Implementing PoE is made interesting by the fact that not every Ethernet device wants power; if you start dumping power onto any device that’s connected, you’re going to break things. The IEEE 802.3af standard states that the device which can source power should detect the presence of the device receiving power, before negotiating the power level. Only once this process is complete can the power sourcing device give its full supply. Of course, this requires the burden of smarts, meaning that there are many cheap devices available which simply send power regardless of what’s plugged in (passive PoE).
[Jason Gin] has taken an old, cheap passive PoE splitter and upgraded it to be 802.3af compatible (an active device). The splitter was designed to be paired with a passive injector and therefore did not work with Jason’s active 802.3at infrastructure.
The brain of the upgrade is a TI TPS2378 Powered Device controller, which does the power negotiation. It sits on one of two new boards, with a rudimentary heatsink provided by some solar cell tab wire. The second board comprises the power interface, and consists of dual Schottky bridges as well a 58-volt TVS diode to deal with any voltage spikes due to cable inductance. The Ethernet transformer shown in the diagram above was salvaged from a dead Macbook and, after some enamel scraping and fiddly soldering, it was fit for purpose. For a deeper dive on Ethernet transformers and their hacked capabilities, [Jenny List] wrote a piece specifically focusing on Raspberry Pi hardware.
[Jason]’s modifications were able to fit in the original box, and the device successfully integrated with his 802.3at setup. We love [Jason]’s work and have previously written about his eMMC adventures, repairing windows tablets and explaining the intricacies of SD card interfacing.
People talk about active and passive components like they are two distinct classes of electronic parts. When sourcing components on a BOM, you have the passives, which are the little things that are cheaper than a dime a dozen, and then the rest that make up the bulk of the cost. Diodes and transistors definitely fall into the cheap little things category, but aren’t necessarily passive components, so what IS the difference?
Continue reading “Active Discussion About Passive Components”
Modern smart keys allow you to keep the key fob in your pocket or purse while you simply grab the handle and tug the door open. [Phil] decided he would rather ditch the fob altogether and instead implemented a passive Bluetooth keyless entry system with his Android phone. It’s probably unlikely for car manufacturers to embrace phone-based keys anytime soon, and [Phil] acknowledges that his prototype poses a landslide of challenges. What he’s built, however, looks rather enticing. If the car and phone are paired via Bluetooth, the doors unlock. Walk out of range and the car automatically locks when the connection drops.
His build uses an Arduino Mega with a BlueSMiRF Silver Bluetooth board that actively searches for his phone and initiates a connection if in range. Doors are unlocked directly through a 2-channel relay module, and an LED indicator inside the vehicle tells the status of the system. A pulsing light indicates it’s searching for the phone, while a solid ring means that a connection is established.
We hope [Phil] will implement additional features so we can make our pockets a bit lighter. Watch a video demonstration of his prototype after the break, then check out the flood of car-related hacks we’ve featured around here recently: the OpenXC interface that adds a smart brake light, or the Motobrain, which gives you Bluetooth control over auxiliary electrical systems.
Continue reading “Passive Bluetooth keyless entry system”
Turns out you don’t need to be Superman to see through walls. Researchers at University College London have developed a way to passively use WiFi as a radar system. Unlike active radar systems (which themselves send out radio waves and listen for them to echo back), passive radar systems cannot be detected.
The system is small enough to fit in a briefcase, and has been tested through a one-foot-thick brick wall. It can detect position, speed, and direction of a person moving on the other side of that wall, but cannot detect stationary object. [Karl Woodbridge] and [Kevin Chetty], the engineers behind the prototype, think it can be refined to pick up motion as minuscule as a person’s rib cage moving with each breath. For some reason we get the picture in our mind of that body scanner from the original Total Recall.
[jefffolly] published some straight forward plans for a passive volume control. It uses a resistive ladder built across the contacts of 12W rotary switches. Each resistor provides a 5dB difference, and he recommends using 0.1% tolerance resistors to maintain accuracy. The use of discrete resistors instead of volume pots means that the output is much more predictable. All of the RCA sockets were connected using oxygen-free copper wire.
This came in on the tipline: [Ville ‘Willek’ Kyrö] wanted to build a fully passively cooled computer. That means no fans at all. He started with scrap aluminum heatsinks, ripped apart a cpu heatsink to get the copper heat pipes, and began surrounding the boards with heatsinks to form a case. Cooling down the powersupply was the hardest part, as it did not lend itself to the flat surfaces of heatsinks. Any passive case with powerful components will inevitably be huge and heavy; this one weighs over 20 kg. He says, “It might not have been worth it, but it sure was weird watching the computer boot up with no sound at all”.