⚠️WARNING: do not touch components connected directly to the AC mains. This post is for educational purposes only, and I am not responsible for the correctness or safety of the information given here!
Many producers of so-called air fresheners, sell the device at cost price or maybe even at loss. The business model is similar to computer printers (many years ago), where they lose money when they sell you the printer but get this money back when they sell the ink.
Most are simply a plug connected to a PTC heat element. Recently, I bought a unit (P&G’s 3volution) for 1.95€ in which three bottles can be plugged and the device turns each one of them for 45 Minutes, the idea is as stated in the product description is to make you notice that smell, because once you are used to a certain smell for long time you won’t notice it!
I searched for a teardown of this product and found one on Youtube, however the PCB looks very different. The version I bought contains an unlabeled microcontroller with SOP 8 package.
The PCB has components on both sides. One side for SMD components and the other for through-hole. The plastic casing states the input as 230v at 50Hz, and this is important as we will see later.
As usual for cheap appliances, the circuit is not isolated so there is no transformer. One would expected the usual capacitor dropper to be used, but instead a Zener diode is used as a voltage regulator. The unknown MCU seems to be working at 5 Volts.
The AC input is rectified with two forward-biased diodes 1N4007 on the TH side and M7 (=1N4007 in SMD package) on the SMD side, diodes in series should enhance the voltage blockage in high-voltage applications (hyperlink).
An RLC filter which utilizes a microstrip inductor (on the PCB) conditions the signal for the Zener regulator which in turn drops the voltage to 5 V.
The pins of the microcontroller indicate that it is a PIC-like (clone…), which could be the now famous 3-Cent processor from Padauk, possible the PMC150. If this assumption is correct, the the clock must be throttled 37 kHz or below in order for the Zener regulator to work properly. If we assume the “ILRC” is selected which is the internal RC oscillator of the microprocessor with frequency of 37 kHz, then the current must be at 18 microA, so the MCU load can be modelled as a resistor of about 300 kOhm. Given the requirements of the product, this frequency looks more than enough 🙂 .
The final part is the power switch transistors with an LED for each PTC element. Each of them is connected to a GPIO of the microcontroller, from which they are switched on and off.
Two of the pins are used as an input and connected to a 3-state switch to select intensity (low, medium and high). They intensities would simply determine for how long (within the 45 Min.) would the heat element be ON.
The last pin is connected to the AC line through two in series resistors R9 and R8 each of 502 Mohm, which seems to be used as zero crossing detector to then be configured to interrupt and used as an accurate clock for the timer.
LTSpice file of the schematics can be found here.
Would it make sense to add Bluetooth functionality in order to control the time of each PTC is turned on?