Cryptanalyse Your Air Con

Infrared remote controls are simple and ubiquitous. Emulating them with the aid of a microcontroller is a common project that hackers use to control equipment as diverse as televisions, cable boxes, and home stereos. Some air conditioners can be a little more complicated, however, but [Ken]’s here to help.

The root of the problem is that the air conditioner remote was using a non-obvious checksum to verify if commands received were valid. To determine the function generating the checksum, [Ken] decided to bust out the tools of differential cryptanalysis. This involves carefully varying the input to a cryptographic function and comparing it to the differences in the output.

With 35 signals collected from the remote, a program was written to find input data that varied by just one bit. The checksum outputs were then compared to eventually put together the checksum function.

[Ken] notes that the function may not be 100% accurate, as they’re only using a limited sample of data in which not all the bytes change significantly. However, it shows that a methodical approach is valuable when approaching such projects.

Thirsty for more checksum-busting action? Check out this hacked weather station.

Measuring Airflow in an HVAC System

[Nubmian] wrote in to share his experiments with measuring airflow in an HVAC system. His first video deals with using with ultrasonic sensors. He found an interesting white paper that described measuring airflow with a single-path acoustic transit time flow meter. The question was, could he get the same effects with off-the-shelf components?

[Nubmian] created a rig using a pair of typical ultrasonic distance sensors. He detached the two transducers from the front of the PCB. The transducers were then extended on wires, with the “send” capsules together pointing at the “receive” capsules. [Nubmian] set the transducers up in a PVC pipe and blew air into it with a fan.

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Custom Cut Pinwheel Makes a Useful HVAC Duct Flow Meter

Everyone is familiar with pinwheels, and few of us haven’t crafted one from a square of paper, a stick, and a pin. Pinwheels are pretty optimized from a design standpoint, and are so cheap and easy to build that putting a pinwheel to work as an HVAC duct flow meter seems like a great idea.

Great in theory, perhaps, but as [ItMightBeWorse] found out, a homemade pinwheel is far from an ideal anemometer. His experiments in air duct flow measurements, which previously delved into ultrasonic flow measurement, led him to try mechanical means. That calls for some kind of turbine producing a signal proportional to air flow, but a first attempt at using a computer fan with brushless DC motor failed when a gentle airflow couldn’t overcome the drag introduced by the rotor magnets. But a simple pinwheel, custom cut from patterns scaled down from a toy, proved to be just the thing. A reflective optosensor counts revolutions as the turbine spins in an HVAC duct, and with a little calibration the rig produces good results. The limitations are obvious: duct turbulence, flimsy construction, and poor bearings. But for a quick and dirty measurement, it’s not bad.

Looking for an outdoor anemometer rather than an HVAC flow meter? We’ve got one made from an old electric motor, or a crazy-accurate ultrasonic unit.

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Measuring Air Flow with Ultrasonic Sensors

Measuring air flow in an HVAC duct can be a tricky business. Paddle wheel and turbine flow meters introduce not only resistance but maintenance issue due to accumulated dust and debris. Being able to measure ducted airflow cheaply and non-intrusively, like with this ultrasonic flow meter, could be a big deal for DIY projects and the trades in general.

The principle behind the sensor [ItMightBeWorse] is working on is nothing new. He discovered a paper from 2015 that describes the method that measures the change in time-of-flight of an ultrasonic pulse across a moving stream of air in a duct. It’s another one of those “Why didn’t I think of that?” things that makes perfect sense in theory, but takes some engineering to turn into a functional sensor. [ItMightBeWorse] is using readily available HC-SR04 sensor boards and has already done a proof-of-concept build. He’s getting real numbers back and getting close to a sensor that will go into an HVAC automation project. The video below shows his progress to date and hints at a follow-up video with more results soon.

Here’s wishing [ItMightBeWorse] the best of luck with his build. But if things go sideways, he might look to our post-mortem of a failed magnetic flow meter for inspiration.

