Electronics

Inductive reactance online calculator & Python code

Use this online calculator to compute the reactance of an inductor in Ω at a specific frequency given its inductance.

Also see Capacitive reactance online calculator & Python code

TechOverflow calculators:
You can enter values with SI suffixes like 12.2m (equivalent to 0.012) or 14k (14000) or 32u (0.000032).
The results are calculated while you type and shown directly below the calculator, so there is no need to press return or click on a Calculate button. Just make sure that all inputs are green by entering valid values.

H

Hz

Formula:

X_L = 2\pi fL

Python code:

The preferred way is to use UliEngineering’s UliEngineering.Electronics.Reactance.inductive_reactance:

from UliEngineering.Electronics.Reactance import *
# You can either pass strings like "150 uH" or values like 150e-6

inductive_reactance("150 uH", "10 MHz") # returns 9424.77796076938

Or get a human-readable value:

from UliEngineering.Electronics.Reactance import *
from UliEngineering.EngineerIO import *

# Compute value as a string
xc = auto_format(inductive_reactance, "150 uH", "10 MHz") # "9.42 kΩ"

# ... or print directly
auto_print(inductive_reactance, "150 uH", "10 MHz") # prints "9.42 kΩ"

In case you can’t use UliEngineering and you want to do it manually, here’s a minimal example:

import math
def inductive_reactance(f, l):
    """Compute the inductive reactance"""
    return 2*math.pi*f*l
Posted by Uli Köhler in Calculators, Electronics, Python

Capacitive reactance online calculator & Python code

Use this online calculator to compute the reactance of a capacitor in Ω at a specific frequency given its capacitance.

Also see Inductive reactance online calculator & Python code

TechOverflow calculators:
You can enter values with SI suffixes like 12.2m (equivalent to 0.012) or 14k (14000) or 32u (0.000032).
The results are calculated while you type and shown directly below the calculator, so there is no need to press return or click on a Calculate button. Just make sure that all inputs are green by entering valid values.

F

Hz

Formula:

X_C = \frac{1}{2\pi fC}

Python code:

The preferred way is to use UliEngineering’s UliEngineering.Electronics.Reactance.capacitive_reactance:

from UliEngineering.Electronics.Reactance import *
# You can either pass strings like "150 pF" or values like 150e-12

capacitive_reactance("150 pF", "10 MHz") # returns 106.1032953945969

Or get a human-readable value:

from UliEngineering.Electronics.Reactance import *
from UliEngineering.EngineerIO import *

# Compute value as a string
xc = auto_format(capacitive_reactance, "150 pF", "10 MHz") # "106 Ω"

# ... or print directly
auto_print(capacitive_reactance, "150 pF", "10 MHz") # prints "106 Ω"

In case you can’t use UliEngineering and you want to do it manually, here’s a minimal example:

import math
def capacitive_reactance(f, c):
    """Compute the capacitive reactance"""
    return 1./(2*math.pi*f*c)
Posted by Uli Köhler in Calculators, Electronics, Python

Volts to dBµV online calculator & Python code

Use this online calculator to convert a voltage in Volts to a voltage in dBµV.

Also see dBµV to Volts online calculator & Python code

TechOverflow calculators:
You can enter values with SI suffixes like 12.2m (equivalent to 0.012) or 14k (14000) or 32u (0.000032).
The results are calculated while you type and shown directly below the calculator, so there is no need to press return or click on a Calculate button. Just make sure that all inputs are green by entering valid values.

V

Formula:

U_{\text{dBµV}} = \frac{20\cdot\log(1\,000\,000 \cdot U_V)}{ \log(2) + \log(5)}

Python code:

import math
def volts_to_dbuv(v):
    """Convert a voltage in volts to a voltage in dBµV"""
    return (20*math.log(1e6 * v))/(math.log(2) + math.log(5))

 

Posted by Uli Köhler in Calculators, Electronics

dBµV to Volts online calculator & Python code

Use this online calculator to convert a voltage in dBµV to a voltage in Volts.

