Marker Distress Light

Brooke Clarke, N6GCE  2001 - 2019

General Information
SDU-5/E Sinks
Lens Covers
Similar Product
46 CFR 161.012 Specifications for Personal Flotation Device Lights
Power Sources
Test Equipment
    Normal Operation
    O-Ring Fit Problems
    Strobe Not Flashing

General Information

The SDU5/E Marker Distress Light is used by people trying to be rescued.  It has been replaced by the MS2000, but the MS2000 requires two hands for operation so the SDU-5/E is still the official marker light for some applications.  It is designed to take the obsolete BA-1574/U Mercury battery.  NSN 6230-00-067-5209

There is some operation instructions in the manual for the  SRU-21/P Survival Vest TM 55-1680-351-10.  The battery has a 3 year shelf life in temperate climates.

The back side of the marker light has a 1 1/2 x 3" patch of Velcro loop material to allow fastening the marker light to a mating piece of Velcro hook material.  This is not how a SDU-5/E ships and was added by a prior user.

The SDU-30 is a smaller Marker Distress Light that uses a 3 Volt flashlight bulb and a special lens.

In the movie "Blackhawk Down" an "IR Strobe" is used to mark an enemy position.  The IR cover is held on with 100 MPH tape.
The hero of Bourne Identity is rescued in the opening scenes of the movie because he's wearing two SDU-5/E strobes strapped one on top of the other, but pointing in opposite directions.  I could not see this on the VHS tape, but can easily see it by capturing a still frame from the DVD.  In the movie "Behind Enemy Lines" near the end Owen Wilson sees a SDU-5/E, picks it up and without removing the battery cap shakes a 9 Volt battery out of it.

Email from Ed in response to a question about the SDU-5/E on the Army Radios Yahoo group:

I have several of those lights and I really like them.  I got some battery adapters from Brooke Clark, (who is on this list), that make these strobes a lot cheaper to operate!  Brooke makes battery adapters for about anything that takes a battery to operate.  Most likely he has a few in the space station, "just in case".    All of his products are top rate, 1st class and I've never had a problem with any that I have purchased from him.
The light that you mention is the old type safety strobe that was used by all sorts of Troops from Pilots to Special Operations guys and sometimes just us Troops on the ground.  I used one in Vietnam that had another cover so we could put it over the white light and make it another color, red or IR I think, to keep it from being mistaken for ground fire when we were trying to call in air support at night and we wanted to mark our position.  I just hated it when our guys bombed us, ruined our whole day!  Thank God it didn't happen that much!  The fly guys did one hell of a GREAT job for us 99% of the time.  When I found one in a bar back when I drank they couldn't pay for a round, not while I was around!!  I still feel the same but I don't go to bars anymore.  Back then we didn't have smart bombs or GPS or any of the neat toys our guys have today so we used what we could to stay alive!!  It has been replaced by the model 2000 I think it's called, I have a few of those too, they come with the shield built in and use two AA batteries to operate.  They are waterproof to a certain depth, I forget what it is, but they are a giant improvement on what we used.  They were great at the time, the very best and have lasted for years but the newer ones had the advantage of taking everything that we bitched about and fixing them to take care of most of the bad stuff.
Go ahead and get an adapter from Brooke and save a lot of time and money and pain trying to find the correct batteries for your strobe.

Best Regards,


From - Tue Apr 22 17:31:16 2008
Hi Brooke,
I was Army, got out in 1971, did two tours in Vietnam, 1968 & 1969, both with the 1st Cav.  The 1st was mostly as a 60 gunner since I'm so big, 6'5" & then around 220 lbs.  2nd tour was in the 1st Cav again, this time in Hueys 1st as a Door Gunner and then as a Crew Chief/Door Gunner, with the 1st of the 9th Cav, 1st Cav Div.  I made it all the way to SSG E-6 which was a neat pay grade as far as I was concerned.
Feel free to use anything that I say about your products.  I call them as I see them, always have, always will and you make some fine products without a doubt.  That's kept me from having as many friends as one would think but if they don't speak the truth and don't want to hear the truth what kind of friend would they be?