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Heat Pump Gets Brain Transplant; Such is Life in Latvia

If you buy a used heat pump that was made in China and try to use it in Northern Europe, there are bound to be issues. If said heat pump ends up encased in a block of ice that renders it ineffective, you’ve got two choices: give up and buy a proper heater, or hack a new ice-busting brain board into the heat pump and get back to life.

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A little too icy.

[Evalds] chose the latter course, obviously, and in the process he gives us a pretty good look at how heat pumps work and how to overcome their deficiencies. In [Evalds]’ Latvia, winters can be both cold and humid, which can worsen an inherent problem with air-coupled heat pumps: they tend to ice up. As the outside coil is cooled to pick up as much heat as possible from the outside air, water vapor condenses out on the coils and freezes. Most heat pumps account for this by occasionally running in reverse, heating the outdoor coils to clear the ice buildup. [Evalds]’  had nothing more than a simple timer to kick off the defrost cycle, and it wasn’t keeping up with the Latvian winter. An Arduino replaced the OEM controller, and wired up to temperature sensors and an IR sensor that watches for ice buildup on the lower part of the coil, the heat pump is now much better behaved.

Of course it wasn’t as smooth as all that — [Evalds] has some hoops to jump through, including EMI problems and a dodgy Arduino clone. But he stuck with it and brought the heat pump back online, likely at far less expense than HVAC techs would charge for a service call.

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Hackerspace Takes Fume Extraction Seriously

At first we laughed at the ridiculously over-the-top fume extraction system this hackerspace built for itself. Then we thought about seriously questionable donation rolls of solder some of the members managed to find and bring in. The kind of roll where the local greybeard assures you that a Californian State Trooper has permission to shoot you if you try to take it into the state, but damn does it solder well. They may be onto something is all we’re saying. But on a serious note, for a communal space like this one, a great air quality plan makes the place a lot more pleasant, if not safer at the same time.

The build uses a regular boost fan for its main suction and pulls the fumes out to a place the members aren’t. Knowing hackerspaces that could be anything from an empty alley to vents on the building’s roof. It’s actually an interesting challenge to solve in a rented space (please share your own solutions for “daylighting” to the outside in the comments).

The frame is made from ducting and dryer hose. Since there aren’t really fittings for this. Most of the joints were designed in OpenSCAD and 3D printed. At each end of the tube a computer fan provides another little boost of airflow. We like the stands to position each end of the hose at the fume source. All of it is powered by a distribution box of their own making with the juice being fed with repurposed Ethernet cables to the fans at the ends of the hose.

It’s a nice build and likely extended the life of a few of the more electronically active members in the space. Especially if the retired radio enthusiasts decide to do their fifty year anniversary garage cleaning and gift upon the space their findings.

Arduino Replaces Bad AC Thermostat, Hacker Stays Cool

Most of North America has been locked in a record-setting heat wave for the last two weeks, and cheap window AC units are flying out of the local big-box stores. Not all of these discount units undergo rigorous QC before sailing across the Pacific, though, and a few wonky thermostats are sure to get through. But with a little sweat-equity you can fix it with this Arduino thermostat and temperature display.

We’ll stipulate that an Arduino may be overkill for this application and that microcontrollers don’t belong in every project. But if it’s what you’ve got on hand, and you’re sick of waking up in a pool of sweat, then it’s a perfectly acceptable solution. It looks like [Engineering Nonsense] got lucky and had a unit with a low-current power switch, allowing him to use a small relay to control the AC. The control algorithm is simple enough – accept a setpoint from an encoder, read the temperature sensor, and turn the AC on or off accordingly. Setpoint and current temperature are displayed on an OLED screen. One improvement we’d suggest is adding a three-minute delay between power cycles like the faceplate of the AC states.

This project bears some resemblance to this Arduino-controlled AC, but it seems more hackish to us. And that’s a good thing – hackers have to keep cool somehow.

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