I’m going down much the same road with my new widgets as I did with the Wixels- depending on the software loaded and the application, a Widget can be a client or controller. A client can be a locomotive, turnout or any other device that is controlled via servos or discrete outputs. The controler can be a hand-held interface and/or a computer. In my case this is my Raspberry Pi, configured as a web server/web sockets host.

To that end, I coded up a quick Xbee interface for the Raspberry Pi in python that works with the datagrams in my set of train/control widgets. It’s extremely minimalistic (of course) but allows you to send standard ‘AT’ commands to the Xbee and also send advanced API datapackets. This code assumes that you have an Xbee module configured with X-CTU. I’m using the Parallax Xbee USB board to do this. You will need to set the interface baud rate of the Xbee to 38400 baud and place the Xbee in API mode. Once configured, move the Parallax board to the Raspberry Pi and that’s it. Destination Xbee modules (configured as above) will receive this data (based on it’s destination address of course) and pass this out the serial port to the host. In this case, the host is (will be) my train widget boards. Once they are available that is, perhaps early March (I hope)

Anyhow, you can now control Train Widgets with the following code or adapt it to your own xbee design. XbeeTransmitDataFrame is the primary control interface. All servo, sound and sensor data uses this fixed length datagram. It’s compact and fast and offers 65K of possible command codes (I’ve implemented 3 so far).

xbeeSendDataQuery is used to do a standard ‘AT’ command using the API mode. For example, you can pass ‘M’ and ‘Y’ to this method and it will return the 16 bit node address of the Xbee. Any of the other AT commands can be used as well but you will have to get the data using serial.read() and know where in the packet the return value and how long it is.

xbeeTransmitDataFrame is used to send the 16 byte packets I’ve made up for my implementation. You can see how these are constructed from the C structures below.

import serial

class xbeeController:
    def __init__(self):
        usbPort = '/dev/ttyUSB0'
        self.sp = serial.Serial(usbPort, 38400)

    def xbeeReturnResult(self, datalength):

    def xbeeDataQuery(self, cmdh, cmdl):
        frame = []
        c0 = ord(cmdl)
        c1 = ord(cmdh)
        frame.append(0x7e)	# header
	frame.append(0)	        # our data is always fixed size
	frame.append(4)         # this is all data except header, length and checksum
	frame.append(0x08)      # AT COMMAND - send Query to Xbee module
	frame.append(0x52)	# frame ID for ack- 0 = disable
	frame.append(c1)	# Command high character
	frame.append(c0)	# low character
	frame.append(0)	        # zero checksum location

	cks = 0;
	for i in range(3,7):	# compute checksum
	    cks = cks + frame[i]

	i = (255-cks) & 0x00ff
	frame[7] = i	        # and put it in the message

	for i in range(0,8):	# send it out the serial port to the xbee
    def xbeeTransmitDataFrame(self, dest, data):
        frame = []
        frame.append(0x7e)	# header
	frame.append(0)	        # our data is always fixed size
	frame.append(21)        # this is all data except header, length and checksum
	frame.append(0x01)      # TRANSMIT REQUEST - send Query to Xbee module
	frame.append(0)	        # frame ID for ack- 0 = disable
	frame.append( (dest>>8) & 0x00ff) # Destination address
	frame.append( (dest & 0xff))
        frame.append(0)         # disable ack for fastest transmission

        for i in data:          # move data to transmit buffer
        frame.append(0)         # checksum position

	cks = 0;
	for i in range(3,25):	# compute checksum
	   cks += frame[i]

	i = (255-cks) & 0x00ff
        frame[24] = i

	for i in range(0,25):	# send it out the serial port to the xbee

This is the xbee header information that is in the Widgets. This defines the three control packets I’m sending with the Xbee. These are the 16 bit fixed length packets that ride in the transport area of the above datagram. By keeping them small, I’m getting a 7ms packet transmit time which is very fast.

