Vicks Humidifier Modification: Adding a Humidity Controller with Hysteresis

One winter night, I woke up and my room was very foggy. I checked my Sunbeam humidity monitor and the humidity is around 80! I immediately turned off my Vicks humidifer (which I bought from Walmart). We all know that molds are the number 1 enemy in a very humid room. Since now, it is summer, I was able to do some modification and a bit of reverse engineering.

First, I opened the Vicks Humidifier and checked all the circuit inside and I did my first prototype and sensor characterization on an Arduino Mega 2560.

Unmodified circuit

Arduino Mega Prototype with DHT-11

Next,

Next, I modified a couple of things inside the Vicks humidifier. I only have 24V DC on the board and I planned on using an Arduino Mini. I checked Arduino Mini's schematic and it uses MIC5205 regulator. From MIC5205's datasheet, normal operation for Vin is from 2.5V to 16V. Since I don't have any regulator in my hand, I just created a 12V power supply using P2N2222A in emitter follower configuration with voltage divider in it. I used this 12V to power my Arduino Mini.

Next, the float switch needs to connect 24V to certain part of a circuit. I modified this and since I don't like to use a relay in this scenario and I want this be have a lower power consumption, I created a PNP/NPN combo to deliver the needed 24V on a part of Vick's circuit and this triggers the ultrasonic mist generator and the internal fan.

Almost Complete

Now it is working. I implemented a hysteresis from 50% to 60% of humidity.

Arduino Mini Mounted

I removed the scent filter since I think it is not that useful and it just consumes additional power. The DHT-11 humidity sensor goes out via the scent filter.