Electronics and science

Digital ultrasonic transmitter and receiver

Information can be transmitted by cable, infrared, telephone, modulating an acoustic tone, radio waves… Today I am going to describe a digital ultrasonic communication system. Some of you may think it is a vulgar and unoriginal circuit. But you like schemes with operational, I know, I see it in the blog stats. I encourage you to read it.

In this article, we will first describe analogically how transmitter and receiver work. Then, in the digital realm, we will program the transmitter’s firmware. As for the receiver, we will first make a dedicated software and finally, we will get into the operating system with some more advanced explanations.

We will use ultrasonic transducers at 40kHz like the ones you can find in these modules to measure distances. I haven’t found the exact reference, but they look a lot like the ones in the 400ST160 datasheet.

Describing an unknown protocol

Imagine you are facing a rock wall and want to climb it. You look for some indication and consult the guide, but no one has done it before. No one tells you how to start or where to go, you just have to watch carefully and find someplace to hold on to go up slowly.

This is the process of analysis in any investigation, from scientific to police. Sometimes you have no idea where it’s going to lead you. You’re just looking for clues and patterns and not knowing if you’re walking in the right direction. Over time, some pieces fit together and you find a foothold that allows you to certify the work done so far. From there you keep going up.

In the previous entry AFSK Demodulate, from scratch, we explain how to demodulate and interpret a known protocol. In this one, I invite you to investigate an unknown signal. We’ll deduct the layers from the radio carrier to the text messages to find out the origin.

There are Universal Radio Hacker type programs, designed to facilitate this work. They will save the professional a lot of tedious work. Since we are not professionals, we will do it in an artisanal way without resorting to specialized software.

Demodulate AFSK, from scratch

Today I want to talk to you about FSK modulation. Sound too specific? It is possible… What if I told you that we were going to talk about radio, retro-computing, trains, sound, mathematics, electronics, and programming sounds better? I guess so. And if you’re looking for nothing else to read. There are very good articles about something else on the Internet.

As I was saying, let’s try FSK modulation, for my taste most pages are too superficial or too mathematical, or they stay in the description or get lost in details without getting to spiral the whole process.

Phone adapter. Frame by Wargames (1983). Source.
These aren’t telecom notes, but believe me: it’s not a light entry you can read while you’re drinking your coffee either. It is a practical demonstration of how digital information began to be transmitted. From start to finish. No formulas, hardly any hardware and no more software than the one we are going to write.

Evaporative Coolers – Psychrometry

Today I wanted to talk to you about an appliance, let’s say intermediate between fans and air conditioners: evaporative coolers. In their portable version, they don’t have as much tradition as domestic refrigeration although you will find it together with the portable air conditioning devices -penguins-, with the claim of being much cheaper, more ecological, lighter, have a lower consumption and do not need installation or hot air exhaust ducts.

But does it cool? Is it worth it? Does it replace an air conditioner? Well, short answers: Yes… it depends and… no. The long answers you have in this article.

First, we are going to introduce some basic psychrometric concepts to use them later. Then we will see the parts of an evaporative and the principle of operation. We will connect an HTU21D sensor to measure the temperature and humidity at the output of the device. And, for dessert, we’ll do some performance tests under different conditions.

Chelsea R. Mansfield

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