Oscillating voltage (0-400mV) from a closed relay

[analogo]

My MCU controls a 5 V Songle relay. The input to the relay is a 9 V battery. The NO output is connected to a LED, the NC output is disconnected. Everything works fine, but I have noticed something peculiar (peculiar only to a beginner like me, probably).

If I measure the voltate from NO and ground while the coil is unpowered, I get an oscillating voltage. It starts from 0 V and ramps up to 400 mV. Then it drops down to 0 V. A sawtooth shaped waveform. If the coil has been energized and de-energized in the last 5 minutes then the voltage will go up to 400 mV in about 5 seconds. After 30 minutes it need 4 minutes to go up to 400 mV.

What causes this oscillation?

Is it normal? Is there a capacitor inside the relay? Has it to do with the coil?

[T3sl4co1l]

Could it be that your meter changes from "Hi-Z" mode to 10Mohm mode when it switches range around 400mV?

Tim

[MK14]

LEDs can make small amounts of electricity when they are in light. A bit like some photo devices, such as solar cells.
Does covering the LED stop it from happening ?

http://sciencewithkids.com/Experiments/Energy-Electricity-Experiments/LED-generate-electricity-experiment.html

Turn on the multimeter and set the dial to read DC voltage. It is the V symbol. Read the voltage value produced by normal room light. Now move the breadboard over to a window or take it outside and watch the voltage values increase!.