St. Cloud, Twin Cities and rural Minnesota's one stop shop for electronic device repair (phone repair, tablet repair, laptop repair, tv repair, etc), Tech support and IT consulting for small/medium business. Maker/Hacker space friendly offering a number of arduino and raspi components in stock.
Search This Blog
RFID / HID / NFC Projects
Inductive Power Transfer
I was paid to produce a board that could emulate hid proxi cards.
What you're looking at (bellow) is a Proxi Max reader in a plexiglass case that I made.
What I'm holding up to it is version 2 of my emulator board. Where the chip should be soldered I've placed an LED instead.
Light emitting diode is the key to understanding how this works.
Any NFC or RFID application the reader actually sends out an AC field.
The LED is soldered across the ends of a coil made by a spiral on a pcb trace. The coil of wire attached to the reader and the coil of copper on the PCB form an air core transformer. Which passes the small amount of across the light emitting diode. Diodes are capable of chopping DC to AC, perhaps a better way to say it would be diodes don't give a s*** if you give them AC or DC you'll get DC out of them. Light emitting diodes happen to emit light mt while they do that.
It's hard to see due to the camera but the LED in the top right blinks green indicating a valid read.
The concept for why the LED lights up is the same way we are able to power the microcontroller attached to the board. Integrated circuit engineers long ago realize that static electricity is a problem that can fry their chips. most modern integrated circuits contain static protection diodes on external pins. You don't see them because they're inside the black case hence integrated circuit... But it just so happens that on this chip the clock pins are protected by such diodes. The beauty is you attach the coil to where you normally connect an oscillator and when you bring it in proximity to the reader which is emitting an AC field the diodes not only clip the signal to form a perfect clock, the excess power is directed to VCC and zero or positive and negative where you normally powered the chip.
To put it simply you end up both providing the chip with power and clock just by attaching a coil.
I think some of the shortfalls could be overcome with clever use of plls but I'll discuss that or look into that later. It's beautiful in design but it lacks in the ability to brute Force proxy codes due to the fact that your clock cycle is at the same speed as the reader field and it's not enough extra clock cycles to work with. You just barely get to spit out the code by modulating the coil.
Powering it externally might sound like the solution
But it brings with it a problem. the problem is you need to be in phase with the reader's signal to produce a valid read.
If you listen closely you might notice the servo sounds jumpy/jittery. This is because the rasberypi lacks a hardware PWM generator .aka servos take precise timing of highs and lows in the signals sent to them. The pi does this with its cpu..or trys to. this leaves a vulnerable to other things running on the CPU interrupting the precise timing or being in front of the command to send the signal at the microsecond it needed to be sent. Which produces the audible clatter from the servo motor. which actually produced enough vibration to knock the whole assembly off the shelf it was on
Here's the same thing with a cheaper reader and a different motor. Actually it's an electric door strike.
At the time I pulled this off the instructions were not there, there was no easy guide for using this reader with the Raspberry Pi. That was like 2016 I'm hoping the situation has improved. At the time I was translating little bits of Russian and mostly cobbling it together on my own.
Do you have a lawn tractor you use to plow snow? Ever wondered if you could make a manual snow plow attachment into a push button? A friend of mine had a riding lawn mower and snow plow blade attachment. Every winter he used the riding lawn mower to plow the driveway. The plow was a kit that bolted on the front of the lawn tractor and it had a mechanicaly actuated adjustable snow plow blade. The issue and this project came about when the arm to adjust the snow plow blade broke. I had just bought a mig welder. I had also watched linear actuators capable of handling 300lbs of force drop to $25 on ebay. Here's what I came up with for converting a riding lawn mower snow plow into an electricity adjusted lawn tractor plow. Repair The lawn tractor snow plow kit had seen better days. The fork that attaches it had bent /deflected and spread. I used a map gas torch and a bench vise to squeeze the tongs back and then welded some L channel angle iron on the sides. Less Play I also tack weld
In this multi part series on diy circuit board manufacturing/fabrication I'm going to cover my diy, vacuum pull down UV LED exposure box. If you are looking for a description of the entire fab process click here UV Exposure Rig The photos bellow are of the uv lithography exposure box I cobbled together. It also features vacuum pull down. The source: A 4.1w High luminous efficiency 365nm UV LED. Sourced from digi key mid 2016. Model LZ1-00UV00
This post is for anyone interested in owning or building a CD welder. I wanted a reliable Resistive spot welder for lithium ion pack construction and or repair. I didn't want to spend 3k or buy from china. I decided to design and fabricate my own. Bellow is my first crack at a Mosfet Switched capacitive discharge mcu controlled spot welder. This post is a work in progress. If you've stumbled across in hoping to build your own cd welder feel free to ask questions in the comments section. I'll provide any help I can. My CD Spot Welder Here's pics of the first working version from 2018