This is a red laser, it's only the casing that is purple. Also, you can just buy them from a dollar store or whatever... I think that the photoresistor you got is perfect, there is still a difference between a laser beam shining on it and the ambient room light. If you didn't know, a photoresistor just changes the resistance of the current that passes through it depending on the lightning. So if you plug it in +5 volts, and measure the current that is exiting on the other side, by hooking it up to an Analog Input, you can get the quantity of light that is shining of it with digitalRead() function, which outputs a number from 0 (0v of 5v, lot of resistance = lot of light), to 1023 (5v of 5v, no resistance = no light at all) so you can set a threshold, so it takes a lot of light to trigger. To pull this value off,  you can either:

-Hook up a pot that sets that threshold, so you turn it until it only detects the light from the laser beam, ideal if you plan to take it to different places, with more or less lightning for easy adjustment

or,

-Hardcode that value in your code, so it's kind of "permanent" (until you change it in the program). To do so, you send the photoresistor values thru serial and you listen to them while playing around to write down your values.

This system just recognizes motion by reflected ir waves on the subject, and for the laser, any cheap laser pointer would do, example: http://www.ebay.com/itm/Mini-LED-Flash-light-Torch-Keychain-Laser-Pen-PURPL-/190732252540?pt=US_Laser_Pointers&hash=item2c6887357c
You just need a laser pointer and a photoresistor hooked up to an Analog Input and +5v, where you can set the threshold for the ambient light in your program. If you need any help for the program I can help you. But your best bet remains the reed switches, if it's not a big deal to put screws on your door frame...
http://www.ebay.com/itm/White-Door-Window-Contacts-Magnetic-Reed-Switch-Sensor-/350611210023?pt=LH_DefaultDomain_0&hash=item51a20e7f27

I dont't think that Ultrasounds are your best bet, I would rather use a laser (any cheap 1$ laser is good) or, simpler, a door magnetic reed switch.

If anyone wonders how to identify keypad pins, I got started there: http://arduino.cc/playground/Main/KeypadTutorial#Identify

Now, I plan on having 19 N Channel mosfets, gate wired to my arduino, collector to a com and source to a pin on the keypad, so I would just have to trigger two outputs simultaneously to connect them, like a keypress.

My only concern is: is there a two-way flow capability on the drain-source when voltage is applied to gate (is it acting like a diode, the current can only go from d to s ? or it can go from s to d too?) ? I don't know that much on transistors, so a little help would be appreciated. Thanks!

EDIT: Got my answer, it acts like a diode.

Here's what I got for the "pinout":

can Analog inputs serve as outputs?

Yes, or with some diodes and charlieplexing, or with a serial->parallel chip

I don't think I am advanced enough for that...

can Analog inputs serve as outputs?

Yes, all the I/O pins can be used as normal (i.e. non-PMW) digital inputs and outputs on all Arduino boards.  Sorry for not specifying that in my initial reply.

That's nice to know, so I could have a 20th pin that controls the servo to cut the paper, and use for example pin 19 and 20 to control the sevo. That seems good, thanks for your help!

can Analog inputs serve as outputs?

ok, thanks for your reply, so I could note which keypress affect which pins, and build an array, buy 19 octocouplers and wire them to each pin and a com, when I activate two simultaneously, they get in contact and simulate the key. But how could I have 19 inputs on my leonardo? Charles

PS: I could also wire the thermal head and the stepper motor to the arduino like you said, which is a good idea, but I will be fine with only the default chip to handle the font and everything

Hi, my name is Charles, I am a 15 years old guy who loves electronics. I own an Arduino Leonardo board, and I got the idea to interface it with a Dymo LetraTag XR label printer that I found for 20 bucks. So here is the main idea:

1-Connect the keypad with the Arduino,
2-Have software to make arduino issue keypresses to the LetraTag (for example: "H E L L O PRINT SHIFT+DELETE")
3-I'll have a little servo to push the blade to cut the paper, but that's not the deal now

Here's my problem: I don't know anything about interfacing matrix keyboard and programming a clock to scan presses and etc....
So, If anyone has done that with a similar keyboard, or anyone that knows the principe and could explain it to me, You are welcome!

Thanks, Charles

Here are pictures of the device and the keypad:
http://i.imgur.com/O3wCe.jpg
http://i.imgur.com/bDyAa.jpg
http://i.imgur.com/lhWHO.jpg