobile phone to power this

what I need help with is to make this safe so I don't burn my computer or my desk
when the relay stay open over the night



Regards

S.S.G

 

[Boomslang]

You can also use the 5V from the USB port of your PC to connect to the relay. Then let your PC go to "Sleep mode" after X amount of minutes, this will switch the USB and your relay off (you will also save some electricity in the process).

 

[sgear1]

Yes I was thing to use this in that way use the 5V from the USB and let the Relay stay open when I am working
and close when the screen saver go on,
then it is no power on the Relay when I go home and the landline don't work until I use my computer day after,

but is it not risky to use USB to power this for 8 or 12 hours over the day and is it change the 5V go into the serial if something happend?

 

[Boomslang]

No, it is not dangerous as long as your relay draws less than 100ma@5V. A transistor might also do the job here, but I am not sure what kind of V and A is on those telephone lines.
The USB port also has protection built in.

 

[sgear1]

The landline is 45V DC when your phone is off but when the phone ring it go up to 90V DC
it can not go from the Relay into the Computer ! well I hope hehe

 

[Boomslang]

No it can't, except if you get some catastrophic relay failure. You can also use opto-couplers if you are very worried about it.

 

[rezer]

The landline is 45V DC when your phone is off but when the phone ring it go up to 90V DC
it can not go from the Relay into the Computer ! well I hope hehe

The ringing voltage is actually 90-130Vac at 20Hz.