I wanted a smaller PIR module than the usual HC-SR501 type for a project.
The HC-SR505 appeared to be ideal because it is would not waste much space on a PCB and they are easily available cheaply (less than £1 each).
I after waiting about a month for them to arrive from China, I received five modules and began testing them.
The first module tested didn’t work very well. It kept triggering even with the sensor covered over!
The other four worked okay. Although they can easily be triggered by changes in room temperature or by any switching noise on the power supply.
It is definitely not as reliable as the type I usually use (HC-SR501). So I decided to investigate why.
Specification and data sheet information about the module is very limited, some websites provide basic specifications and some also provide a circuit diagram – but is actually a diagram for the HC-SR501 type which uses a different chip.
As the 8-pin chip used in this module is unmarked, I decided to trace the PCB circuit to help identify it.
It seems to correspond very well with the manufacturers application circuit for an EG4001 PIR controller.
The EG4001 is an 8-pin chip made by EG Micro and the datasheet is only available in Chinese.
A google translation of the main features are:
- 8-pin infrared pyroelectric chip, the external circuit is simple, low cost.
- Static power consumption is small, 3V power supply when the power is less than 45uA, 5V power supply when the power consumption is less than 75uA, ideal for battery-powered systems.
- High input impedance operational amplifier, with a variety of sensor matching, signal and processing.
- Two-way discriminator, can effectively suppress interference.
- Built-in reference voltage for internal comparator and operational amplifier reference voltage.
- Built-in delay time timer and block time timer, change the oscillator frequency to set the delay time.
- Low external components, only need to configure the gain of the first op amp and RC oscillator components that can work reliably.
- Working power supply + 2.3V ~ + 6V
- Package: SOP8 and DIP8
Here is the basic circuit of the module:
I had to guess the value of some SMD capacitors as they aren’t marked.
Note that the board has a polarity protection diode in series with the + supply connection but a polarised 22uF capacitor is connected BEFORE the diode and is therefore not protected from any accidental reverse voltage supply and could be destroyed if that ever happens!
The circuit is much simpler than that used for the HC-SR501 and although it has a 3.3v regulator, it does not have as much smoothing for the supply to the PIR sensor. This could be why it is more susceptible to false triggering?
The HC-SR505 mini PIR module is acceptable for basic movement detection and available very cheaply but is susceptible to false triggering.
In my search for a better small PIR module I have now ordered a new type which uses an AM312 sensor that has all the PIR digital processing built-in. I will review it when it arrives…