Raspberry Pi Forums

Hey guys, I'm having an interesting/annoying problem with a baby monitor. When I plug my cell phone in to charge, everything sounds fine, the baby monitor has no problems. The moment my wife plugs in her charger though everything starts buzzing. What's interesting is that my cell phone charge is actually connected to the same physical outlet and her's is connected to a different one (same circuit though).

This is also true for her computer charger. Any time she plugs in her laptop we start getting a buzz off the monitor on the same circuit.

To me, this sound like I'm getting some noise created by the cell phone/laptop chargers that's dirtying up the AC line. I'm guessing the AC->DC converter attached to the baby monitor isn't doing a good job at smoothing out the DC line.

Does anyone know what I might be able to do to fix this short of buying a new AC->DC plug for the baby monitors or cell phone or laptop chargers?

EMI noise can be tricky to figure out, but I would start the trouble shooting by trying each charger one at a time to find out if it is the charger that is noisy. The noise could be at any frequency, but my guess is that it is at hundreds of MHz so it is radiating using the wiring and charge cables as antenna.

if the baby monitor is picking up the noise though its cable you could try to use different length of cable if possible. another way is to destroy the antenna by adding a dampening ferrite bead clamp around the cable. you see these black cylindrical ferrite beads on many USB cables and power cables that come with electronic equipment. these are used as a last minute fix to get the product out to the market while it does not quite meet the radiated emissions regulations yet. later products that are improved will not ship with these clamps.

in this case where the problem happens with several chargers it is likely that it is the baby monitor that is sensitive to noise. but it may also be that the chargers are really emitting too much noise. if the noise is high frequency the ferrite bead clamps may work. if it is lower frequency that travels along cables you could try to add an EMI filter to the mains outlet that you plug the monitor into. the filter contains common mode inductor, series inductor and EMI capacitors that block most of the noise coming along the cable. you can buy these filters in many sizes, and you may have to mount the filter in a metal box and add an ac outlet. the metal box needs to be connected to protective ground, so you must use a grounded outlet. The outlet in the box does not need to be grounded, but it is important to add a fuse or circuit breaker in the box that matches the current rating of the EMI filter. so you don get a fire if you plug in a much larger load to the filter box.