Best Regards,

Ed Kirkley SSG E-6
US Army, Medically Retired
Vietnam 1968 & 1969


Electrical Check

Using a Fluke 87 DMM (or other DMM with a diode test function, but check lead polarity) and testing a good SDU-5/E by touching the Red (+) test lead to the threaded brass contact and the Black (-) lead to a small metal screw driver sitting on the pin in the bottom of the battery compartment the Forward Voltage drop is 1.07 Volts.

Note: Using the Ohms range you get an open circuit indication with the Black to Brass and Red to pin.
          With Black to pin and Red to Brass you get about 2.5 Meg Ohms.
          Be careful not to touch both probes at the same time since your skin resistance is also in the Meg Ohm range.

Functional Check

You can install a couple of 123 photo batteries with the positive contacts pointing up and then jumper the positive terminal to the brass ring on the strobe.  You need to wait a second or two for the first flash.  If no flash press the switch one time and try again.  Aluminum foil or a screw driver can be used if you don't have a cap.  With the strobe aginst your ear you can hear the whine as the cap gets charged just like on a camera strobe flash.

Switch Boot

The rubber boot covering the on-off switch can be removed by just unscrewing it.  There is a small ridge molded in the rubber that acts like part of an O-ring to make a water tight seal.  But if the boot is partially unscrewed then it would leak.  You might be sure it is snug and/or apply a small amount of Radio Shack Lube Gel or other Silicon grease to the mating surface if you are concerned about water tightness.


TM 55-1680-322-12 although listed on ETM it is restricted
Y1-01-0353 Installation, Operation, and General Characteristics, September 15, 1973. (authorized per TSO-C85) for the ACR-4F

If you have a manual let me know.

SDU-5/E Sinks

The SDU-5/E is designed to be attached to a Personal Flotation Device (PDF) and it does NOT float in fresh water.  With a stock BA-1574/U Mercury battery it weighs 6 oz in air and 2 oz when completely submerged.  When the 5BA battery adapter and two 123 type photo Lithium batteries are installed the strobe weighs 6 3/4 oz in air and 3 oz when completely submerged in fresh water.  This is why the SDU-5/E has a lanyard that exceeds 30 inches in length 161.012-7k


Model No. FED SDU-5/E
Serial No. 020595
Type Light Marker, Distress
Cont No. DLA-400-89-F-0490
NSN 6230-00-067-5209
Fed of Handicapped, N.Y. 10011, U.S. (212) ? their pay phone is (212) 929-5919
I have seen that other SDU-5/E lights were made by Neo-Flasher in N. Hollywood, CA.


After carving the outer case off the the internal potted assembly I can say how the SDU-5/E was assembled. The circuit can not be traced but it looks like a 2.2 uF-250 V cap is the main energy storage unit.  The PCB has a transformer that is part of the oscillator that steps up the 6 volt battery supply to one or two hundred volts.  There are a couple of transistors, one for the oscillator and one to do the timing of the flashes.  The trigger coil is close to the flash tube partially in the air space.
Strobe Lights and Design Guidelines, Useful Circuits, and Schematics - has some circuit diagrams that will be close to this one.

Lens Covers

Flash guard/Blue Filter FG1C NSN6230-00-401-2285

When the SDU-5/E is used in a combat area there are important considerations relating to both friendly and unfriendly combatants.  The Blue Filter addresses some of these.   This Blue filter is part of the survival vest SRU21. First it causes the flash to appear Blue in color rather than white.  This will make it clear to friendly forces that it is not muzzle flash and that they should not shoot back.  Second there is a shroud that allows the light output to be aimed at the rescuer, assuming that you know where to point it.

The Flash guard can be stowed over the SDU-5/E when not in use.  The Blue filter material is flexible plastic that will deform in or out so that this type of storage can be done.  Note that with my 5BA Battery Adapter installed the Flash guard will still fit onto the SDU-5/E.

Another consideration when trying to be rescued and at the same time evade an enemy is night signaling.  For this purpose there are Infra Red filters that can easily be seen with night vision equipment but not by enemy troops with their eyes alone.

