Decoder Circuit Additions

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Two Function Bias Circuit for 1.5v bulbs

Bulbs: Miniatronics 1.2mm, 1.5v, 15 ma
Decoder: TCS T1.

2 function bias circuit

Typical uses: GPs, SDs and U-boats with standard dual lamps front and rear, as well as RPP CF7s as shown here.

1.5V bulbs need more than the half power provided for Rule 17 dimming to show adequately when dimmed. In fact, most 1.5V bulbs are not usually visible at half power. This circuit provides a half-power bias at all times, so the the lamps receive 3/4 power when Rule 17 dimming is applied.

The bulbs used in this circuit are 1.5v, 15 ma "grain of rice" type. For bulbs rated at 1.5V and 30 ma, use 750 ohm resistors. For single 15 ma bulbs, adding 100 ohms to the 1500 ohms in series with the white and yellow leads may be needed. For single 30 ma bulbs, increase the 750 ohms to the higher value of 820 ohms.

The two diodes (1N4000 series up to 1N4004 are ok here) are needed for most 2-function decoders to provide a negative common supply for the bias currents. For other decoders, such as a TCS A1 or T2, that have an extra function, you can turn it on all the time and connect the common there. For decoders that bring out the common lead, that will work just fine also. In either of these cases, the diodes would not be needed.

The example shown below uses the Rail Power Products CF7 shell and frame with Athearn drive train.
The front of the unit is to the left in both of these photos.

The anodes of the two diodes are shown connected to the rail bus wires. Two resistors go in each direction fore and aft from the common junction point. The wires are controlled by segments of two-hole coffee stirrers, and the small segments, over the right-hand flywheel, will be used to help control the positioning of the TCS T1 decoder, holding the red and orange wires on the right side and the black and gray wires on the left

Shorts are avoided by liberal use of shrink-fit tubing at wire-to-wire joints, soda straw segments covering the two resistor strings and a final taping at the center. Each lamp wire will be folded once when installed in the unit, so in this picture, the forward lamps stick out to the rear and vice versa. The lamps will be taped in place individually inside the shell.

The results...

CV settings:

CV	Value
49	40
50	40
61	32
33	1
34	4

Bright Forward	Dim Reverse or Function 4

Circuit, text and photo ©2004 by Jim Fuhrman. Used with permission.
Dual Fleet Lighter for dummy F7A units This circuit uses 2 FL "Fleet Lighter" decoders to provide the main headlight along with a white gyralight (both used going forward) and red gyralight. (used going backward). The fourth function is used to turn off the forward gyralight when the F4 button is engaged. Since the forward and reverse gyralights, powered by FL #1, are mutually exclusive and are both 15ma 1.5V bulbs, they can share a common 750 ohm dropping resistor, which is shunted to the negative common by the fourth function lead (yellow on FL #2). FL #2 controls the main headlight, which is a 12-14 V 30 ma bulb. A 47 ohm resistor in series with that bulb helps to limit inrush surges when the lamp is turned on. FL #1 was programmed first then locked. FL#2 was then programmed out of the unit and subsequently plugged in, and did not need to be locked. Active only when going forward, the main headlight is turned on by F0 and dimmed by F4. The gyralight is turned on by F1, and shunted off by F4.
Yes, those old rubber-band drive frames you see at swap meets can be put to good use!

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