Mobile Short-Arc Searchlight

A High-Pressure Mercury Short-Arc Searchlight delivering approximately 20,000 lumens and 9,300,000 candela, achieving a beam range of up to 6 kilometers.

#Disclaimer#

You will be working with high voltages (340VDC and up to 15KV AC) and you will be handling a bulb that gets very, very hot and emits lots of UVC

Tools Needed:

1. Soldering iron and solder
2. Multimeter
3. Screwdrivers
4. Wire strippers
5. Drill and bits
6. Safety equipment (gloves, goggles)

Materials Needed:

(Anything underlined can be scrapped from a projector)

1. Light Source: 250W short-arc mercury bulb with parabolic reflector - i will cover how to source this below
2. Power Supply:
3. Full bridge rectifier
4. ~470µF 400V capacitor
5. 1MΩ resistor
6. 240VAC to 12VDC adapter
7. Ballast: Compatible with the chosen bulb
8. Housing Materials:
9. Wooden boards (e.g., planed pallet wood)
10. suitable crate/briefcase/box
11. Cooling System:
12. 12V blower fan X2
13. Large heatsink/CPU cooler
14. Control System:
15. Generic Arduino
16. 12V switches
17. Generic diode and MOSFET
18. 12V SSR
19. Wiring: Appropriate gauge wires and connectors
20. Miscellaneous:
21. Screws and brackets
22. Adhesives
23. Heat shrink tubing
24. DFplayer pro and suitable speaker
25. 3 state switch

(A detailed guide to finding UHP bulbs for cheap)[https://github.com/ruckusmattster/searchlight/blob/main/find-projectors.md]

Short overview:

Look for Older Projectors:

1. Search for projectors using UHP lamps, these will have a high voltage and heat label somewhere on the body.
2. Common sources include eBay, local auctions, or surplus stores.

Preferred Models:

1. Try to find one with a parabolic reflector and a suitable housing

Inspect Before Purchase:

1. Check photos or descriptions for verification that the projector does power on and light comes out, you can disregard image quality issues

The only parts you want to keep from the projector will be the bulb, ballast, power supply, fans, and wiring. Everything else can be binned/recycled.

Everything else

Everything else can be sourced from amazon or hobby stores

I used a hard-shell briefcase to make this, the only modifications needed were 6 holes, 2 for the switches, 1 for the LED, 1 for the AC wire inlet, 1 for the bulb, and 1 for the cooling fans' intake.

I mounted everything with "sticks like sh*t" which is a silicone adhesive, its very secure but also very difficult to service.

I freehanded most of this with a dremel and carving attachment but a hole saw would've been easier. I added two metal plates in front of the bulb, these aren't necessary but i liked the aesthetic and it helps to cover some dead space.

Something to consider is that the weighting of the overall case will really impact comfort. Mine is back heavy, so it naturally points towards the sky, reducing strain on my wrist.

For my intake fan i used a blower fan taken from the projector, it provides cold air from outside to the bulbs housing where its used for cooling.

It's wired to the 12V power supply via a switch.

Build the Power Supply

1. Solder the full bridge rectifier with the 470µF capacitor (or other suitably large capacitor from the projectors power supply) and 1MΩ resistor in parallel onto a PCB or "deadbug" to create a stable 340V DC output from the 240V AC input.
2. Test the output voltage with a multimeter to confirm proper operation before connecting it to other components.
3. Insulate all exposed connections using heat shrink tubing or insulating tape.
4. The full bridge rectifier converts AC into DC and the capacitor smooths the output, the resistor is for safety and will slowly discharge the circuit after powering off.
5. This will connect directly to the ballast so make sure to reuse the correct plug from the original projector when connecting it.

Configure and Install the Ballast

1. Typically the ballast wont turn on unless controlled by the projectors motherboard, but this is easy to bypass
2. Look for 3-5 optocouplers near the input of the ballast, you will need to short the output of this, this is covered in the github
3. Use the SSR and a 12V switch to connect and disconnect power to the ballast

Safety mechanism

1. You can use a diode connected between the two switches to make sure that when the bulb is powered the fans will receive power too. This ensures the bulb is always cooled.

12V Systems

1. Check that all your fans, arduino, and LED are wired to the 12V supply and check everything with a multimeter. The fans should be in parralel.

The secondary LED is controlled with the arduino and a mosfet, i use PWM to set the brightness, its not the best method for sure but its cheap and easy. A better solution would be a buck/boost module with constant current. to control the speaker I connected a dfplayer proi via uart to the arduino, it has built in storage for all the media files - right now it plays the ghostbusters theme song

Power everything on and be careful not to stare into the bulb, its generally unsafe due to the brightness and intensity of light. You can use a wattmeter to measure the power usage and if it's close to the bulbs rated power then youre all good, if its close to half you've probably enabled eco mode and need to rejig your optocouplers.

Make sure all your fans spin up and provide sufficient airflow, check that the ssr correctly opens and closes, check that your switches open and close, and check that no components are getting too hot (USE A THERMISTOR OR IR CAMERA NOT YOUR HANDS)

Depending on the diameter and power of your bulb it can have anywhere from 2-10km of range realistically. If you want to measure the range you will need a lux meter and ~20m of space but i won't cover that full process, you can find guides online like this thread https://www.reddit.com/r/flashlight/comments/wv0ucy/how_is_the_throw_measured/

Here's some photos of my testing, we got very lucky with heavy fog.

Since the build process really heavily depends on what parts you get its difficult to write a comprehensive guides. i have included some links to my build thread, some photos, and my youtube to help you along the way.

https://github.com/ruckusmattster/searchlight

https://budgetlightforum.com/t/finished-ruckusmattster-s-12th-annual-old-lumens-challenge-entry-hand-made/225844/4

https://photos.app.goo.gl/rBuQnaPYmjDiugSS7

https://www.youtube.com/@Ruckusmatter