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by BudBennett
Tue Aug 06, 2013 8:57 pm
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

I have been busy as well. My Plan A and Plan B PCB arrived last week and I have been evaluating the two circuit approaches (see postings above). The bad news is that Plan A doesn't work. It has a serious flaw – the LTC4425 shuts down the PFOB comparator when it VIN pin goes below 1.5V. I thought tha...
by BudBennett
Wed Jul 24, 2013 4:52 pm
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

paulv, Two things: 1. You can't save the batteries by asserting the SHUTDOWN pin of the LT1302 - there is a DC path from the batteries through the inductor and Schottky diode. You must put a series switch in there somewhere if you need to disconnect the batteries. I suggest it be located at the outp...
by BudBennett
Wed Jul 24, 2013 2:44 am
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

Nelly26, I am using a Python script to debounce the GPIO input and shutdown the Pi. Go to this link: http://raspi.tv/2013/how-to-use-interrupts-with-python-on-the-raspberry-pi-and-rpi-gpio-part-2 It should answer all of your questions. The only other thing that I did was to create a bash script in /...
by BudBennett
Wed Jul 24, 2013 2:36 am
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

I want to address the last post by Tage. I have somewhat radical idea: Why not just let the Schottky diodes handle the load switching? There are already two of them and their cathodes are tied together. Don't use a switch. Here's a schematic of what I'm talking about. LT1302.png The LT1302 is pretty...
by BudBennett
Mon Jul 22, 2013 12:45 am
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

This is my "Plan A" circuit. A simpler version of this was posted here: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=41&t=42956 but it did not work correctly. These types of circuits are not for the newbie. I originally tried to breadboard it (simulation would have been preferred, but LT doesn'...
by BudBennett
Sun Jul 21, 2013 8:04 pm
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

This is my "Plan B" circuit, which I designed after a lot of frustration using the LTC4425 supercap charger IC. The 4425 is replaced with the LTC1541 - a simple opamp/comparator/reference IC, which is pretty easy to understand. There are still some issues/problems with this solution: 1. The supercap...
by BudBennett
Sun Jul 21, 2013 3:14 pm
Forum: Automation, sensing and robotics
Topic: Controlled Shutdown at Power Failure
Replies: 72
Views: 53959

Re: Controlled Shutdown at Power Failure

I, too, have a Raspberry Pi heating system that needs to stay operational after a power outage. I have given this problem a lot of thought. There are certain requirements for a circuit/system with this function: 0. Manage you reserve power source. 1. Detect the loss of power and signal the RPi. 2. H...
by BudBennett
Sat Jun 22, 2013 5:14 am
Forum: Automation, sensing and robotics
Topic: Logging data to a web server
Replies: 17
Views: 36501

Re: Logging data to a web server

I am doing something similar. You can use sqlite3 in python for your database. It works quite well. I am writing sensor data with a timestamp to the database once per minute and I have had no problems with the SD card wear out - it has been running for several months now. I believe that the SD write...
by BudBennett
Tue May 28, 2013 12:22 am
Forum: Automation, sensing and robotics
Topic: Controlling 12VDC from GPIO
Replies: 25
Views: 13836

Re: Controlling 12VDC from GPIO

$5 gets you 5 of the SOT23 adapters (plus some others) which can mount 2 devices each - all of 10 of your drivers. You can hunt for through-hole devices, but the vast majority of new tech is SMT, at lower prices.

The polarity of the GPIO doesn't matter...
by BudBennett
Mon May 27, 2013 8:52 pm
Forum: Automation, sensing and robotics
Topic: Controlling 12VDC from GPIO
Replies: 25
Views: 13836

Re: Controlling 12VDC from GPIO

Be very careful using an octal darlington. The darlington will not saturate with a low VCE voltage and you will get a significant amount of power dissipation per driver. I calculate PD = 0.5Watt for VCE=1.6V and IC = 320mA. If you try to apply this to all 8 drivers your going to get 4watts! The thet...
by BudBennett
Sun May 26, 2013 1:24 am
Forum: Other projects
Topic: Providing power for intermittent power loss
Replies: 15
Views: 8155

Re: Providing power for intermittent power loss

Here's the second draft of the circuit, after a lot of thought and cuddling with the data sheets for the LTC4425, LTC4411 and LTC4415. My first draft wouldn't work at all. I'm about ready to order parts from Digikey unless somebody spots a serious flaw in the design. What this circuit does: The RPi ...
by BudBennett
Fri May 24, 2013 4:41 am
Forum: Other projects
Topic: Working out the pins for a Li-Ion battery
Replies: 13
Views: 5136

Re: Working out the pins for a Li-Ion battery

It appears to be a smart battery. T is probably a thermistor to sense the temperature of the battery as a failsafe to the charger. D and C, as noted previously, are most probably data (SDA) and clock (SCL). If it is a smart battery then it uses the smbus to communicate and follows the protocols of t...
by BudBennett
Thu May 23, 2013 6:45 pm
Forum: Other projects
Topic: Providing power for intermittent power loss
Replies: 15
Views: 8155

Re: Providing power for intermittent power loss

My Pi is controlling the heating system in my house. I need a circuit that will ride through short power glitches 1-2 secs, but also hold up the 5V input until the Pi is shutdown properly. It then needs to re-enable power (or perform a restart) once the power supply is back up. Here's the first pass...
by BudBennett
Fri May 17, 2013 3:43 pm
Forum: Automation, sensing and robotics
Topic: Raspberry Pi Home Automation Ideas...
Replies: 30
Views: 35698

Re: Raspberry Pi Home Automation Ideas...

I have implemented a multi-zone (multi-room) heating control system for my house. The Pi senses thermistors in 9 zones and controls the zone valves and boiler for the in-floor heating. The interface is a secure website on my local network that is accessible from any device with a browser. There are ...

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