is there a cheap device to test level of humidity ...

Yes, you can get a 3in1 soil meter prob very cheap at a number of places. I got one a few years back on Amazon for $10. It’s just 2 long metal probes that are stuck down into the soil with a top reading piece and usually give you a reading for PH, your moisture level and light levels. Or you can stick your finger in the soil an inch or so and if it’s dry it’s time to water. Also if you can pick up your plant you can tell by the weight of it dry and wet. The finger in the soil method does disturb the top root zone though, and the pick up method requires the plant to be able to be moved, but the probe you can put in and leave. There cheap enough you can buy 1 for each plant pot to leave in and they also give you your soil ph levels as a bonus. The light level is not really helpful unless you want to know the light intensity at you soil level for some reason...lol Hope this helps. Cheers 💨