#define CONTROLLER   0x0000				// XbeeCTRLPacket{}
#define RFIDCOMMAND  0x0010				// XbeeRFIDPacket{}
#define SOUNDCOMMAND 0x0011				// XbeeSOUNDPacket{}
typedef struct
	uint16_t destinationAddress;		// Xbee destination address for this client
	uint16_t sourceAddress;				// This node's address
	uint16_t commandID;					// 16 bit command ID
	uint16_t Sound0;					// 16 Bits of Sound Triggers
	uint16_t Sound1;
	uint16_t Sound2;
	uint16_t Sound3;
	uint16_t Sound4;
} XbeeSOUNDPacket;

typedef struct
	uint16_t destinationAddress;		// Xbee destination address for this client
	uint16_t sourceAddress;				// This node's address
	uint16_t commandID;					// 16 bit command ID
	uint16_t RFID0;						// 12 bytes of ascii RFID info
	uint16_t RFID1;
	uint16_t RFID2;
	uint16_t RFID3;
	uint16_t misc1;
} XbeeRFIDPacket;

typedef struct
	uint16_t destinationAddress;		// Xbee destination address for this client
	uint16_t sourceAddress;				// This node's address
	uint16_t commandID;					// 16 bit command ID
	uint16_t Servo0;					// Servo 0
	uint16_t Servo1;
	uint16_t Servo2;
	uint16_t Discrete;
	uint16_t misc1;
} XbeeCTRLPacket;


This is the prototype control widget. I originally called them ‘train widgets’ but that is a bit limiting as they should be able to control pretty much any surface vehicle. I wouldn’t use them for flying devices however as the range is only about 300 feet- fine for earthbound machines but a bit too limited for anything that flies.

Nevertheless, since my primary purpose is controlling G scale model trains, this will do nicely. As you can see from the photo, these are very simple devices. An Xbee series 1 and an Atmel Attiny 1634 micro controller. I also have a suite of software modules written to control or monitor each of the functions mentioned in the diagram, plus do all the Xbee network message passing.

Each widget can control servos, motor controllers, discrete outputs (lights etc), a sound card with 8 channels of up to 1000 sounds, an RFID reader (for position sensing) and a configurable set of I/O pins for inputs (speedometer, current sensor, etc) or SSI/SPI to expand the I/O.

The Xbee allows bi-directional radio communications using the 802.15.4 protocol, so any node can communicate with any other node in real time. One Xbee in a hand-held configuration can control multiple locomotives, turnouts and animatronics, while a computer can also control and/or monitor the same (or other) locomotives or devices.

Anyhow, I’m working on a whole series of designs based on this, I have ordered the first set of PCBs, they should be here in a couple of weeks. After that I’ll be designing PCBs for the handheld, working on the sound card (I have a new one, see this link for more info) and expanding the software modules.

Well, after much debugging and trouble shooting I still cannot get either of my Android Interface prototypes to work. I have tickets open with the support guys but so far they have not been much help. This is rather disappointing but perhaps it’s a good thing. Time to move on. I have all of my control and sound s/w and h/w working, now I need to concentrate putting it into widget form with some custom PCB designs. I have the client widget designed, next are the hand held PCBs which will entail a display board and a keyboard circuit. The computer interface is covered, a parallax Xbee to USB board will be used for that, I just have to get the python drivers together and interface it into my websockets design.

I plan on releasing all of this as a series of several components, or widgets as I like to call them. The first will be the client widget. This will be for installation in a locomotive, turnout or animatronic device. It will interface to servos, motor controllers, my sound board, discrete outputs (ex lights). I will also provide inputs for various things like current sensors and feature an RFID reader port (for position sensing).

The second will be a couple of small PCBs, this will be the hand-held widget. I’ll also offer a resin cast/3D printed housing for this. And like the client widget, the s/w will be open source.

The third widget is the mp3 sound player. This will require the client widget to function.

The last widget will require only s/w as it’s based on the parallax Xbee interface board. You will need one of these anyway to program the Xbees for each of the clients and the hand-held. It will then double as a computer control point for the raspberry pi.

I’ll link all this together with some of the software I’ve done for the Wixels, at least on the HTML5/websockets and server side.

The basic idea is two fold. You can drive your trains with the hand-held and control all client nodes with that (locomotive or whatever) AND/OR control them with the computer or some combination of the two. I’ll open source everything so you have something working right away, yet also have the ability to alter the code to whatever you need. I’ll keep the cost of the PCBs to the absolute minimum and offer them bare, as a kit, or populated and programmed. Stay tuned.