Amherst Drop Zone carries the Blue  filter - out of business
Wardens Supply Co -

IR Lens Cover

Designed for signaling at night with an enemy nearby.  Is used as a marker IR strobe as in the movie Blackhawk Down.  This is an Infra Red filter that completely hides the visible flash, even when looking directly at the strobe when close.  If the IR cover is placed over the end of a TV remote that uses IR the remote will work.  When viewed with M18 IR binoculars you can see very bright flashes when the strobe is in the pouch.  In daylight the IR flash is dim when viewed in a camcorder, probably my camcorder has an IR filter. The IR flash is just as bright when the strobe is in an inside pocket when viewed by the M18 binocs.  There are no markings on the IR Lens Cover.

Amherst Drop Zone carries the IR filter

Red Lens Cover

Red Filter
It was made for MP/AP use to mark runways and LZ's at night.

Similar Product

ACR sells the Firefly 4F and Firefly 4G strobes that appear to be equivalent.
The 4G has a magnetic slide switch and is intended for use underwater.
Some 4F specifications:
Waterproof:          4F - Waterproof to 50 ft (15 m) and shock resistant
Visibility:              5 mi (8 km) line of sight on a clear night from overflying aircraft
Certification:        Complies with SOLAS 74/83; USCG 161.012/1/0, 161.112/2/0, Canadian CG approved; 4F is also FAA (TSO-C85) approved
Operational Life:  Minimum 8 hours continuous to meet USCG specifications

SDU/SDK - Signal Devices also includes the SDU-30 and SDU-30 Marker Distress Lights

46 CFR 161.012 Specifications for Personal Flotation Device Lights

46 CFR 161.012 PDF -
storage for 24 hours at 149 degrees F, allowed to cool to room temp,
then at -22 degrees fahrenheit for 24 hours per 161.012-10f, allowed ot warm up to room temp,
then submerged in 59 degree fahrenheit water and operating for 8 hours and flash between 50 and 70 flashes per minute - 161.012-11c

Power Sources

5BA Battery Adapter

This is a cap designed to hold 2 each commercial 123 photo batteries.
See my Products for Sale for pricing


The BA-1574/U ia a 5.2 Volt Mercury battery has the brass cap with threads as part of the battery, not a separate cap.  The end that goes into the marker light has what appears to be a white rubber surface.  The marker light battery compartment has a pin that penetrates the rubber to make contact with the battery.  NSN 6135-00-073-8939 [0.645" dia x 2.275 long  to the o-ring]
The markings on the battery are:
Battery, Dry
Battery Assm, Inc.
Bohemia, N.Y. (11716)

02 90


This is a modern Lithium Manganese Dioxide battery for the SDU-5/E.  It is made by Bren-Tronics (BT-70430) and Unicor 6135-01-455-9846).  This is the same chemistry used for the 123 photo battery that is used with my 5BA battery adapter.  These batteries can NOT be tested using test sets designed for the BA-1574/U Mercury battery.  The physical appearance of the BA-5374/U is just like the BA-1574/U, only the label information is different.  The metal cap is still part of the battery.

K-304 Submarine Alkaline battery

K-304 Alkaline Battery Installed on SDU-5/EMercury and LiSO2 batteries are not allowed on board submarines.  If there was a fire and a Mercury battery was involved, poison gas would be released, not a good thing on a sub.  A LiSO2 battery can vent SO2 gas for a number of reasons.  This too is a bad thing on board a sub.  So an alkaline battery was made for the SDU-5/E.  It is marked:
Model No. ACR K-304, FSCM-18560
p/n S4-01-0003A
Type: Battery, Alkaline
CONTR. No. (blank)
NSN:  6135-00-877-1659, Data of Mfg. (blank)
ACR Electronics, Inc.
Fort Lauderdale, FL 33312

There is a 5/8" diameter protrusion at the top about 1.5" long that goes into the battery compartment.


A replacement might be made from a PX28L? battery with it's positive end in the cap and a 7/8" spacer to make the length correct.  This is described in the book Power Up.  But the PX28L is also a Mercury battery that is no longer available.

Vehicle Power Adapter NSN 5995-01-335-3369

  SDU-5/E Vehicle Power
This is a kit of parts made by ACR Electronics, Inc. that contains an adapter that screws into the battery compartment and has a cable with red and black wires inside.  In addition there is a separate voltage regulator that will drop vehicle DC power (either 12 or 24 volt systems) down to 5 Volts DC to power the marker light.  There are also numerous wire joints, a pair of battery clamps, tie wraps, Velcro hook and loop materials.

