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GrowFAQ :

DIY: How can I build a compact hood with an intergrated light trap?

  Added by: MarvinM92  Last edited by: MarvinM92  Viewed: 449 times   Rated by 17 users: 8.44/10
Contributed by: MrE

The following are somewhat detailed instructions for constructing a shielded hood/reflector with an integrated light trap designed for a small cabinet… It can certainly be scaled up for larger lights, but a 70w HPS (Home Depot security light) was used here…

Final Product

*Note: While researching on other hoods/reflectors (Jackerspackle, Nimby, NGB, Tick, etc…) this problem often popped up… How do you bend sheet metal w/out a proper press brake? This method works like a dream and requires only a few pieces of scrap wood…

Primary Tools:

· Tin snips
· Rivet gun
· Square / Straightedge
· Utility knife
· Drill
· Good marker for layouts


· Sheet Metal – Picked up large sheet of decent gage sheet for $8 at The HomeDeopt…
· HPS Light
· Glass Shield
· Plug and Wire
· Paint – White & Black – Use HIGH TEMPERATURE only… (For BBQ grills/Engines/Radiators?)
· Weather Striping
· Foil Tape
· Scrap Wood


*Note: The dimensions are for a 70w HPS bulb… If you are building this to use with another size bulb you must alter the dimensions accordingly… So scale/adjust to suit other bulb wattages…

Illustration for the Light Trap’s Layout…

Illustration for the Hood’s Layout…

The Light Trap Subassembly

Start out by transferring the dimensions from the layout to the sheet metal with a magic marker… Try to be very accurate and definitely use a square…

In this image you’ll see A) the sketch on cardboard in the background used to plan hood to fit in the small filing cab, B) the dimensioned flat pattern layout on paper, C) a section of sheet metal with the layout already transferred…

The pattern is then cut with tin snips and “scored” for bending… The scoring is important to get a nice crisp bend… Use the utility knife, and make like you?re going to cut on a “fold line” and take a few passes… The score should be on the outside of the bend…

Folding technique – using 3 scraps of plywood… Two sheets sandwich the section of sheet metal that will be stationary during the bend… The third sheet is placed as shown below the section to be bent, and butting up against the bottom of the sandwich, meeting right on the bend line… Be careful not to get any of the 3rd sheet below a section not to be bent yet…

While standing on the “sandwich”, pull the 3rd sheet up with confidence… The resulting bend is shown and is very crisp, accurate, and straight due to the score… Don’t bend completely; just as shown… The remaining degrees in the bend will be completed by hand later…

Continue bending… Here, you’ll stand where the hammer is sitting…

A narrower piece of scrap wood is used here for easier bending…

The final bends are made by hand – simply grip the sheet metal with the V in the fold facing your palms and squeeze… It will bend easily, and again, the score will help here… A bottom view of fully folded trap…

Note that the tabs have been drilled and riveted… This would be a good time to seal the seams with high temperature foil tape… I didn?t do this at first (I was trying for 100% metal unit) and ended up with tiny light leaks, so I had to add the tape later… With the airflow through the hood it runs fairly cool anyway… A top view…

The Hood & Insert Subassembly

Transfer the dimensions from the layout to the sheet metal with a magic marker… Again, try to be very accurate and definitely use a square…

The pattern is then cut out with tin snips and scored for bending… Remember, the scoring is important to get a nice crisp bend… Here is the scored and cut sheet ready for folding…

Take note of this image… It is a rectangular section used as an insert, bent into a V shape, for reflecting light above the bulb… This is an important element of the design… Hopefully you can see the detail of the scored lines here… BTW- the fancy Kett Power Snips were borrowed from AF and worked well, but hand snips work just fine, and are required for getting into the little corners anyway…

Image shows the hood and V Insert folded… The bar on the top is used to secure a 90-degree bracket on the inside for holding the light fixture (note the wires sticking out)… It was salvaged from the security light housing…