Also see Volts to dBµV online calculator & Python code

TechOverflow calculators:
You can enter values with SI suffixes like 12.2m (equivalent to 0.012) or 14k (14000) or 32u (0.000032).
The results are calculated while you type and shown directly below the calculator, so there is no need to press return or click on a Calculate button. Just make sure that all inputs are green by entering valid values.

dBµV

Formula:

U_{\text{Volts}} = \frac{10^{\frac{U_{dBµV}}{20}}}{1\,000\,000 \frac{V}{µV}}

Python code:

def dbuv_to_volts(dbuv):
    """Convert a voltage in dBµV to a voltage in volts"""
    return (10**(dbuv/20.))/1e6

 

Posted by Uli Köhler in Calculators, Electronics

How to install picamraw using pip

First try installing it normally:

sudo pip3 install picamraw

In case that fails with this error message (like for me):

Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple
Collecting picamraw
Could not install packages due to an EnvironmentError: 404 Client Error: Not Found for url: https://www.piwheels.org/simple/picamraw/

download it and install it manually: Copy the link of the most recent .whl file from https://pypi.org/project/picamraw/#files, download it using wget and install it using pip3, e.g.:

wget https://files.pythonhosted.org/packages/1e/47/4efb0d0ab5d40142424e7f3db545e276733a45bd7f7f9095919ef30c96b3/picamraw-1.2.64-py3-none-any.whl
sudo pip3 install picamraw-1.2.64-py3-none-any.whl

 

Posted by Uli Köhler in Python, Raspberry Pi

How to capture Raspi Camera image using OpenCV & Python

First, install OpenCV for Python 3:

sudo apt install python3-opencv

Here’s the code to acquire the image and store it in image.png:

#!/usr/bin/env python3
import cv2
video_capture = cv2.VideoCapture(0)
# Check success
if not video_capture.isOpened():
    raise Exception("Could not open video device")
# Read picture. ret === True on success
ret, frame = video_capture.read()

cv2.imwrite('image.png', frame)
# Close device
video_capture.release()

Run it using

python3 cv-raspicapture.py

 

Posted by Uli Köhler in OpenCV, Python, Raspberry Pi

How to fix Raspi camera ‘mmal_vc_component_create: failed to create component ‘vc.ril.camera’ (1:ENOMEM)’

Problem:

You are trying to acess the Raspberry Pi camera using raspistill or raspivid, but you see an error message like this:

mmal: Cannot read camera info, keeping the defaults for OV5647
mmal: mmal_vc_component_create: failed to create component 'vc.ril.camera' (1:ENOMEM)
mmal: mmal_component_create_core: could not create component 'vc.ril.camera' (1)
mmal: Failed to create camera component
mmal: main: Failed to create camera component
mmal: Camera is not enabled in this build. Try running "sudo raspi-config" and ensure that "camera" has been enabled

Solution:

The most common issue here is that you don’t have the camera interface enabled. Read How to enable Raspberry Pi camera using raspi-config for instructions on how to do that.

If the camera interface has already been enabled (remember that you need to reboot for the changes to take effect), the most likely reason for error messages like this is that the camera is not connected correctly to the Raspberry Pi:

  • Is the CSI cable inserted the right way? The silvery contacts need to face away from the Ethernet connector!
  • Is the CSI cable fully seated?
  • Did you insert the CSI cable into the Display connector? It needs to be inserted into the CSI connector, which is the one closer to the Ethernet connector.
  • Is the other end of the CSI cable correctly attached to the camera board?
Posted by Uli Köhler in Embedded, Raspberry Pi

How to enable Raspberry Pi camera using raspi-config

You can enable the camera interface by running

sudo raspi-config

Select 5 Interfacing Options and press Return.

Now select P1 Camera and press Return.

Select Yes and press Return.

Now that the camera interface has been enabled, press Return.

