Howards Astronomical Adventures

: Tales From The Night Sky

GMN Camera Rebuild

The standard build for a GMN camera uses a POE cable integrating a small buck converter molded into the cable at the RJ45 jack. This drops the power over Ethernet (POE) voltage from about 48vdc down to 12vdc. I have found these integrated POE cables are unreliable; I have now experienced two failures. The latest being February 4th, just forty days since being replaced. From my reading this is the most common part to fail in a GMN camera setup. Not wanting to go through this again I have rebuilt my camera to use an internal POE power board.

This is what the camera board with a POE board looks like while being bench tested:

IMX291 Camera module and POE converter board

This particular board I ordered (AliExpress: Shucheng Electronics Co) is well made with nice clean soldering. What is interesting is this type of separate POE board use a different design circuitry: a flyback topology. My research suggests the board I am using is built around a Silan SD4932B DC/DC controller chip. This is a guess as my example board carries a HUNUK chip part number but the board is actually silk screened with SD4932B name. It appears many of the AliExpress vendors sell boards using this flyback topology. Compare that to the integrated POE cable built using a simple buck converter design with a Techcode TD1670 chip (or similar). I don’t know the details well enough to debate the design decisions but for those looking to learn the basic difference there is an excellent TI video comparing the various topologies.

For my GMN camera I use a recycled analog security camera housing. These old Speco Technology cameras have a superb aluminum body with o-ring seals. The assembled internal parts of my camera are pictured below. Pity I have a limited supply of these as they are obsolete, but are a superior build compared to the model recommended (but readily available). They do have a smaller internal volume compared to the recommended housing. You will not put a Raspberry Pi 4 inside them! The one difficult part of using them is feeding the network cable through the housing base plate. To do that I have to remove the connector(s) from the network cable, feed the wires through, and then reassemble the connector(s). This is a bit tricky to avoid damaging wire connectors so hopefully this new board will last several years.

Bench testing rebuilt camera assembly.

While I understand the basic GMN camera build is easy to assemble using integrated POE converter cables the GMN online documentation should feature an optional section outlining other methods of powering it. Like using a separate power board. This is a bit more complex as one has to figure out the correct wiring. As opposed to just plugging in the current recommended POE cable. However I would recommend anyone deploying remote cameras not easily serviced avoid the integrated buck converter POE cables. Either feed 12v directly into the camera cable (bypassing the converter) or use the separate power board design as I have. The standard GMN camera housing is plenty big enough to house the second power power board just needing four standoffs and nuts.

After assembling my camera I added one other change: a vinyl mask over the outside of the front housing glass. A 60mm diameter circle was cut with a 30mm diameter center hole. The goal was to cut down on reflections from stray light sources (usually headlights do to my location). The original Speco design had something similar but used an internal coating on the front glass. This seems to have worked very well so far.