The hole is 25mm and 10cm from the bottom of the box.
Internal dimensions are about 12cm by 10cm.
The roof is covered with some old bits of lead flashing and the sides are waterproofed with bits of roofing felt. A small camera is mounted in the top of the box and illumination provided by six infrared LEDS. The cable can be seen going off to the right of the box.
There is an infra red beam across the hole which is broken every time a bird passes through it. This is detected by a small electronic assembly underneath and a tone is transmitted to a receiver indoors. This in turn is monitored by a Python program running on a Raspberry Pi computer.
Each time the beam is broken, the Raspberry Pi records the time in a text file. The name of the file is the current date. So at the end of the year I should have a file for each day, each with a record of every time a bird passed through the hole! This assumes of course that a bluetit starts using the box. 🙂
This is a view of the hole from the inside of the box.
The slots are cut to hold aluminium tubes which will contain the infrared led and the photodiode.
The tubes protect the components and shield them from the infrared light coming from the camera in the top of the box.Another view inside the box with the aluminium tubes in position.
The vertical tubes carry the wires from the infrared LED and photodiode to the underside of the box. Previous observations have told me that bluetits peck the inside of the box and the tubes protect the wiring. The wood under the hole is roughed up because I read that this helps the young get to the hole when its time to fly the nest.
For more detail: Bluetit Monitor