Press Tab twice to select Finish and press Return. You are now asked if you want to reboot:

Select Yes and wait for your Raspberry Pi to reboot. Your camera interface will be enabled after the reboot.

In case you are not asked to reboot, your camera interface was already enabled. In this case, select Finish on the main raspi-config screen by pressing Tab twice and pressing Return:

After pressing Return, check your shell again. Reboot (sudo reboot) so that any previous settings will take effect and then try

sudo raspistill -o myimg.jpg

in order to test your camera.

Posted by Uli Köhler in Embedded, Raspberry Pi

How to fix Raspi camera ‘failed to open vchiq instance’

Problem:

You are trying to access the Raspberry Pi camera using raspistill or raspivid. Although you already enabled the camera interface using raspi-config, you see this error message:

* failed to open vchiq instance

Solution:

Your user does not have the permissions to access the camera interface. As a quick fix, you can run raspistill or raspivid as root using sudo, or add your user to the video group to acquire the required permissions:

sudo usermod -a -G video $USER

After doing this, log out and log back in again for the changes to take effect – or close your SSH session and connect again. If in doubt, try to reboot.

Posted by Uli Köhler in Embedded, Raspberry Pi

How I fixed my Arduino ISP ‘Device signature = …’ errors

Problem:

I was trying to program my Arduino using an AVRISP MK II programmer, but every time I tried to program it (using avrdude or the Arduino software), I encountered errors like this:

avrdude: stk500v2_command(): command failed
avrdude: stk500v2_program_enable(): bad AVRISPmkII connection status: MOSI fail, RST fail, SCK fail, Target reverse inserted
avrdude: initialization failed, rc=-1
avrdude: AVR device initialized and ready to accept instructions
avrdude: Device signature = 0xb058e2
avrdude: Expected signature for ATmega2560 is 1E 98 01
avrdude: NOTE: "flash" memory has been specified, an erase cycle will be performed
         To disable this feature, specify the -D option.

Solution:

In my case, the reason for this issue was that there was a shield connected to the SPI bus.

Since the SPI bus on the Arduino is shared with the ICSP header for In-System Programming (ISP), which is the protocol used by external programmers for programming AVR chips, connecting anything else to the SPI bus may cause issues.

Try to disconnect anything from the SPI bus. If the CS (chip select) signal of the extra device on the SPI bus is low, it might interfere with the communication with the AVR chip!

Posted by Uli Köhler in Arduino

How to fix Arduino ‘U8glib.h: No such file or directory’

Problem:

You want to compile an Arduino sketch, but you see an error message like

In file included from sketch/ultralcd.cpp:96:0:
ultralcd_impl_DOGM.h:46:20: error: U8glib.h: No such file or directory
compilation terminated.
exit status 1
U8glib.h: No such file or directory

Solution:

In Arduino, click on the Tools menu, then click on Manage libraries. Now enter u8glib into the search bar an press Enter.

Scroll down to the U8glib library, hover over it with your mouse and click Install on the bottom right. Now the error should be gone.

Posted by Uli Köhler in Arduino

How to fix SDCC ‘at 1: warning 119: don’t know what to do with file ‘main.o’. file extension unsupported’

If you use SDCC to compile your C code for your microcontroller and you encounter an error message like this:

at 1: warning 119: don't know what to do with file 'main.o'. file extension unsupported

you have to configure your build system to use the .rel suffix for object files instead of the standard .o. SDCC expects built object files to have the .rel extension! See How to change CMake object file suffix from default “.o” for details on how to do that in CMake.

