Shoestring-Budget mercury vapor collecting lamp construction

(or How To Be A Real Moth Collector On clinton-era Military Enlisted Pay)

Joel Szymczyk, Las Cruces NM

 You are visitor number   since 29 Aug 2001 

Anyone who's seen one working knows that a Mercury Vapor lamp is the "way-to-go" for nighttime insect collecting. Cost is the prohibitive factor, along with the need for a portable generator ($400 minimum) or access to commercial AC power (hard to find an extension cord long enough to stretch up into Pena Blanca canyon from Nogales.) If you have the power source problem solved, it's a simple matter to build your own merc-lamp for collecting and save LOTS of money. At current prices in the BioQuip catalog, you would spend just about $200 for a light and extra bulb. Depending on what items you may already have laying around your house or garage, you can get the same result for under $40 and an hour or two of work.

I built mine from parts purchased at Home Depot and a few odds-and-ends laying around the garage for a total cost of $39.24 including tax. If I had to buy a few bolts, hose clamps, etc, the whole cost still wouldn't exceed $45.

Granted, the end result will not be as pretty as a BioQuip unit, but it will be no less effective.

First you will need to buy a "Medium Duty" outdoor mercury vapor lamp assembly for $21.95. This contains a 175W clear merc lamp, socket, ballast, and housing to mount on an outside wall. The only difference between this one and the "Heavy Duty" model is the housing and the price.

You'll throw away the housing, most of the hardware, the photocell (unless you want to incorporate it into your lamp for remote setup) and wiring. What you'll have left is the ballast, socket, and lamp. Pick up an extra 175Watt clear merc lamp just to have on hand for $9.95. Also pick up a 50-foot orange outdoor-type extension cord for $4.95.

That mysterious ballast.

The "mysterious" part of the whole thing is the ballast. All it amounts to is a center-tapped step-up transformer that develops the needed AC voltage from the input standard 115VAC. No moving parts, no active components to fail. Once the merc lamp is up to it's full operating temperature (usually about 5 minutes after you turn it on) the voltage going to the lamp is right around 150VAC for a 175W lamp. The voltage slowly increases from about 20VAC at turn-on, to the 150 or so as it warms up to it's full capacity, and conversely the current draw drops. At full operation after warm-up the 175W lamp draws under 2 amps. Ohm's law in action. It does get warm, but not overly hot if you stick to using one lamp per ballast, and one ballast per container. I used an old 30 cal ammo box to house the ballast.

An ammo box seems perfect, but you could also use a large metal "project case" from Radio Shack, or even a large coffee can.

Make sure you drill a vent hole in the housing if it's a water/airtight box, to allow for expanding heated air to escape and the reverse as it cools down. A small hole is all that's needed.Two holes are needed for the wiring, one from the generator and one to the lamp socket. It's an ungrounded (2 wire) system from the ballast to the light, so ignore the ground (green) wire in the part of your extension cord that goes to the lamp socket when you cut it and strip it down. The piece of cord coming into the ballast box from the AC source (generator or outlet) should be grounded to the case. That means you should connect the GREEN wire in the extension cord to the metal box. The most convenient way is to just strip half an inch or so of insulation off the end, and connect it to one of the bolts you'll use to secure the ballast with. This way, if either of the hot or neutral wires somehow shorted to the metal case, the breaker would trip on your generator or outlet. If this were to happen and you didn't have the incoming AC cord grounded, you could get a very nasty shock if you touched the box with the unit running. The connections are simple; both neutral (white) wires, one from the generator and one to the lamp socket, are attached to one end of the coil; the incoming hot (black) wire goes to the center tap; and the hot (black) wire to the lamp socket goes on the other end of the coil.

You can either solder the wires on the terminals or use crimp-on spade connectors.

Same thing applies to the terminals on the bottom of the lamp socket.I used rubber cable grommets in the 3/8-inch holes I drilled to run the extension cords through, to protect against abrasion. I also used a little clear silicone RTV sealant to secure them. The grommets I had laying around the shop... If you don't have any they are about $0.99 for a handfull at Radio Shack. Run enough wire through so you can easily wire up the ballast before securing it in the box with a couple of bolts and the original bracket clamp, the one that held it in the commercial housing. Pull the excess wire back out before sealing it up with the RTV (sealing it with RTV is not really necessary if you use the grommets.)To enable mounting the lamp on a camera tripod, I simply cut the head off of a long 1/4-20 bolt, epoxied a "threaded-rod joint" (it's just an overly long nut, $0.72 for a pack of 3 at Home Depot if you have to buy them) on the end, and secured the whole works to the ceramic socket with a couple of hose clamps. Of course you can omit this if you plan to just hang the lamp from some support in front of your sheet.

This set-up is 100% effective, and built with the absolute minimum of parts and trouble. You could spiff it up a little with some quick-disconnect connectors on the input and output lines, or a terminal block inside the case for wiring the ballast, or paint the case a pretty color, or whatever you want.

If you are fortunate enough to live somewhere with good collecting right outside your door, you could mount the photocell on the box so you don't have to worry about shutting the lamp off before going to bed. Just for info, the lamps I bought have a rated life of over twenty-four THOUSAND hours... Set it up, plug it in, and away you go.The left-over $160.00 you saved will buy a lot of gas to get to new collecting areas, or buy something nice for the wife to make up for the nights you spend out chasing bugs.

Why is it that the Mother-in-law can't ever really accept the fact that her daughter's husband is out all night REALLY catching moths?

Comments from Eric Hossler (

I built an MV lamp according to Joel Szymczyk's directions. However, I have one possibly useful modification.

His recipe calls for a 30 cal ammo box, which would house the step-up transformer. I used the next larger size ammo box; it might be a 50-cal or something. Anyway, there are two advantages to using this only slightly-larger size.

One is, with a few inches more "elbow" room, it's easier to connect the wires if you're no electrical engineer.

Second and more importantly, there is just enough space within the box to "store" your 175-watt MV bulb. I've got a 2'x2' piece of bubble wrap (like you use for shipping) that I just wrap around the bulb. Then the bubble wrap fits snugly into the larger ammo box. It closes nicely and the bubble wrap keeps the bulb from rattling around. So I don't need a separate container for the bulb. The only drawback is that after using the unit for a night, you can't just stick the plastic bubble wrap in the box; it would melt. So I let it cool for 30 min and then put the whole unit away.

I find the larger size very convenient on trips for keeping my bulb safe, never having to worry whether I remembered to pack it or not.

Thanks again for those wonderful directions. I could never have afforded an MV light without them!