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Daniel Spilker

November 8th, 2008

Wireless S88 Feedback Module

My temporary model railroad layout stretches from the office through the hall into the living room. The PC which I use to control the trains is in the office. How do I detect that a train arrived in the living room without any wires running across the hall? I designed a RF transmitter module and a RF receiver module to monitor contacts remotely. The transmitter can sense 4 contacts, encodes the state to a serial data stream and transmits the data. The receiver decodes the serial data into 4 separate lines and feeds them into the S88 bus. The receiver is a semi-complete S88 module and can be connected directly to the S88 bus, but has to be the last module on the bus. The transmitter takes power directly from the track and needs no additional power supply.

It uses a bridge rectifier and a voltage regulator to transform the track voltage to +5V and decouples the sensed contacts with an optocoupler from the encoder/transmitter part. I choose the Linx ICs and modules because of the small package, simple surrounding circuitry and interoperability. The transmitter/receiver modules have the advantage of 8 selectable channels. I hope to reduce the size of the transmitter so that it will fit under Märklin C-track. The transmitter module is connected to a SMD chip antenna and the receiver uses a 1/4 wave length wire antenna. It works well over a distance of 5m and through 1 wall.

The advantages of this design is that it the data is fed directly into the S88 bus so that no modification to existing control hard- and software are required. Furthermore it requires no microprocessor programming so that you can put the parts together and you are ready to go. By adding more transmitter/receiver pairs it is possible to monitor up to 32 contacts.

On the downside, one need to have an extra transmitter/receiver pair for every four contacts to sense. It will get rather expensive when many contacts should be monitored. Despite the advantage that the data is feed directly into the S88 bus, it is also an disadvantage. The S88 is rather unstable and I am not certain how many current one can pull from the S88 bus. The receiver module pulls 20mA which is more than a usual S88 module pulls. But it works fine on my Märklin 6051 Interface.

Transmitter Module

Connect pin 1 of JP1 to the brown wire of the digital power circuit and pin 2 to the red wire. Connect JP2 to the contacts to monitor.

Parts

B1 Bridge Rectifier 50V 1.5A DIP-4 DigiKey DF005M-ND
IC1 Voltage Regulator 78L05 TO-92 DigiKey MC78L05BP-APMSCT-ND
IC2 Optocoupler AC 4-Channel DIP-16 DigiKey 751-1372-5-ND
IC3 Linx LS Series Encoder DIP-8 DigiKey LICAL-ENC-LS001-ND
JP3 Linx RF Transmitter 900MHz 8-Channel DigiKey TXM-900-HP3-PPO-ND
C1 Capacitor Tantal 330nF 35V DigiKey 399-4593-ND
C2 Capacitor Tantal 100nF 35V DigiKey 399-4596-ND
R1, R2, R3, R4 Resistor 100k 0.25W Metal Film DigiKey 100KXBK-ND
R5, R6, R7, R8 Resistor 200k 0.25W Metal Film DigiKey 200KXBK-ND
ANT1 Ceramic Chip Antenna 916MHz SMD DigiKey ANT-916-CHPCT-ND

Receiver Module

Connect JP1 to the S88 bus. Pin 1 is +5V, pin 2 is reset, pin 3 is load, pin 4 is clock, pin 5 is ground and pin 6 is data.

Parts

IC1 4041 8-stage shift register DigiKey MC14094BCPOS-ND
IC2 4043 Quad R/S Latches DIP-16 DigiKey MC14043BCPGOS-ND
IC3 Linx LS Series Decoder DIP-8 DigiKey LICAL-DEC-LS001-ND
JP2 Linx RF Receiver 900MHz 8-Channel DigiKey RXM-900-HP3-PPO_-ND
C1 Capacitor Tantal 100nF 35V DigiKey 399-4596-ND
R1, R2, R3, R4 Resistor 100k 0.25W Metal Film DigiKey 100KXBK-ND

Future

The current design has some drawbacks due to the instability of the S88 bus, the limitation to 4 data lines per transmitter/receiver pair, the high current consumption on the S88 bus and because it is operating on a frequency which is not free in Europe.

I already have some ideas to use a ZigBee network for data transmission. The XBee modules from Digi can monitor 12 digital inputs and the data can be transmitted from multiple module to a single receiver module which is connected to the PC via USB. The range of the network can be extended by adding router modules.