FRED rear end LED - with optional power sources

Chapter-3e Page 15

A Flashing Tailend Light

The flashing rear-end device, part of the "FRED" box, is mounted on the end of North American freight trains that run without a caboose. One can be constructed from a small LED, mounted in a model TIBs box on the last wagon. It could then be wired back to the electronics and battery etc. hidden out of sight in a box car for example.

There was an Integrated circuit available a few years ago that flashed an LED from a 1.5 volt battery and needed only a capacitor to control the flash rate. The battery had a life of several weeks so it could be left running all the time. However it is not available now and a 1.5 volt battery will not work an LED on its own with sufficient brilliance. This flasher will do the same job with two more components and can be run from a variety of sources and recharged from the track power.

It can be used for a hazard light on the cab roof of a diesel loco, a road hazard light or wherever you need a warning light on. If you run on DC the 1K charging resistor should be changed to 680 ohms but the loco will need to be run at more than half speed to get full charge current. There are several ways to provide the battery power. Two standard AA or AAA cells could be used without the recharging part. On test one has been running for seven days on two recharged alkaline cells. A good source of small 3 cell batteries giving 3.6 volts is an old cell phone, provided it wasn't thrown out due to weak batteries. Or you could buy a cell phone or computer mother board rechargable Li-ion cell, they tend to be a little expensive but should last for many months if switched off when not in use. The new metal hydride type batteries now becoming available do have a higher capacity and will need a higher charging current which can be got by changing the 1K resistor to 560 ohms. A 5 volt 1 Farad capacitor with a 3.4volt zener diode across it will act like a short term battery and charge up in minutes when set on the track and last for an hour or two when taken off the track. The timer I.C. used is a TLC555 CMOS low power type that will run on 3 volts and it directly drives the LED. A switch is included in the circuit as the power drain is about 5 to 8ma. The flash rate can be varied by changing the 20K ohm resistor. As shown it will flash about 3 times per second. With a 27K resistor the rate would be 3 flashes in 2 seconds. Construction is on a small strip

board and the 3 pairs of wires coming from it are for the battery, switch and DCC connection. If you are using it as a tail-end flasher then the power will have to be picked up from the bogie wheelsets with small wire wipers contacting the metal wheel rims or the axles.

The strip board diagram shows the I.C. TLC555 shaded so the link wires underneath can be seen. The pin 1 position is shown . The I.C. will have a small dot or depression beside this pin The capacitor can be bent over to lie on top of the 20K ohm resistor and link wires so the overall height is kept as small as possible. Observe the + lead position, the longer lead wire is usually an indication of the + (positive) lead on the capacitor case. Do solder the link wires in first, but if you don't they can always be soldered underneath later.

Chapter 3e-2 Page 16

The component layout for the full circuit. The 10ufd electrolytic capacitor has been Laid into the space above the 555 IC

If only a battery will be used then the size of the board can be smaller. The battery leads are on the left (Red and Black) and those to the switch are on the right. Note the capacitor is a tantalum type which is a lot smaller. With smaller components and some redesign the board could be made even smaller Resistors 1 of 1000 ohms, [1K0] 1 of 20,000 ohms, [20K] 1 of 100 ohms [100R] Capacitors one of 0.1 ufd one 10ufd 16 volt tantalum or 10ufd electrolytic capacitor a TLC555 integrated circuit timer one diode 1N4004 a small on/off switch 10 by 10 hole strip board.

If you are not charging a battery

or running on a DC track then the circuit board can be modified by not installing the diode and 1000 ohm resistor and the board size reduced. A smaller cut down PCB can then be made by removing the last 3 rows. The battery connects to row 3 hole 1 positive and the negative to row 4 hole 1 without changing any other components.

The LED shown is a 2 mm base with a 1.2 mm stud. Which when scaled is a 18 centimeters square and 10 centimeters stud. Close to what is needed for a Fred Box.