Improved Current Load

Some Free Advice!

(Update - July 2019 - see the end of this post regards how not to blow this up!).

Back when I wrote about my failure to build a current limiting circuit for my Alarm Panel Interface, Zega was kind enough to offer me some advice.

Long story short, half the oscillations I was chasing in my limiting circuit were due to my quick and dirty Constant Current Load.  

Here's the Problem...

Multi Channel Meter, Part 2

From Idea to Implementation

What I really love about my hobby projects is that I can develop answers to questions that I can't explore at work.  20kW HF transmitters, Radar Installations all over the world, and Cutting Edge elemental analysers?
No worries.

Digitally Controlled Gain, Isolated PSU Choices?  Fergedaboudit.

In part 1, I detailed my path from the concept of a multi-channel volt meter, to a working system based on individual modules.  Here I'd like to talk about how I refined the design into my first prototype and then look at where I'm going from there.  Like Art, Engineering is never finished, you just chose a point to stop...

More on Alarm Panel Interfacing

I've Been Busy...

I haven't had the chance to update things for a while, but for those interested, I'd like to share some build pictures of my Alarm Interface.  And yes, it's working and in service, with a nice programming GUI. (Links at the end - code will be up "soon").

Teaser Render!

Finishing Up

Final Tests and Code

Listening to Others

I've made no secret that I took my inspiration from Lior's Alarmino project, and I was quite pleasantly surprised when he contacted me on one of my posts.  He suggested that I take a look at the value of the filter cap I'm using on the 4V rail supplying the SIM900.

In his design, the bulk storage cap was charged by a linear regulator that delivered a maximum of 1A, but as my design uses a switch mode that's (nominally) rated to 3A I figured I could go smaller with my filter cap as my circuit can deliver a higher rate of charge - even with the substitution of the eBay sourced LM2596 circuit.

Thank You eBay!

Design and Test, The Third

Filter Testing and Options

IN THEORY

In my previous posts, I'd debugged my power supply, and got my Arduino Leonardo clone up and running.

I'm getting closer to finishing but there's still one functional block that I'd not yet tested and it's my active filter.  Again, a big hat's off to Lior for the inspiration behind my build, but I have done some things my own way.

For example I used a Sallen-Key low pass active filter to get a 1.5kHz Low Pass Filter.

Thanks to the internet, rather than hand crank the numbers, I used this on-line calculator from OKAWA Electric Design. From that design I whipped up this LT-Spice File.

LT-Spice Circuit (Active Filter)

Design and Test, Part Two

Testing Day Two

As mentioned before, I'd tested and debugged my hardware to the point where it's time to start integrating the systems.  Or in other words get the micro talking to the rest of the board.

Notice Something Strange?

Design and Test

Testing, Day One.

It's been a while (a large part was waiting on parts - I'm looking at you Futurelec!) since I posted about my Alarm Monitoring system but as I've finally been able to get some build and test underway I thought I'd share the progress.  Which is a nice way of saying I'm not finished yet.

Step By Step

With this design I've incorporated a fair few new circuit blocks I haven't tried before, being:

1. FET Current Limiter Circuit
2. 4V, 3A (TWELVE WATTS BABY!) Switch Mode power supply
3. AVR microcontroller (yes, never used 'em before)

So to start, I loaded only the power supply section before placing other parts.  This is a good practice when prototyping as if you stuffed something up, you don't blow up the rest of your board.  I've actually designed in zero ohm links on the power supply rails which could also prevent the same destruction but through experience I prefer to do it his way.

Zero ohm links are good for when you are setting up production boards.  You can adjust anything that needs adjusting in isolation before installing the links.

But for prototypes - Why load everything on your first prototype when a SNAFU in any area could make the whole exercise pointless?

Partially Loaded

Polyswitch Current Limiting

Polyswitch Testing

Just a quick little video I took while testing my Raspberry-Pi protection circuits.

To recap, I was testing the current drawn when a 5V1 zener clamp is protecting an input from an over volt condition.

Power Supply Protection for Raspberry-Pi

Defining the Problem, Again

The power rails of the Raspberry-Pi are bought out on the GPIO header as well as the IO pins.  This is both a blessing and a curse.

The advantage of this is that you have 5V and 3V3 rails available to power your circuitry that you are hooking up to your GPIO pins, but again, as the Raspberry-Pi doesn't have any protection circuitry you run the risk of killing your Raspberry-Pi through:

• Short Circuiting the Rail
• Reverse Biasing the Rail
• Over-Volting the Rail

GPIO Protection For Raspberry-Pi

Or How Not to Let the magic Smoke Out

Previously I've written about my love / hate relationship with Polyswitches, and discussed some input and output pin protection methods.  I was looking into this so I could make a breakout board / protection device for experimenting with my Raspberry-Pi and not let out the magic smoke.

I could say it's to help kids / beginners protect themselves from basic mistakes, but even after practicing Engineering for nearly 20 years I have learn that Murphy still bites, so it's also a lesson in self preservation :)

Testing

Why We Test

The Magic of FETs

I've never had a lot to do with FETs before so I've been keen to experiment with and learn about FETs on my home projects.

As I wrote before in Basic Protection I like the simplicity of using a P-FET to provide reverse bias protection, and with my Alarm Interface I was planning on combining this reverse bias protection with current limiting.

As usual, I got it wrong the first time....