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Thu May 24, 2018 3:19 pm
Forum: Beginners
Topic: Building a Wheatstone bridge
Replies: 7
Views: 1137

### Re: Building a Wheatstone bridge

Hi Creamy,

Perhaps you can use this HX711 board, see discussion about "rpi3 and SparkFun Load Cell Amplifier - HX711".

https://www.raspberrypi.org/forums/vie ... 3#p950998
Tue May 22, 2018 7:38 am
Forum: Beginners
Topic: Building a Wheatstone bridge
Replies: 7
Views: 1137

### Re: Building a Wheatstone bridge

Z80 Refugee,
The concluding post in that thread looks rather like hope over practicality, flying in the face of advice further up. Just throwing any old op amp at a strain bridge won't cut the mustard.
Explain your post!

How would you measure such small voltages?
Sun May 20, 2018 11:49 am
Forum: Automation, sensing and robotics
Topic: Alarm voltage detection
Replies: 6
Views: 695

### Re: Alarm voltage detection

Hi ralfyM,

Did you connect the alarm gnd and pi gnd together?
I should use an optocoupler. Google for examples.
Tue May 15, 2018 3:06 pm
Forum: Automation, sensing and robotics
Topic: temp sensor ds18b20 problems
Replies: 8
Views: 854

### Re: temp sensor ds18b20 problems

Hi blazbrajer,

A good starting point is:
https://www.maximintegrated.com/en/app- ... mvp/id/148
Mon May 07, 2018 11:54 am
Forum: Beginners
Topic: Electrical Resistance Calculation Help
Replies: 61
Views: 4642

### Re: Electrical Resistance Calculation Help

Hi vvarior, Use a programmable current source circuit/ic like LT3092. Google for application notes. Measure the voltage accross your sample and divide it with by the current of the current source. You may set a current by using a dac. You will find on google many examples. You may start a current fr...
Wed Oct 04, 2017 10:15 am
Forum: Advanced users
Topic: RF 433 MHz Communication Between Raspberry Pi & Arduino Sending Data
Replies: 6
Views: 7233

### Re: RF 433 MHz Communication Between Raspberry Pi & Arduino Sending Data

Hi MichaelCropper,

Another fine transceiver is the HC-12. It has a wide voltage range and 100 channels. For more information see:
https://www.banggood.com/HC-12-433-SI44 ... 73522.html

Success with your project!
Sat Aug 05, 2017 8:41 am
Forum: Interfacing (DSI, CSI, I2C, etc.)
Topic: How to get stable 2KHz square wave with MCP4725 on RPi3?
Replies: 9
Views: 2541

### Re: How to get stable 2KHz square wave with MCP4725 on RPi3?

Hi rollyng, 3 choices: 1) use in this case a micro controller like an arduino because it has no operating system. Timing is much better defined. Or 2) Use c instead if python, because timing is well defined in c. Or 3) connect a arduino, running 2 kHz, to a gpio pin. Write a small interrupt program ...
Tue Aug 01, 2017 1:43 pm
Forum: Automation, sensing and robotics
Topic: PIR motion sensor alternatives?
Replies: 7
Views: 3575

### Re: PIR motion sensor alternatives?

Perhaps a radar sensor like rcwl 0516 could replace a pir sensor. You don't have to drill a hole for these type of sensors. See aliexpress.com or youtube for more information. If I'm right the output is 3.3 V.
Mon Jul 03, 2017 6:51 pm
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

What is your exactly your problem? If you start rc.local proposed in my previous post than your problem has been solved. The only thing is you have to do some tweaks. Modify analitico.py for systemd 1) Add time.sleep(20) 2) Remove the while loop and try except stuff inside the while loop too. You do...
Mon Jul 03, 2017 2:54 pm
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

I think I found your problem in your original setup (rc.local). Your network is not yet running after a reboot, so you have to wait a couple of seconds. analitico.py add the following line just after import time time.sleep(20) rc.local edit the line "python3 /home/pi/Documents/PY/analitico.py &" int...
Mon Jul 03, 2017 10:20 am
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

I have read that already. I cannot test your application because I have no mqtt server running on my pi. I used the server and application of eclipse.org and that runs. See my previous post of @lxterminal -e './home/pi.../ Adafruit writes: The mosquitto mqtt server is started with systemd and the <y...
Mon Jul 03, 2017 9:51 am
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