Test Equipment

BT-2B Battery Tester

This tester will test the BA-1574/U as well as other Mercury batteries designed for survival radios like the PRC-90.

TS-23 Light Output & Battery Tester


Normal Operation


There is a pin in the bottom of the battery compartment that will cause a dent on the bottom of the bottom battery.  This is normal.  When finger tightened the stock O-ring should be slightly compressed.

O-Ring Fit Problems

There are a couple of reasons the stock O-ring is not compressed.

Bad SDU-5 Threads

To check:  Remove both batteries and remove O-Ring from cap.  Screw cap into empty SDU-5.  Use stock O-Ring as feeler gauge to see if gap is wider than O-Ring.
If gap is too wide return SDU-5 to me with return postage and I'll chase the threads in the strobe.

SDU-5 Tolerances

If your strobe threads are OK (see above) but there's still a small gap with the stock O-ring you need the optional Fat O-Ring.  Ask and I'll send one to any of my customers.

Strobe Not Flashing

s/n: 05170
c/n: D5A-400-75-F-4009
MFG: Federation of Handicapped

A clip lead from the bench supply positive to the brass threads on the strobe.  A clip lead from the negative supply to a brass rod sitting in a plastic tube, where the plastic tube has an ID slightly larger than the diameter of the point in the bottom of the battery compartment.  Working the switch shows the off position at no current and the on position was at about 180 ma, yet there was no flashing.

After sitting powered up for a few hours there were periods of flashing with about a 4 second interval (two seconds is more normal) and there was no "snap" noise with the flash.  After a few days the interval is close to two seconds and there is a "snap" with each flash.

The theory is that the High Voltage cap electrolyte has lost it's polarization and needs to see HV for some time to reform.  When the strobe is running the HV ramps from near zero to maybe 350 volts and back to near zero, it does not stay at the HV value.  So it takes much longer to reform than it would if the HV could be applied as a DC value.

This also suggests that instead of leaving the strobe on the shelf for many years and expecting it to work when needed it would be much better to run the strobe until the batteries are dead.  This might be done now if it has not been run for some time, and once every five years thereafter.

Note:  It may take 20  seconds to an hour without any flash for the strobe to start flashing if the HV cap needs to be reformed.

2 Dec 2008 - This strobe has been running for a month.  When the power is removed for a few hours and then turned back on it takes 80 seconds before the first flash.  So the long operation time has not reformed the capacitor., or maybe it has reformed as far as it can?  Must likely the HV capacitor is bad.

There are about 42 flashes per minute on my strobe.


2500809 Automatic radio transmitting apparatus, Fennessy Edward, Roberts Graham Estyn, Decca Record Co, 1950-03-14, - custom record for each vehicle.

2552969 Telltale radio signal device, James P Holman, 1951-05-15, - transmits upon impact.
2803838 Aircraft
                  crash marking device, Jr Nathaniel B Wales,
2803838 Aircraft crash marking device, Jr Nathaniel B Wales, 1957-08-27, - not SDU-5. - flashing light that floats
Here because Wales has patents on the Bazooka and Proximity Fuze.
2192450 Position indicator for airplanes - tethered balloon is released & inflated
2380587 Distress signal device for aircraft, etc.- tethered balloon is released & inflated
2459267 Self-contained emergency signaling device adapted to be operated automatically after being launched - water dye packs
2674726 Ambient light-controlled flashing signal

2478908 Electric light-flash-producing system, Harold E Edgerton, 1949-08-16 -
Strobotac, Sonar & Krytron
Harold "Doc" Edgerton

3102982 Air rescue transmission apparatus, Walter C Stypulkowski, Kenneth C Jacobs, Aeronad Electric Corp, 1963-09-03, - light or radio beacon.

3623681 Aircraft location marker, Clifford E Ashline, Res Q Mark International, 1971-11-30, - streamer


Mike Murphy - often has the SDU-5/E as New Old Stock
Bren-Tronics - BT-70430 is a LiMnO2 replacement
ACR Electronics - LLB1 Lithium battery comes with a seperate cap. Directly replaces mercury 4RM1B battery
Flight Helmet  - Survival Gear - remember to press "NEXT" at the bottom of a page
Amherst Drop Zone sells the IR Lens Cover - out of business
Flight Helmet - Carries a lot of aircraft survival items

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