Magnetic stripe card spoofer

Step 3 Build the amplifier

Because a personal media player's headphone jack is not strong enough to drive our electromagnet, we must build a simple amplifier to drive it.

I just threw together this circuit from stuff I had lying around my desk. You can use pretty much any amplifier as long as it has enough gain to drive the electromagnet before it starts to distort. If you really don't want to build this circuit, you could even get some old powered computer speakers and replace one of the speakers with your electromagnet.

Anyway, to build the amplifier I'm using, you need the following items:

6-pin DIP socket
IC1 - LM386 op-amp
C1, C2 - 0.1 uF ceramic capacitors
C3 - 220 uF electrolytic capacitor
R1 - 10 ohm resistor
R2 - 5k trimmer potentiometer (actual value doesn't really matter, just as long as the max is large)
Audio cable with 1/8" phono jack on one end (I just cut the cord off some old headphones)
9V battery clip
Small switch
Prototyping board
Assorted jumper wires

Assemble the circuit as shown in the schematic. Make sure that the ground lead of the phono cable is connected to ground of the circuit. It does not matter if you connect the left or right channel of the phono cable to the input of the amplifier. Also, it does not matter which end of the solenoid is connected to ground, as long as one of them is.

I just soldered some jumper wires for the coil outputs that I connect up later with alligator clips later in order to keep the system modular.

Comments

[fub1985]

you say that a old powered computer speakers can be used.. what do you mean by this?

[seth3d]

WIP: I've created a PCB trace. The yellow lines signify where I'll be using wire jumps. If anyone is interested I'll post my results, negs, and Digi-Key part list.

[macgeek10]

There is an error in your pcb layout if i am not mistaken, the potentiometer is not hooked up as an adjuster, but rather as a simple 5k resistor, you need to change one of the traces to that center post instead.  I recommend Eagle for your pcb layout, it is lightweight and free, give it a try.

[brokentesting]

Seth3d, This would be excellent. But seeing as it has been going on 2 months since your last post, I still hope you provide the details :) Nonetheless, I have subscribed to your Instructable area, just waiting on you to create a Instructable with the details of your PCB board. In the interim, I will try and contact you otherwise. Thanks for offering this Seth3d :) Keep us the cool work :)

[seth3d]

OK, here are the files in the zip file. The express PCB layout has top layer flipped for your printing convenience. I used the oxy-hydrogen-chloride etching method described here.

I haven't had much time recently to work with this much more. The amp is essentially a simple "square wave" modulator. I'm not 100% certain that the wave files would even work without the amplifier though others have stated it does with the right miniaturized electromagnet.

Additional materials not included in the part list include some solid core wire to use for jumpers, copper clad, an audio jack which was wrenched from the end of a broken headphone, and an LED with resistor to display power state. The LED can be excluded.

[seth3d]

Here are both layers to the PCB, no sense wasting two sheets of transfer paper.

[brokentesting]

Seth3d, You are the MAN! Very prompt reply indeed with all the necessary details :) Going to send you a Email shortly regarding some other stuff I think you'll find interesting. Thanks much Seth3d. You DA MAN!!!!!!!!!!!!!!!!!!!!!!!!!!! :)

[seth3d]

Small correction, should read 8-Pin DIP socket not 6 pin.