As we enter 2014, I thought I would post up a recap of what I’m working on. Primarily I’m developing a wireless method for controlling moving vehicles. In my particular case, this is Garden Scale (G) model train locomotives. However the principles and widgets I’m designing could be used to drive almost anything via real time wireless up to a distance of about 300 ft (100 meters).

The key difference between my method and say standard FHSS (or similar) 2.4Ghz Radio Control is that I am oriented toward a computer driven network, in my case, 802.15.4 as implemented with the Xbee Series 1 modules.

Why a computer network? Well, for one, it gives me fine control over the exact data I’m passing back and forth. Second, it’s bi-directional. The network is oriented as a single to many so any device can talk to any other device. Each Xbee module has a unique 16 bit ‘node address’. You just form a message, add the address of the node you want it to go to and send. The network takes care of getting it there.

This is quite powerful. You can be sending control data to your locomotive and then, as it passes over an RFID tag, it could send that position data back to the computer (or your hand-held, or another device, etc) in real time. Additionally, other data such as speed, current draw, etc could be returned from the locomotive as well. Just because this is a computer type network, it isn’t slow. I’m getting data packet transmission times in the 7ms range which is quite fast. (For reference, your average servo scan time is 20ms)

Anyhow this level of potential automation is, by electronic design standards, fairly simple. I have an Xbee and an Attiny 1634 Microcontroller as the main components of my ‘Train Widget’ and that’s pretty much it. Basically about $22 worth of electronics as the ‘brain’ of all of my devices.


This is a picture of the basic train widget. Xbee does the communications, the Attiny handles the processing and drives the servos and motor controller. It also sequences the mp3 players (2 stereo channels) so you can play sounds via commands sent over the network or by throttle control positions. The two cards are just to the left of the speaker which I’m driving with a generic LM386 amplifier chip and some caps from Radio Shack.


Above is a shot of the front end showing the coupler connected to a micro servo, the ‘back’ coupler servo sitting beside it and the RFID reader in the upper left. I have all of this working including the transmission of the RFID data to the hand-held.


This is a picture of my stand alone ‘simple’ hand held throttle prototype. I also have all of this working, however only at a basic level of controlling one locomotive at a time, I don’t have any multiple unit code in there yet. I’m also thinking of going with a graphic display instead of this ASCII only one and also designing a ‘cradle’ or appliance of some sort so I can use my tablet as a user interface instead of a LCD. (See the post below this one)


Finally, this is a pic of my uncoupler device. One of these per car with the locomotive in charge of each one. The handheld would allow you to address the locomotive and then each car individually to uncouple anywhere. Each of these consist of one Attiny Microcontroller and a small 2.4Ghz transceiver. The locomotive has the ‘master’ transceiver and can talk to each car in the train that has a ‘slave’. The Attiny then controls the servo motor to move the coupler. This is clocking in at about $2 for the Attiny and $8 for the transceiver. Add $4 for a servo. Not super cheap but not too bad. $18 per car? I guess. Hmm, need batteries too…


I’ve retrofitted one widget and am building another from scratch, both have 18.432Mhz crystals driving the clock instead of the internal resonator for some experimentation and prototyping. Adding a crystal takes two pins but what can you do eh?

I have one running and so far and everything seems a bit tighter, but not dead nuts on like I was looking for. Note the couple of nanoseconds off on the first pulse. Main servo cycle is also 15ms instead of 20ms. Not sure I like that.

I think I’m missing something in the algorithm aspect of the servo drivers so I may refactor it for the third time. The above is a hack, I’m not fond of it but it works.

The big revelation here is that the timers must have some intrinsic slop which you have to compensate for. I found this while fixing the s/w uart so perhaps some servo driver adjustments are prudent.

Speaking of, the s/w serial out is now dead on, I get zero errors out at 4800 baud. So now I have one dedicated h/w serial link to the Xbee, a dedicated h/w serial RX from the RFID reader and a s/w tx to the sound boards. Sweet.

