So you have a little electronics skill. Or maybe not, but it might be worth it to learn. Why not automate your setup a little? There are plans out there which range in complexity. If the do it yourself way is not for you, there are also several products sold for a variety of purposes, which can be used to automate a terrarium. First one must figure out if doing so would be worthwhile doing. Though some automation solutions may not be cost-effective for some situations, in other situations is desirable to have this kind of accuracy in controlling an environment, for example, for species requiring very precise conditions good
The problem has been encountered before, of how to automatically control appliances (such as heaters and humidifiers) to keep a terrarium within a desirable range of temperature/humidity. Homemade devices have been seen in terrarium setups in the past, including ones that operate independently and just control the appliances, to devices that interface with a computer or rely on a computer for operation. The standalone types typically use a microcontroller, while using a computer to read sensors or control outlets may be cheaper to do.
The biggest issue a mycologist might run into is finding software to run one of these homemade devices.
There are a few commercial products out there already, and they may be found through the threads linked in this section. Most of them are targeted towards horticulture, but some can be made to work with ideal conditions for growing mushrooms.
A problem with some hygrometers is that above a certain point, they lose their accuracy. It is important to keep this in mind when choosing a hygrometer/humidity sensor.