Rivet the V Insert in place… You can see the 90-degree light bracket on the left… The rivet gun has a fancy swivel head that was more useful than expected, but any one should work ok just fine…

Finishing Up

· Join the two sub-assemblies together… Used a small piece of sheet metal at back end (end of bulb pointing toward it), and fashioned another piece for the end closest you in the picture…

· Paint assembly with flat white HIGH TEMPERATURE paint…

· Install the electronics (ballast, starter, power cord to timer)…
*NOTE: For details on wiring a security light check the “Lighting” folder in the Grow FAQ’s…
The ballast is shown up front here and worked fine for 1st grow, but have since relocated the ballast to the back of the unit where the exit is located, thought it would add a slight bit less total heat, maybe, and no longer use carbon scrubber cartridge…
Looking at the final image at the bottom, you can see the exit with weather stripping around it that mates to exhaust port in cabinet… It is in here that the ballast is now located… Secured ballast with a sheet metal strap… I had a bit of issue with ballast humming… Wedging a metal shim under the ballast to make it snugger eliminated the vibration and it’s practically silent now, even at startup…

· Now might be a good time to test your hood assembly!!! Yeay…

Unit Assembly Installed

· The remaining triangular shaped space on right side is used for power junction box and in front of that the timer… Keep all electronics upstairs and away from fluids… No separate utility room need for this cab!! There is an air space above the hood…

· Install glass shield… Tempered glass is best but I used regular 1/8″ glass; as it’s fairly small wattage and little stress is placed on it… A gap of 3/4″ or so is left at far end for air to enter, pass across bulb, and then flow out hood and over ballast on way out of box… You might note the two channels bent along either side of hood for glass to slide in… Add another inch to the glass width to account for channel it slides into…

· Seal the front of the assembly… Used a material called ” Reflectix”. Version 1 of the cab had Reflectix on the walls and door, and when closed sealed off the front… Now I?m using Mylar, so it?s shown for current setup…

Light On!

The camera was placed on the floor of cab for this shot… Each image is simply a different exposure… The left image gives a good idea of where most of the light goes… It is said little light is emitted from end of bulb, and this picture makes that fairly clear… The right image shows the imbalance in my light distribution a bit more clearly, due to about a 3/16″ misalignment of the bulb over the V Insert… Will tweak that before next grow when glass is removed for cleaning – however, it does demonstrate how important the V Insert is and also that accuracy does matter for some parts of the setup…

Carbon Scrubber Insert

Abandoned this for now…
This little module was made with a flouro light fixture grill cut to triangles and stacked and glued to achieve about 4″ length… Channels are filled with activated carbon for scrubbing odors… It was inserted into back of cab through the opening, and then slid in… It caused too much static pressure for the single fan being used… (I had to go with active intake) So now use stacked scrubber modules on back of cab – oh well, so much for 100% containment with-in the little cab – still stealthy…

Buds in First Grow

Just a shot to show that this light and tiny cab will produce some decent smoke… Goal is to achieve 2oz in this cab, which seems do-able…


Well, there you have it, an air-cooled hood assembly with integral light trap – all for less than $100… This took me a full day and well into the night to design and build, however, the design part was probably 80% of the work… Also doesn’t count a day of searching OG to see what others had done… Could probably build one of these in an afternoon now – or maybe just a bit longer than the time is took to prepare this post!

Works for me! On third grow with this setup and it performs well… The temperature increase is about 6 to 7 degrees above ambient… More fan power should bring this down; the flow is probably a bit low now (only 1 Rotron fan on intake at this point, another tiny one below glass in corner for circulation)… An active exhaust would be an improvement, as there isn’t airflow through the hood when the door is removed… Didn’t have fans that would fit at the time this was built… Have since picked up some smaller Rotron DC fans that will be used to add an active element to the exhaust at some point… I’d recommend this from the start…

  Last modified: 20:13 – Feb 22, 2003  
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faq:1440 “DIY: How can I build a compact hood with an intergrated light trap?”