K-27 Electronics

Tender electronics

There is a dummy board in the socket on the main board. There are several decoders that will plug in.

Also, there is a row (at the top of this picture) where you can solder in wires to connect to the board if you have a decoder that does not plug in. (I wonder if the spacing would allow soldering a bank of screw terminals)

Below is the famous Bob Grosh schematic, but it appears that he did not label the row of connections, unfortunately.

Interfacing to the chuff circuitry.

The Bachmann chuff circuitry has 2 optoelectronic sensors triggered by a baffle in each cylinder.

(see the links below for the schematic and wiring diagram)

The resistors are pullups to positive voltage, and then are "overridden" when the phototransistor is illuminated by the led in the sensor. When illuminated, the phototransistor conducts and pulls the voltage to ground.

So, you have positive supplied when no piston, and the output is pulled to ground when the piston is passing the sensor.

To connect the 2 sensors together, you typically use diodes. The bachmann circuit uses them in a way that would not be the "normal" way. This connection causes the output to source voltage most of the time, and be in sort of a neutral state when the piston passes the sensor. So, what you would connect here would be an input that can "pull itself" to ground, i.e. an input to a sound card that reads high as being supplied voltage, and the absence of voltage as low.

Again, normally low, goes high when you want a chuff.

Most sound cards want to be "pulled low" for a chuff, so in these cases you need to invert the sense of the chuff output, you can do this with a transistor, an NPN one. Put the chuff output (J1-5) to the base of the transistor, and the emitter to ground (J1-7). Then you use the collector to provide the low going input to the sound card.

A 2N2222 or other small signal "switching" transistor can be used.