This is what I use... http://burglarbomb.com/Repulsar-IV.aspx
a little bit pricey ....(I managed to get mine wholesale but still $$$)
unit is a gravity enhanced design.. ie they use a standard pepper spray grenade(bug bomb type nozzle and catch) and the can is mounted in a tube inverted over a well with a notch/lever designed to trip same(the bug bomb nozzle) when the can encounters same , on top of the bottom of the pepper spray can is a 1 lb metal weight, the can is held above the well with a solenoid activated release allowing the can to descend into the well(s) X 4 and release their contents one can after the other.. I will get you some closeups of the whole mechanism as my roommates did NOT want me deploying it yet(we are on the edge of the xxxxxxx ghetto) We chose dobermans instead
is one source for the type of pepper spray can you will need, the search term "pepper spray grenade" on google will get you a LOT more sources.
Do use VERY clear signage to warn LEO and emergency personel of the breathing hazard and DEFINITELY consider/design a loud VOICE alarm warning of the pending deployment..., and get a very good criminal defense attorney...
ie Shomer-tec.com used to sell a unit for about 129.00 but no more.. if access to the cans is obtained then a cylindrical trip mounted in the end of a tube of PVC/ABS sized to let the can slide accurately but freely and a DC/AC solenoid turned on/off by a power MOSFET/relay would give you almost the same effect for less than 30.00 or so(including the bug bomb type pepper spray and solenoid as well as mosfet) however you will lack the additional time delay of the commercial unit(which is designed to be added to a regular alarm system and spray after that has activated for a time...)...
I LIKE the following system A LOT... very pricey and VERY MASSIVE/EFFECTIVE
A very low cost unit is a DC 4" bilge blower and ducting mounted over a small AC hotplate with a well heatsinked logic level AC SSR to turn on the heatplate, a 30.00 PID controller and thermocouple could be added for totally accurate vapor temps or simply pid code in your arduino program and a thermocouple interface/thermocouple) and of course a handful of dried habernero/scotch broom or Bhutanese Ghost Peppers(most potent of the weapons grade peppers) in a small pan on top of the hotplate .(http://www.chilepepperinstitute.org/cart/product/bhut_jolokia/
) . When the entry sensor is tripped then the hot plate is heated slowly and turned off when the entry code is entered.. when an intrusion sensor is tripped a FULL on heating scheme is chosen until vapor temp is reached and the bilge blower is activated directing the pepper vapor/smoke into the area to be denied(depending on hotplate can be ready in as little as a minute) the whole assembly should be built into a metal cabinet that you can duct easily...(vapor temp should be the vapor temp of capsicum)...
This last design of course is totally impossible to control or regulate by the authorities
.. use code to time the on period /temp below combustion levels and turn off well in advance of that as it will be unattended.
Dont forget high intensity sound , high intensity strobing LEDS etc...and locking the perps into a given space...
and an M17 gas mask for yourself along with the integral headcover...SOMEONE has to go in to open windows and turn on fans and you will have little bits of weapons grade pepper over everything for months so DONT use this around food/clothing(you get the idea) my wife still forbids any more experiments...NEVER EVER TEST 1 LB Pepper Foam Cannister(s) in the house by attempting to contain the sprayed foam in a large handful of paper toweling(YOU WILL FAIL!) trust me on this one! and that goes triple for bear spray(pepper gas)...
ANY of the bear bombs/sprays or any POLICE type spray grenades are NON flammable by Federal standards(They have to be safe alongside Taser deployments without setting the criminal on fire)...
hope all this helps
and pepper spray/gas/vapor will kill MOST garden pests for you indoor garden types
ps flamers and too curious types >/dev/null (ie what he is trying to protect is his property and himself..I dont want to make comments about the illogic of such questions or my personal disgust with those who would tell us its illegal to defend our property(its not) at least where I choose to live) A good use for these devices is to lock yourself into a saferoom(bedroom with a solid core door/reinforced frame) in the event of home invasion(s)(they occur quite frequently in xxxxxxx) and then flood the remainder of the house with pepper/tear gas until Law enforcement comes..