Well, after much poking about, taking scope traces and trying various prototype code, I’ve come to the conclusion that a s/w driven uart to control the sound boards is just not going to work reliably. I’m not sure why, but the timers on the Attiny 1634 seem to be off a bit sometimes. Quite often actually. I noticed some of this on the servo control code, but the servos are more forgiving so they don’t mind much and work fine. At even 4800 baud though, the bits are off by enough to cause the wrong data to be clocked into the sound card- Yuck.

I can use one of the onboard h/w uarts and it’s rock steady but I really didn’t want to do that. One of the h/w uarts is dedicated to the Xbee and the other I use for the RFID reader. It’s a shame the RFID reader wants to run at 9600 and the sound boards wants 4800, otherwise I would be set. However I don’t think there is a way to change either of them so I’m kinda stuck.

Anyhow, I’m guessing it’s because I’m running the chip on the internal clock, so perhaps an external crystal is the ticket. I have a couple of 20mhz xtals around but it looks like the best ‘baud’ friendly crystal is 18.432 which gives 0% error on all the rates. They are a buck from mouser but will consume two pins on the chip. Oh well.

Looks like the best bet is to do a 3.0 design with the crystal and then move all the I/O off to the SPI interface, that way I can control 32 bits of I/O for about $3. This lets me control the sound plus the 915mhz radio for the automatic uncouplers too. Hmm.


I refactored the software for both the master and client nodes over the last few days. I cleaned up some of the prototype code, added some basic structures to lay over the xbee transmit packets and moved things around just a bit. As far as functionality, I can now control three servos and send the RFID tag reading back to the master. If you look closely at the display you can see the raw hex code from a read in the lower left of the display. The ID20 from Sparkfun is a really nice reader, it catches the card passing by very quickly at about 40-50mm.

This also shows some experiments with the Kadee coupler controlled by a servo. Seems to work well, but this is on a bench test setup, not a real locomotive- but I’m optimistic. All I did was drill a very small hole in the ‘tab’ on the coupler. This is a standard Kadee 787 coupler meant for the Aristocraft RS-3. The actuator wire will be replaced by a chain or something more cosmetically pleasing.

The sound cards are the next step. Right now I’m feeding the power from the same bus as the servos so I get a bit of noise mixed into the amp output. I’ll clean that up later, for now all I hear is the servo when it moves, other than that it sounds fine, good enough to get the s/w working.

I’m getting ready to refactor the software for both the client and master so I decided it would probably be good to shoot a bit of video showing everything working and driving servos before I break it for a while. This shows servos on channel 1 of both clients moving via the knob on the master. There is a bit of dead space on both ends of the knob rotation but other than that it tracks really well. Note that I click the somewhat silly keyboard to select between the two clients. Anyhow, you can see other posts I’ve made of the timings and message passing but I wanted some real world video to show just how well it’s working. So here ya go. Next up is getting the RFID via Xbee and the slave sound card up and running. Almost there. That’s up next.


The two current widgets. The Xbee client is in the upper right hand corner- it sports wireless bidirectional communications following the 802.15.4 protocol. FCC approved, 300 ft range. It controls servos, motor controls, discrete outputs- lights, solenoids. Also has a serial input for an RFID reader. Additional inputs are available for current sensors or distance/speed sensors. This is my locomotive Frankenstein. I am really happy how this has turned out. I’m working on a set of simple PCBs so all of this can be just soldered in. (soldered and wired breadboards are a pain sometimes)

In the center is the sound widget. Up to 200 stereo mp3 files can be triggered from R/C inputs or discrete buttons presses or switches. You can put ridiculously large SD cards into it. Like 32G of sounds. You can trigger music, sound efx, whatever you want. A 1 watt amplifier is onboard- more than enough to drive a large speaker.


Finished up another client, I can now control two from the handheld, just push one of the keyboard buttons to change to the next locomotive (Xbee Address). Point to multipoint. I like it. I also got the RFID reader working, quite nice. Super simple interface. This is the ID20LA from Sparkfun. I’m using the big thin credit card sized tags and they work really well, even at speed. Put a reader on the locomotive and embed the tags under the track and I think this will work great.