Reddit mentions of Mosuch Battery Holder for (8) AA with Standard Snap Connector : BH383 (1)

Pi A+ -$23 (http://www.amazon.com/gp/offer-listing/B00PEX05TO/ref=dp_olp_new?ie=UTF8&condition=new)

monitor - $13.69 (http://www.amazon.com/Rearview-Monitor-Automobile-Support-Resolution/dp/B008BFGDMU/ref=sr_1_1?s=car&ie=UTF8&qid=1425707624&sr=1-1&keywords=backup+LCD+monitor)


RFID scanner - $6.17 (http://www.amazon.com/Contactless-Proximity-Reader-EM4001-Windows/dp/B00M3V438E/ref=sr_1_11?s=electronics&ie=UTF8&qid=1425706532&sr=1-11&keywords=rfid+reader)


Battery box - $5.79 (http://www.amazon.com/Battery-Holder-Standard-Snap-Connector/dp/B000LFRTIK)


mini USB keyboard - $6.04 (http://www.amazon.com/AGPtek%C2%AE-Protective-Leather-Keyboard-Tablet/dp/B008JBPP9I/ref=sr_1_79?s=electronics&ie=UTF8&qid=1425707892&sr=1-79&keywords=7%22+tablet+usb+keyboard+case)


5V UBEC - $6.87 (http://www.amazon.com/Zjskin-Receiver-Servo-Power-Supply/dp/B00PANR6EK/ref=sr_1_1?s=electronics&ie=UTF8&qid=1425708123&sr=1-1&keywords=5V+UBEC)


So ... $61.64, starting from scratch, and not trying all that hard. Probably need $8 Wifi and a $5 USB hub ...


I feel like if you really tried, you could shave $10 off that. If I were me, I'd use a UBEC off my desk, and 3D print a battery box ... but, then, I think I have that entire setup in my parts bin, except the RFID reader, so that's not fair.


Still, yeah, I think for the price of a few lunches out it's doable.

EDIT

You could cut the monitor and keyboard (20$), find a cheaper battery box, and tape it together as a handheld scanner ... you'd just need an indicator LED for when you successfully scanned, and someone would have to connect to your serial port and log in to see if you were caught.


It'd be nice if you could find a reader that cheap that could talk to an arduino ... then you could do a 'dumb' scanner a lot easier.

I got a great deal on some HMEC466s on ebay, but they a were more standard 6-pin lemo with built-in battery pack. Looking at the pin-out, you should be able to make your own battery pack or have a socket put in to hook it up to ship power.

pin out is in the manual for those headsets. https://www.manualslib.com/manual/151043/Sennheiser-Noisegard-Hmdc-322.html?page=31#manual

pin 1 is your DC+ (12-35V according to that manual)
pin 2 is ground
pin 3 is No connect



8 AA battery holder: https://smile.amazon.com/Battery-Holder-Standard-Snap-Connector/dp/B000LFRTIK
xlr 3-pin mic extension cable: https://smile.amazon.com/Male-Female-Microphone-Extension-Cable/dp/B006ZQ49UA

could probably cut one end off the mic extension cable, hook it up to the battery pack as the manual indicates. not pretty. not even an on off switch, but might solve it for less than $10

Depending on the solenoid it'll probably take more current than a normal 9V battery can provide.

If it doesn't need to be portable, there's plenty of options for cheap 12v power supplies.

If it's going in a car you'll want to just run the wires from the battery.

If for some reason you need it to be portable and don't want to plug it in you're going to need a decent sized battery. If it's a small enough solenoid (i.e. an amp or two max) you could get a battery holder. There's also plenty of 12V RC batteries.

As others have said, you can just use 12 volts from almost anywhere.

Easiest would be 8AA batteries in a holder like this. Use rechargeable batteries or this might get expensive.



If you wanted to get fancy you could find bigger and better battery packs. Maybe even a nice lithium ion pack....but that could get expensive and complex.


Reviews on those lights you linked to are pretty harsh. Not sure if they'll have enough light output for what you want to do...but what the hell, give it a try!

https://smile.amazon.com/Battery-Holder-Standard-Snap-Connector/dp/B000LFRTIK?sa-no-redirect=1