Not every project needs a microcontroller... My intention was to design an motion detector to turn on the light and a water drinking fountain for my cat as he approaches his feeding place. This positively influences the fountain's pump life time, saves energy, and reduces its often fill-ups with water because of a faster evaporation of the moving water. Such a device can be built with a PIR (Passive Infra-Red) sensor available for about $2. However, to increase its sensitivity one also needs passing Fresnel lens, comparators, and other components, so the project price quickly goes up to $30 and more. However, I discovered by chance an assembled and ready to use PIR module in WalMart for just $6.50. The module is manufactured by Meridian Electric and its part number is PHMO01.
| The PHMO01 module | Front view of the main board | Back view of the main board | ||
The left image above shows how it looks like. The module is supposed to turn on a built-it LED in the dark only, which is achieved by a small photo-cell. In my application, however, the drinking fountain should be turning on during the daytime too. So, I just disconnected the photo cell. The images above with Fresnel lens cap removed demonstrate this. The middle image shows the module PCB, the PIR sensor and the LED. The heart of the unit is the chip BISS0001 (datasheet). Most part of the module's actual circuit goes almost along the lines with the Application Example in the datasheet. The largest difference is the photo cell interface that uses 2 transistors, several resistors, and a diode, and works like a latch. But this part of the circuit is disabled in my case. Also, the output is connected to an LED instead of a relay. The white wire on the right photo is my addition and connects the output of BISS0001 to my board shown below.
| The switch board | The unit in action | Schematic | ||
The module's LED provides a sufficient (for cats) lighting, but it cannot directly handle the water pump of the drinking fountain. This pump in my model works on AC and to switch it on and off I used a solid state relay AQV252GA manufactured by Panasonic. The relay is an opto-pair consisting of a controlling LED and two MOSFETs, which can switch AC or DC up to 2.5A (the water pump draws about 200mA). Although the LED requires just 0.5-3mA in the ON state (about 1mA in my case), I decided to control it with another MOSFET transistor in order not to overload the BISS0001. The logic-level transistor I use for that (RFD14N05 by Fairchild) is way too powerful for turning on just the relay, but I might use it later for switching other devices. The high-state voltage on pin 2 of BISS0001 is 4.5V by powering it from 5V. The remaining components on my PCB are intended to provide 5V to the main board, which eliminates the batteries. The entire unit is powered from the water pump power supply. The secondary board is assembled in the sensor's battery compartment, the back cover has a small hole for incoming wires (not shown on the images). The sensor is mounted on a reverse T-shape wooden construction and is hangs low enough to detect the cat from about 10 feet. Although the light (and the fountain) go up for about 20 seconds according to the datasheet, slight movements of the cat during drinking/eating are sufficient for turning them on again. My first impression is that the cat appreciates the new technology on his property.
Last modified:Mon, Jan 23, 2023.