Automatic Fragrance Dispenser Circuit


I am building an automatic fragrance dispenser (AFD) from scratch and, because I am a little crazy, I want to build the system myself as well. The idea is to drive a processor (CPU) cooler fan with a passive infrared (PIR) motion sensor. This touch-free, sensor-controlled fragrance dispenser can be supplied directly with a standard 9-V (6F22) battery. This little AFD fans a fresh scent through the air and can be placed on walls, on stands, in hallways, and in public restrooms.

My AFD is comprised of two equally important sections. The first one is the “fragrance engine” — a combination of a standard processor cooling fan and an air freshener. The second one is the “controller” — a passive infrared motion-sensor-based fan switcher built around a handful of inexpensive components. As shown below, the cooling fan is placed on top of the air freshener (thanks to its stiff frame), while the cooler fan cable is extended to the controller circuit. This arrangement ensures clear visibility of the content level without opening the controller box packed with delicate electronic components.


Fragrance Engine Parts (used by me):

• Odonil Jasmine Mist Fragrance Block x1
• AMD CPU Cooler Fan x1

At the heart of the controller is a popular passive infrared motion detector, a type SB0061. The PIR (passive infrared) sensor is a pyroelectric device that detects motion by measuring changes in the infrared levels emitted by surrounding objects. The device contains a special filter called a fresnel lens, which focuses the infrared signals onto the element. As the ambient infrared signals change rapidly, the on-board amplifier wakes up the output to indicate a valid motion. This can be seen as a high-level signal at the output pin.

The output (OUT) of the motion sensor module (PIR) is connected directly to the base of transistor T1 (the sensor module has a suitable bias resistor onboard). When motion is detected, the sensor output goes high to about 3.3 V. During idle conditions, transistor T1 is cut off and the collector output is at a high level. When motion is sensed, the high output from the sensor module saturates transistor T1 and the voltage at its collector drops to activate the driver transistor (T2). Now the cooler fan (FAN) is powered through T2 for a limited duration, determined by the time delay setting of the PIR sensor module (decided by its on-board “delay” preset pot). The red lamp (LED1) is an indicator of the fan activity.


Controller Components (used by me):

  • PIR1 = SB0061 PIR Sensor Module x1
  • T1 = BC547B x1
  • T2 = SK100B x1
  • LED1 = 5-mm Red x1
  • R1,R3 = 1K ¼ w x1 each
  • R2 = 10K ¼ w x1
  • BAT = 9-V/6F22 Battery x1
  • FAN1 = 12-V/180-mA, 80-mm CPU Cooler Fan (part of an AMD CPU Cooler) x1


The standard IEC-6F22 9-V battery has a typical capacity of only 400 mAh. Therefore, to save the battery (and the fragrance block), delay of the motion sensor should be adjusted to its minimum value (~5 s, when the pot is at its CCW position). Also note that in a three-wire fan, the first two wires (black and red) are for the power supply (as usual), and the third wire is an open-collector output from the built-in Hall sensor of the fan. This tacho wire output pulses in tune with the rotation of the fan. It seems that a yellow, green, white, or blue wire is often used as the tacho pulse wire in a three-wire fan.

If you want to build your own AFD featuring an individually programmable fragrance release with motion sensor and timed interval, you can add this ready-made 555 module to the controller electronics. As this takes time and patience, I have not yet tried it, but when I do work on it, I will share my experience.

555 module

Furthermore, I should mention that I don’t exactly recommend using my concept of the fragrance engine since there are many ways to improve it (assuming you have an artistic mind), but hopefully it can serve as a starting point for your own design. I would love to hear from you regarding your experiences with this project or if you need any technical assistance!