Using FET transistors as switches

When over a year ago I published this post about using transistors as switches, I described only how to do it with bipolar transistors. And bipolar (NPN and PNP) transistors have a small problem: they are current driven, so they consume current when they are switched on. They consume much less than they drive, but still…

Field Effect Transistors (FET), and especially the ones from MOSFET family, work in a slightly different way. Instead being current driven, they are voltage driven. Than means, to conduct between Source and Drain terminals, specific voltage has to be applied to Gate terminal. More than that, FET transistor consumes current only during switching. It works kind of like a capacitor. When Gate is charged, it stops conducting electricity.

And that, in case of low voltages and low currents (3.3V – 5V logic level and few hundred miliamps), removes the requirement of Gate resistor. Small MOSFETs like 2N7000, BS250 and other, can be directly connected to microcontroller outputs. Cool!

Switch with N-channel MOSFET

N-channel MOSFET is ON when positive voltage (comparing to Source) is applied to Gate. So, Load will be powered when HIGH state is applied.

N-channel MOSFET as switch

Switch with P-channel MOSFET

P-channel MOSFET is ON when negative voltage (comparing to Source) is applied to Gate. So, Load will be powered when LOW state is applied. It acts like an inverter.

N-channel MOSFET as switch

Gate resistor

In case of bigger currents and voltages, gate resistor might be required since FET gate acts like a capacitor and passes electricity until charged. Consult transistor data sheet.

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FrSky SmartPort Telemetry on AnyFC F7

STM32F7 CPU family present in newest, experimental, flight controllers like AnyFC F7 (as well as upcoming AnyFC M7 with smaller STM32F722) simplifies many things. For example, comparing to F4 boards, SmartPort or S.Bus connection is extremely simple and can be done on any free UART. No more hardware hacks, external inverters and other “special” ways of doing things.

It’s super simple again, and here is how to do it in Betaflight (Cleanflight 2.x) and INAV

Hardware

The only required hardware is a cable to connect SmartPort enabled receiver with free UART port on F7 board. This will work on X8R, X6R, X4R, X4RSB, XSR and any other. The trick is to connect S.Port pin with UART TX pin only.

How to connect F7 flight controller to SmartPort

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Talking Taranis – teach FrSky Taranis to talk to you

One of the most important features of modern, computerized, radios is that you can make them talk to you. After all, with setup telemetry link from UAV, radio should “know” things. Things like battery voltage for example. Why not make FrSky Taranis (or Horus or Taranis X Q7) talk to you and report LiPo voltage in a smart way?

Before we proceed, you have to setup SmartPort telemetry or “legacy” FrSky telemetry. This post does not covers this topic. You might want to take a look here and here.

Let’s go

In OpenTX menu navigate to last page called Telemetry and check if VFAS is reporting proper value.

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Tutorial: replace battery in RunCam HD

My RunCam HD died after 5 months in a drawer. Reason: battery destroyed. It had enough energy for approx. 2 seconds of operation. So I had a choice: buy original battery from RunCam for $9.99 plus shipment, or fix it DIY style. I’ve choces the second option.

Luckily for us, RunCam used generic 803035 LiPo 1S, 3,7V battery that can be purchased in specialized stores. I’ve paid around $6 for mine, but if you look hard enough, you should find something even cheaper. Also, luckily (or not) there is a battery plug: 2 pin JST 1.25mm.

Runcam HD inside

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How to connect APM Power Meter to Cleanflight and INAV

This topic was eluding me for some time now. It’s time fix the problem and finally present a short tutorial how to connect 90A APM Power Meter for flight controller boards like Naza32, SP Racing F3 or any other running Cleanflight / Betaflight / INAV software and equipped with Current Meter ADC input.

I will not show where to connect APM Power Meter to flight controller, since this differs from board to board. Some boards have dedicated pins, on some boards PWM input pins are used for Current Meter ADC. You have to refer FC documentation and / or flight controller software documentation.

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Programming ESP8266 with Arduino IDE

One of the best things about ESP8266 ESP-01 WiFi modules is that they can be programmed
using popular and well known Arduino IDE and act as stand alone board with WiFi
capabilities. Thanks to ESP8266 group process of integrating ESP8266 and Arduino IDE
is pretty simple.

First step is to add http://arduino.esp8266.com/stable/package_esp8266com_index.json to Additional Boards Manager URLs in Configuration in Arduino IDE.

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