DT Tx21 Transmitter Kit

This kit includes the core components to make a Tx21 transmitter.
Tx21 is a compact wireless transmitter intended to control model railway train engines.
It has controls for Speed, Direction, Inertia, Coupling and Lights (depending on how used).


1Small plastic box
12-way Direction toggle switch
1On/Off pushbutton with Led
1Bind pushbutton
210k pot 300° rotation with center click (detent)
1Medium Speed knob
1Small Inertia knob
210k resistorBrown-Black-Orange-Brown
12k4 resistorRed-Yellow-Black-Brown-Brown
17k5 resistorViolet-Green-Black-Brown-Brown
1PP3 plug
50mm30mm heatshrink for Tx2
1Plastic sheet for battery compartment
1Wiring diagram
1Drill template
1This page (www.DelTang.co.uk/tx21-kit.htm)

PP3 9v battery, hookup wire.
Solder, Iron, Multimeter, Glue, heat-shrink tubing, Drills, Files, Knife, Screwdrivers etc.

Tx21 circuit

Tx21 layout

ASSEMBLY EXAMPLE (click to enlarge):

1. The circuit diagram shows the required connections. The photos are an example of how it can be done.
2. The 'Tx2' radio module is powered with 9v. Tx2 has an on-board regulator which provides a 3.1v reference voltage to the other components.
3. 2 resistors can be installed on the Tx2 module to control Pin2. This allow Tx21 to be used with receivers that have 'Selecta' enabled. Selector is controlled with Ch2 and these resistors hold Ch2 at a fixed position.
4. The 2 longer wires in the middle of the on/off switch are 'Led2'. The shorter of those is marked '-' and goes to negative.
5. The on/off switch has an anti-rotation ridge that makes its diameter larger at that point. File a notch in the box to avoid crushing the switch.
6. The pots have a protrusion to prevent body rotation. These are not needed and can be cut off with side-cutters or hacksaw.
7. The Tx2 module can be protected with with the heatshrink covering provided.

The motor control pot (Ch1) has a 'click' at the center position. When Throttle is used 'center off' it needs to center accurately so that the 'click' on the Tx matches the 'off' position in DelTang receivers. Calibration is done using the Bind button while Tx21 is on. After pressing the Bind button for ~20s Ch1 is calibrated.

1. Switch Tx21 on. The led will come on.
2. Rotate the Inertia button fully anti-clockwise (left) so it has no effect on Throttle.
3. Center the Motor pot.
4. Press and hold the Bind button for ~20s until the led goes off. Ch1 should now be calibrated.
5. Release the Bind button. The led will come back on.

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