Posted by Uli Köhler in Build systems, Embedded

A working SDCC STM8 CMake configuration

If you have been looking desperately for a working CMake example for the SDCC compiler for STM8 microcontrollers here’s my take on it:

cmake_minimum_required(VERSION 3.2)

set(CMAKE_C_OUTPUT_EXTENSION ".rel")
set(CMAKE_C_COMPILER sdcc)
set(CMAKE_SYSTEM_NAME Generic) # No linux target etc

# Prevent default configuration
set(CMAKE_C_FLAGS_INIT "")
set(CMAKE_EXE_LINKER_FLAGS_INIT "")

project(STM8Blink C)
SET(CMAKE_C_FLAGS "-mstm8 --std-c99")
add_executable(main.ihx main.c)

# Flash targets
add_custom_target(flash ALL COMMAND stm8flash -c stlink -p stm8s105c6 -w main.ihx)

This will build main.ihx from main.c. main.ihx is a Intel Hex file which can be directly flashed using stm8flash.

The last lines setup make flash ; you might need to use the correct microcontroller (stm8s105c6 in this example, run stm8flash -l to show supported devices) and the correct flash adapter (stlink, stlinkv2, stlinkv21, stlinkv3 or espstlink).

The setup example shown here is for the STM8S eval board.

I suppose it can be easily modified for other microcontrollers, but I haven’t tried that so far.

Posted by Uli Köhler in CMake, Embedded, Hardware

What’s the formula for 2/3/4 parallel resistors?

Formula for 2 parallel resistors R1 and R2:

R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2}}

Python code:

rtotal = 1. / (1./r1 + 1./r2)

Formula for 3 parallel resistors R1 and R2:

R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}}

Python code:

rtotal = 1. / (1./r1 + 1./r2 + 1./r3)

Formula for 4 parallel resistors R1 and R2:

R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \frac{1}{R_4}}

Python code:

rtotal = 1. / (1./r1 + 1./r2 + 1./r3 + 1./r4)

 

Posted by Uli Köhler in Electronics, Python

Computing crystal load capacitance using Python & UliEngineering

When you implement a crystal oscillator circuit, one task that is both essential and often overlooked is how to compute the load capacitors of the crystal.

Tip: In many cases it is easier to use an integrated crystal oscillator for which you don’t have to care about load capacitors. Crystals are usually recommendable for low-power applications and high-volume manufacturing where oscillators are too expensive.

In this example, we’ll compute the load capacitors for a Abracon ABLS-16.000MHZ-B4-T (specified with a load capacitance of 18 pF according to the datasheet).

It is paramount to understand that 18 pF load capacitance specification does not mean that you can use 18 pF capacitors or 2x 9 pF capacitors. You need to actually calculate the correct values.

You need the following information:

  • Load capacitance of the crystal, specified in the crystal’s datasheet.
  • An estimate of the stray capacitance, i.e. the capacitance of the PCB traces from the crystal to the microcontroller. If you don’t know this, I recommend to use 2 pF
  • The pin capacitance of the microcontroller (or whatever device you want to connect your device to) as specified in the microcontroller’s datasheet. Often the capacitance of oscillator pins is not the same as for other pins and is hence specified separately. 3 pF is a good first estimate for most modern ICs. The value must be specified per pin!

In order to install UliEngineering (a Python 3 library) run:

sudo pip3 install -U UliEngineering

Now we can compute the correct load capacitors using:

from UliEngineering.EngineerIO import auto_print
from UliEngineering.Electronics.Crystal import *

# Print load capacitors: prints "29.0 pF"
auto_print(load_capacitors, cload="18 pF", cpin="3 pF", cstray="2 pF")

In this case, you need to have two load capacitors of 29.0 pF each.

You should use the closest available value of load capacitor, but always check the resulting frequency if you have high clock accuracy requirements.

Tip: You can pass both numbers (like 18e-12) or strings (like 18 pF or 0.018 nF) to most UliEngineering functions. SI prefixes like p and n are automatically decoded

If you want to use the value programmatically, call load_capacitors() directly:

from UliEngineering.EngineerIO import auto_print
from UliEngineering.Electronics.Crystal import *

# Coompute the load capacitor value (for both of the two load caps): 
load_caps = load_capacitors(cload="18 pF", cpin="3 pF", cstray="2 pF")
# load_caps = 2.9e-11

 

Posted by Uli Köhler in Electronics, Python

Computing resistor power dissipation in Python using UliEngineering

In this example we’ll calculate the power dissipation of a 1 kΩ resistor with a constant current of 30 mA flowing through it, using our science and engineering Python library UliEngineering.

