Slot Car Speed Display

[rlno...@gmail.com]

I'm trying to build a flow that will display the speed of a slot car on a track. IR sensors on RPi inputs would be used to start and stop the timer as the car passes through.

I got a basic flow started using the Inject node and a timestamp but that uses a click on the screen instead of a gpio input. So there must be another way other than using a timestamp?

The desired output would be in miles per hour but the speed would not need to be very precise.

[Zenofmud]

Why not have the gpio nodes each feed a change node that sets up for a join. The join feeds a function which does the math - distance / (stop-start) to give you your value. here is an example

[{"id":"66551b75.8c9e2c","type":"debug","z":"d6840804.ac19a8","name":"","active":true,"console":"false","complete":"true","x":675,"y":222,"wires":[]},{"id":"e6116318.50c0b8","type":"inject","z":"d6840804.ac19a8","name":"","topic":"","payload":"start","payloadType":"str","repeat":"","crontab":"","once":false,"x":105,"y":97,"wires":[["cb6d762a.e13508"]]},{"id":"74baeca9.d741bc","type":"join","z":"d6840804.ac19a8","name":"","mode":"auto","build":"string","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":"false","timeout":"","count":"","x":441,"y":138,"wires":[["9e087c9e.e91848"]]},{"id":"3c2caeb6.78aaf2","type":"inject","z":"d6840804.ac19a8","name":"","topic":"","payload":"stop","payloadType":"str","repeat":"","crontab":"","once":false,"x":119,"y":172,"wires":[["4d8ff2a9.3a2f2c"]]},{"id":"cb6d762a.e13508","type":"change","z":"d6840804.ac19a8","name":"Start","rules":[{"t":"set","p":"parts.id","pt":"msg","to":"timer","tot":"str"},{"t":"set","p":"parts.index","pt":"msg","to":"0","tot":"num"},{"t":"set","p":"parts.count","pt":"msg","to":"2","tot":"num"},{"t":"set","p":"payload","pt":"msg","to":"","tot":"date"}],"action":"","property":"","from":"","to":"","reg":false,"x":283,"y":99,"wires":[["74baeca9.d741bc"]]},{"id":"4d8ff2a9.3a2f2c","type":"change","z":"d6840804.ac19a8","name":"Stop","rules":[{"t":"set","p":"parts.id","pt":"msg","to":"timer","tot":"str"},{"t":"set","p":"parts.index","pt":"msg","to":"1","tot":"num"},{"t":"set","p":"parts.count","pt":"msg","to":"2","tot":"num"},{"t":"set","p":"payload","pt":"msg","to":"","tot":"date"}],"action":"","property":"","from":"","to":"","reg":false,"x":285,"y":169,"wires":[["74baeca9.d741bc"]]},{"id":"9e087c9e.e91848","type":"function","z":"d6840804.ac19a8","name":"","func":"var start = msg.payload[0];  // time at start\nvar stop  = msg.payload[1];  // time at end\nvar distance = 10; // distance in feet\nvar elapsed_time = (stop - start) / 1000; //convert to seconds\nvar speed = distance / elapsed_time; // speed ft/sec \nmsg.payload = 'the speed is ' + speed + ' ft/sec';\nreturn msg;","outputs":1,"noerr":0,"x":607,"y":141,"wires":[["66551b75.8c9e2c"]]}]

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[Simon Walters]

You can just add a timestamp to a GPIO event with a change node

[{"id":"3c56ee42.bfa332","type":"rpi-gpio in","z":"7698c679.9464c8","name":"","pin":"7","intype":"up","debounce":"25","read":false,"x":210,"y":1320,"wires":[["f036db48.4fbb98"]]},{"id":"f036db48.4fbb98","type":"change","z":"7698c679.9464c8","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"","tot":"date"}],"action":"","property":"","from":"","to":"","reg":false,"x":400,"y":1320,"wires":[["b054b4b0.7da138"]]},{"id":"b054b4b0.7da138","type":"debug","z":"7698c679.9464c8","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":670,"y":1320,"wires":[]}]

Simon

[Dave C-J]

You may be ok - but when it comes to timing things like this you may want to consider external hardware instead - such as arduino or esp8266. While they run code - it is not a full blown stack with all it's overheads that the Pi does -  most notably in the node.js situation being the garbage collection it does, and maybe other operating system overheads that may distort any timing. which when converted to speed may give wildly inaccurate results...  By running dedicated hardware without these overheads you would get more consistent results.

Of course for a proof of concept it may not matter - just a heads up.

[Zenofmud]

And to add what Dave says, you could have the arduino send a MQTT message to NR and do the calculations and display there...or just use the arduino to take the measurements and calculate the result and use an OLE to display it. All sorts of options :>)

On Feb 22, 2018, at 4:40 PM, Dave C-J <dce...@gmail.com> wrote:

You may be ok - but when it comes to timing things like this you may want to consider external hardware instead - such as arduino or esp8266. While they run code - it is not a full blown stack with all it's overheads that the Pi does -  most notably in the node.js situation being the garbage collection it does, and maybe other operating system overheads that may distort any timing. which when converted to speed may give wildly inaccurate results...  By running dedicated hardware without these overheads you would get more consistent results.

Of course for a proof of concept it may not matter - just a heads up.

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[rlno...@gmail.com]

Paul, your flow is working fine for my application, thank you.

The display on next on my list. At first I thought to just display the Dashboard on a monitor connected to the RPi maybe using a projector. However adding more and more equipment is something I want to avoid.

I need something visible from about 20-30 feet so I thought about a large LED display. Probably not the cheapest solution.

What options are there to convert the payload data to a display?

[Zenofmud]

You could use the ui to display it then send the output of the computer to a projector

Paul

To view this discussion on the web, visit https://groups.google.com/d/msgid/node-red/61601c9f-0228-4397-913f-6c893841d6d8%40googlegroups.com.

[rlno...@gmail.com]

A projector with hdmi input would be nice.

But if I wanted to send the speed payload to a multi-digit seven segment display, would the Serial node be the way to send the payload out of the RPi serial port?

[Dave C-J]

Depends on the display.  

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Sent from phone.

[rlno...@gmail.com]

This display driver should drive my display https://www.sparkfun.com/products/13279

But after hours of searching this forum and google, I can't find any examples of NR driving a 7 seven segment display.

[Simon Walters]

I think you'd have to work the logic out from reading Sparkfun's Ardunio code and then re-writing it

It seems to just need 3 pins being switched high/low in particular sequences

I've ordered the chip its uses to have a play with one and see what can be done

[Zenofmud]

Just to make the project bigger, you might want to attach the display to an arduino and have it run as an MQTT client which will display the numbers, then have NodeRed publish the value to display via MQTT. 

You should be able to find a tutorial on using an arduino to display the numbers.

Also, tis flow might be useful: https://flows.nodered.org/flow/c07fb7be8fbff592a8aa

Paul

On Mar 7, 2018, at 5:22 PM, rlno...@gmail.com wrote:

https://www.sparkfun.com/products/13279

[rlno...@gmail.com]

I was hoping to keep the parts count down and just use the RPi to drive the display instead of adding an arduino.

But when I couldn't find any tutorials or even mention of RPi using NR to drive a pair of 7 segments, I started to wonder if there was a technical reason why it couldn't be done?

[rlno...@gmail.com]

Simon, let me know what you figure out. I'm not very good from the engineering side to get things to work. I rely on the experts here to guide me in the right direction then follow their suggestions.