I am using an 8.5 V DC power supply from my home telephone system to supply an Arduino Uno board by VIn. Now, I get some noise on the telephone, probably by interference from the Arduino back into the phone system. Does anybody have an easy circuit to suppress those interferences to put between the phone system and the Arduino?
It is called a filter. I would at the very least put a large (10uF) and a small (1 to 10nF) caps on the input. Otherwise you need to figure out the frequency range of noise, and implement an appropriate filter. It sounds like you just need a low pass filter, but a standard RC one won't respond fast enough (and the appropriate sized R would probably limit the current too much). Wikipedia has some decent articles on this.
When I pick up the phone there is noise in the line. I can hear it. I just connect to Arduino VIn and GND to the DC power supply of the home telephone system (it's an old analogue home system by the company "Quante" of Wuppertal, not available anymore)
Digital circuits, especially CMOS, put a lot of switching noise on the power and ground lines.
Install the 10uF and 0.1uF capacitors right at the Arduino end of the 8.5V line.
cjdelphi's solution is much more straightforward. Other than power, are there any other connections between the Arduino and your phone system? It is also possible that radiated energy is getting into the phone.
Grumpy_Mike:
As well as the capacitors you need a seriese inductor. Add caps each side if the inductor. It should be as big a value as you can get hold of.
This is what I was suggesting as well, but at the very least, add the small and large caps on the power input of the Arduino. That alone I imagine will drop down the noise by a lot.
but to no avail. Then I noticed that my FritzBox WLAN Router is connected by ethernet to the Arduino with GND connection over the ethernet cable. So most probably I have something like a ground loop between the WLAN Router, Arduino and phone system. I now use the wall power supply of the WLAN router and have no more noise on the phone line.