In order to install UliEngineering (a Python3-only library), run

sudo pip3 install -U UliEngineering

Now we can compute the resistor power dissipation using power_dissipated_in_resistor_by_current()

from UliEngineering.EngineerIO import auto_print
from UliEngineering.Electronics.Resistors import *
# Just compute the value:
power = power_dissipated_in_resistor_by_current("1 kΩ", "30 mA") # power = 0.9

# Print value: prints: prints "900 mW"
auto_print(power_dissipated_in_resistor_by_current, "1 kΩ", "30 mA")

Since the result is 900 mW, you can deduce that you need to use a resistor with a power rating of at least one Watt.

Note that you can pass both numbers (like 0.03) or strings (like 30 mA or 0.03 A) to most UliEngineering functions. SI prefixes like k and M are automatically decoded

If you know the voltage across the resistor, you can use power_dissipated_in_resistor_by_voltage(). Let’s assume there is 1V dropped across the resistor:

from UliEngineering.EngineerIO import auto_print
from UliEngineering.Electronics.Resistors import *
# Just compute the value:
power = power_dissipated_in_resistor_by_voltage("1 kΩ", "30 mA") # power = 0.001

# Print value: prints: prints "1.00 mW"
auto_print(power_dissipated_in_resistor_by_voltage, "1 kΩ", "30 mA")

So in this case the power dissipation is extremely low – only 1.00 mW – and won’t matter for most practical applications.

In many cases, you can also pass NumPy arrays to UliEngineering functions:

from UliEngineering.EngineerIO import format_value
from UliEngineering.Electronics.Resistors import *
import numpy as np

# Compute the value:
resistors = np.asarray([100, 500, 1000]) # 100 Ω, 500 Ω, 1 kΩ
power = power_dissipated_in_resistor_by_voltage(resistors, "30 mA") # power = 0.001

# power = [9.0e-06 1.8e-06 9.0e-07]

 

Posted by Uli Köhler in Electronics, Python

How to fix raspivid ‘* failed to open vchiq instance’

Problem:

You want to capture a video using raspivid, e.g. using a command like

raspivid -o vid.h264

but you only see this error message:

* failed to open vchiq instance

with no video being captured.

Quick solution:

Run raspivid using sudo:

sudo raspivid -o vid.h264

Now either the video will be captured or you will see another error message if there is another issue with your camera setup.

Better solution:

Add your user to the video group so you don’t have to run raspivid as root:

sudo usermod -a -G video $USER

You only need to do this once. After that, log out and log back in (or close your SSH session and connect again). If in doubt, restart the Raspberry Pi. See What does ’sudo usermod -a -G group $USER‘ do on Linux? for more details.

Now you will be able to capture video as normal user:

raspivid -o vid.h264

 

Posted by Uli Köhler in Embedded, Linux

How to enable SSH on Raspbian without a screen

You can open the boot partition on the SD card (the FAT32 partition) and create an empty file named ssh in the root directory of that partition. Ensure that the file is names ssh and not ssh.txt !

If you are in the correct working directory in the command line, use

touch ssh

On recent Ubuntu version, this will switch to the correct directory and create the file (but you need to mount the directory manually e.g. using your file explorer:

cd /media/$USER/boot && touch ssh

Don’t forget to unmount the boot drive before removing the SD card. Once you restart the Raspberry Pi with the modified SD card, SSH will be enabled without you having to attach a keyoard or screen to the Pi.

This approach was tested with the 2018-11-13 version of Raspbian and works with Raspberry Pi 1, Raspberry Pi 2 and Raspberry Pi 3.

Credits to Yahor for the original solution on StackOverflow!

Posted by Uli Köhler in Embedded, Linux