Sun Jul 02, 2017 6:50 pm
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

Did you try:
sudo journalctl -u <your application>.service > <your log file>
Sun Jul 02, 2017 3:05 pm
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

Perhaps you may try this:
edit in your home directory the file autostart in the directory .config/lxsession/LXDE-pi
And add the following line:
@lxterminal -e './Documents/PY/analitico.py'

Reboot your machine.
Sat Jul 01, 2017 11:49 am
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

The execute bit is not set
chmod 777 <your file>
Sat Jul 01, 2017 10:41 am
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

Your are not the owner of that file
sudo chown pi <your file>
Sat Jul 01, 2017 8:13 am
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

Take a look at the first line in analytici.py and fanspeed.py

One file is referring to python 2 the other file is referring to python 3. Perhaps this is the problem. I cannot test at the moment.
Sat Jul 01, 2017 6:41 am
Forum: General discussion
Topic: how to trigger raspi by the doorbell
Replies: 27
Views: 6806

### Re: how to trigger raspi by the doorbell

Hi komarek,

I forgot to tell you that this is a very nice project.
It has some tough problems but I think you can handle this.

If you (nearly) have finished your project please post it here.
Sat Jul 01, 2017 6:34 am
Forum: General discussion
Topic: python script at startup
Replies: 81
Views: 10481

### Re: python script at startup

Perhaps you may post fanspeed.py too.
Then you can compare both files.
Fri Jun 30, 2017 8:51 pm
Forum: General discussion
Topic: Is there anyway that I can monitor deep cycle batteries?
Replies: 4
Views: 1264

### Re: Is there anyway that I can monitor deep cycle batteries?

Hi treichhart,

Search for 'coulomb counter'. It is a micro controlled counter used in ebikes, ecar batteries a.s.o.
Perhaps you can use it.

Follow this link for a simple circuit. It is just an idea.
https://learn.sparkfun.com/tutorials/lt ... okup-guide
Fri Jun 30, 2017 1:04 pm
Forum: General discussion
Topic: how to trigger raspi by the doorbell
Replies: 27
Views: 6806

### Re: how to trigger raspi by the doorbell

A voltage of a transformer without load is ~ 15 V RMS. 12 V full load. That means a top voltage of 15 V * 1.4 ~21 V max. You can measure this with a scope. The voltage across A and B is 21 V minus the voltage drop of your diode. If R2 is 1K ohm than R1 must be 6K ohm. The voltage on C is ~ 3 V. This...
Fri Jun 30, 2017 11:20 am
Forum: General discussion
Topic: how to trigger raspi by the doorbell
Replies: 27
Views: 6806

### Re: how to trigger raspi by the doorbell

I am referring to your post of 11:15 am i will try to send a signal with 2.5/3 V to the gpio (using a diode and a voltage divider circuit), but the current intensity will be the same as the bell... you're sure that this current won't be too high for the gpio so that there's a risk to damage the rpi?...
Fri Jun 30, 2017 10:39 am
Forum: General discussion
Topic: how to trigger raspi by the doorbell
Replies: 27
Views: 6806

### Re: how to trigger raspi by the doorbell

komarek: what is max. the voltage across the cathode of your diode and ground/common?
Fri Jun 30, 2017 8:49 am
Forum: General discussion
Topic: how to trigger raspi by the doorbell
Replies: 27
Views: 6806

### Re: how to trigger raspi by the doorbell

Read this post of jmgdesign for an idea:

http://www.avrfreaks.net/forum/sense-do ... ng-pressed
The current flowing in the gpio port is very low, it is about 500 micro Amps.

Instead of an opto-isolator you may use a relay proposed by HardwareJon.

Make a diagram and post it here.
Thu Jun 29, 2017 7:44 pm
Forum: General discussion
Topic: how to trigger raspi by the doorbell
Replies: 27
Views: 6806

### Re: how to trigger raspi by the doorbell

I suppose your chime transformer is installed in your adjunction box (gas/power closet). So you may install a multiple socket in the box. 1 socket for your chime and 1 for a rpi power supply. Now it is very easy to make a board with a optical isolator/resistors/diodes or just a relay and connect it ...

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