[Dr_C2]

It’s time for another system diagnostic and fun project.

This time it’s a speed trap with large digital read-outs. 

The plan is a two lane system which reports car ID and transit speed as the cars pass the speed trap position.

Having worked on 0.2 inch (5mm) digital displays for 1/32 scale windscreens, it’s time to go bigger for track-side displays :)

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ID detection, speed detection and large display readout to follow.

[Dr_C2]

For anyone interested in the technical details of this project, the plan is as follows:

1/ An ATtiny3226 will act as the core processor reading ID sensor pairs on each of the two lanes.

2/ The ID sensors will give arrival times as well as ID detection. From the timing information, and knowing the sensor spacing, the vehicle speed is calculated.

3/ The ATtiny will then communicate with the dispay modules (in this case 4inch vertical height) using a three signal serial bus. The dispays will be driven by 74HC596 serial-to-parallel shift registers.

All the hardware and firmware for each of the system components have been designed and tested. All that remains is the design of the pcb back-plane for the display modules and this has been tested, already, using smaller displays - yes, 0.2 inch ;)

Hopefully this project shouldn’t take too long to complete.

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[Dr_C2]

As mentioned, I hope this will be a simple project to complete.


The approach is as follows:

1/ Two ID sensors will be positioned on each of two lanes with a known separation.

2/ A sensor hub using firmware which has already been developed will read the arrival time at each sensor, plus the car’s ID. IDs are in the range 1-36.

3/ From this information, the transit speed and ID can then be displayed as cars traverse the dual sensor zone.

4/ As mentioned the plan here is to use 100mm tall (4 inch) 7 segment displays. Two digits for ID and then four digits for speed in m/s in the format ‘ZZ.ZZ’.

So all that is left to complete this project is the design and construction of the display modules. 

I will be using a simple ‘clocked serial data’ approach which is fairly standard. This approach typically uses a 74HC596 or if higher current drive is required then a TPIC5D596. I will be using the latter.

This has become a nice little high school grade project :)

A simple test set-up is shown below. The red dual display is a low cost commercial product. The small white display uses one of my PCB designs, and the large green display is using stand-alone TPIC5D596 driver board. I will design a simple back-plane PCB for these large displays to keep wiring looms to a minimum.

   

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[Dr_C2]

The display module under test prior to PCB design.

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https://youtu.be/HoTivG-_WPo?si=iyuxnmCvfQdsTY0k

[dvd3500]

[Dr_C2]

   

Here is the back-plane PCB design.

The first module in the sequence will have optically isolated inputs for serial data, clock and latch. The module will be powered by a 12V PSU snd includes a 5V regulator for the logic ICs. Again this is only present for the first module in the sequence.

All modules will have the TPIC5D596 shift register and the LED biasing resistors.

That’s the design completed - next the PCB manufacture, component assembly and system test :)

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[Dr_C2]

These PCBs are in production so I can now turn some project attention to what the finalised display should look like.

I have these 4inch seven segment displays in three colours: white, red and green.

So the plan is to assemble 9 of the display modules in a row:

From left to right, 2 white, then 5 red and then 2 green.

The white digits will display car ID.

The red digits will display speed in m/s as the cars cross the sensor zone and will use the format ‘ZZ.ZZZ’ m/s. (I added one extra digit since the earlier post).

The green digits will display laps total, laps to go and/or maybe the car’s finish position.

This display should be great as a track-side speed trap, a basic multi-car lap counter or perhaps a drag-strip timer?

The 4inch display modules should be readable at quite a distance. Hopefully they will be useful at public events?

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[Dr_C2]

At last the PCBs for this project have arrived  so now the project can proceed to a successful conclusion.

The PCBs are the same width as the 4inch seven segment LED modules with a matching plug and socket on each side. The idea here is that a series of modules can be assembled into a row by plug-and-play. This perfectly matches the ‘daisy-chaining’ circuit architecture.

The first seven segment module in the series uses optical isolation on all of its digital inputs. This is to eliminate any risk of circuit damage or circuit malfunction due to transient noise pick-up or ground loop noise.

I plan to use this new large scale and modular display in conjunction with under track sensors to calculate and display vehicle speed as well ID read-out.

A simple but fun project.

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[Dr_C2]

A few more technical details:

1/ Optical isolation of the input digital signals is achieved using a Broadcom ACSL6400.

2/ As the ACSL6400 inverts the logic signal, this is followed by a 74HC04 hex inverter IC.

3/ The approach now uses two voltage regulators, one at 12V to drive the displays and one at 5V to drive the onboard logic.

4/ The system itself will be powered by a 1.2A 15V Hornby PSU (borrowed from an ARC AIR powerbase).

5/ The segments are driven by a TPIC6C596 with one IC for each digit.

6/ The plan is to use a combination of white, red and green display modules, and since each colour has a different forward voltage drop (e.g. 11.5V in the case of green segments), this will require different series resistor values for each of the three colours in order to ensure brightness levels are equalised.

So far all is progressing to plan :)

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[Dr_C2]

Update:

The PCB backplanes correctly fit the 4inch 7 seven segment displays.

The dual voltage regulator circuits function correctly - the multi-digit display is powered by a 15V Hornby PSU. This is regulated down to 12V for the display common anodes and it is further regulated down to 5V for the control logic. All now functioning correctly.

Also, the optically isolated input correctly reads the three input signals (Serial data, serial clock and Latch). This consists of a correctly biased four channel ACSL6400 opto-isolator followed by signal inversion using a 74HC04. All working nicely.

Next task is to assemble all the required modules i.e. 2 x white digits, 5 x red digits and 2 x green digits.

Then the final stage: connecting the display to the track sensors and the system microcontroller.

The fully assembled 9 digit display will have a width of 80cm. Perfect as a slot track accessory… large and retro ;)

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