I built this few days ago for #iceboxchallenge. Tested for few days and it worked well in my old smelly fridge. I tried others material like coffee, lemon, and tea leaves with limited results , I need to put a lot of such material in the fridge with large container to take effect. To reduce the deodoriser size, active charcoal or baking soda works more effective if air flow slowly through it several minutes, and several times per day. I also tried passive device with a container for charcoal or baking soda without fan, it need a lot of vent holes, and bigger container. Thats why I end up with this final design with just a small palm size container, and it can be fixed into everyone's ice box without any fridge modification. It can be easily manufacture and put it on web store for sale. Here's how it works: The bottom section contains 4XAA rechargeable battery and also simple circuit using 555 timer. the center red color container fill with active charcoal, or a paper cup of baking soda. ( baking soda last for about 1 month, and charcoal for several months) I used BBQ charcoal here. The top portion of the design consist of a fan, and a LDR sensor. When the fridge door is open, light shine on LDR and switch on the timer. When fridge door closed, the fan keep spinning for about 5 minutes to circulate air into the container slowly , switch off after 5 minutes, and stand by for next light shine on the LDR sensor when door is open again. Although a programmed timer without LDR will work well too, I choose LDR sensor because it seems more easier to build. Although charcoal or baking soda can work as deodoriser without the slow spinning fan with timer, the fan version works much more effectively by slowly suck surrounding air in the fridge into the container for deodorise process. I used 12V 40mm fan ( rated 0.2A, ) and since I supply it with 4.8V - 6V , it spin slowly. High speed fan can't be used here, not only because of drain out battery faster, the fast moving air seems too fast for the charcoal or baking soda take effect on pass-by air. I used light sensor just because of the fan actually need to operate just a few times per day, for few minutes each time, to circulate the air into the container, and once the fan turns slowly and air blow out from the device, the fridge circulation fan will bring all deodorised air and circulate in the fridge. The installed fan orientation must be inhale from top of the container and blowing down to charcoal or baking soda. I measured the fan current and it draw only 22mA , and less than 0.2mA in stand-by mode if I use CMOS 555 such as TC555, normal NE555 can be used but standby current will be around 3mA. Sanyo Enerloop rechargeable battery recommended here, for lower self discharge rate, or any other brand of low self-discharge battery can be used, and according to calculation, the 4 AA battery will last for about 2 months. Each time when the fridge door opened, the small LED light will turn on immediately, if its not turn on, thats means the battery run flat already. The preset in the schematic was for LDR light sensitivity adjustment, I used fixed resistor of 560k ohm and it works just nice for my fridge . update: I included an accessories STL files- baking soda container + lid. If user choose baking soda as odor absorbing material, user can pour baking soda into this container, close the lid and put the container inside the deodoriser. If charcoal as absorbing material, no need to use this accessories container, can place directly into the deodoriser. I just wish to remind that , only red, yellow, green led works here, blue and white LED can't be used because of higher voltage drop. The LED act as pilot light, and also voltage limit for the transistor, when each AA battery voltage drop to around 0.9V , the LED and also fan circuit stop functioning, so that we will never over drained our rechargeable battery. Print top, bottom , and the middle container, install electronics and battery ( I used double sided tape to secure battery holder. ) on bottom section, and install 40mm fan and LDR on top cover. wire can be pass through the center container . LED - use any LED ,red one work best, green and yellow works too, blue and white not working. LDR- RS component # 596-141 LMC555 CMOS timer - RS component # 534-4002 SS8050 transistor RS component # 739-0307 Fan - 40cm 12V rated 0.14~0.2A should work nicely. the rest of parts such as battery holder, PCB etc should be easy to get in most of electronics hobby shop.