Thursday, March 1, 2012

TNG traffic light - epilogue

Each of the two LED boards on top of the signal tower traffic light are joined to the "platform board" via 9 solder joints. I certainly would not rely on them to hold the boards together nor am I deluded enough to think the corners of the boards won't chip off when the kids start playing with the toy and quite predictably knock it over ... and over. Those boards simply aren't going to survive this school of--literally--hard knocks. So as was planned I've potted the copper side of the boards with clear epoxy. Not only will the plastic make the whole three-board structure rigid and strong, the epoxy will also seal the copper tracks, solder connections and component leads and protect them from corrosion as well as minimize if not prevent possible injury to the kids if they go about running their fingers on the LED pins.

The two sort of cream colored rings are more than conspicuous. For aesthetic reasons I actually would rather not have them except that they're absolutely necessary to soften the impact of any falls. Without them the canister and its cap and maybe even the colored acrylic(?) lenses will surely crack/break/shatter. The rings are made of soft rubber tubing (silicone I think) with an outer diameter of 10mm. To build and install the rings I first cut one piece equal to the circumference of the canister--50mm. I then glued the ends with cyanoacrylate superglue. After letting the glue set for a couple of minutes I coated the sides of the canister where the ring will be positioned with superglue. I slipped the ring on the canister and pressed the ring inwards to make sure there's good contact between the ring and canister. After it was securely bonded, I coated the top portion of that ring with superglue and placed the second ring on top of it. Hence, the upper ring is bonded only to the lower ring, not to the canister. This way I can push that top ring down when the canister cap has to be screwed off. The rings have to be as close to the cap as possible to make sure the face of the 7-segment LED does not hit the ground when the tower tips over. 

I've tested the light with power on and knocked it down a couple of times and for now at least it can take a lickin' but keeps on tickin.

Just a couple of facts I failed to mention in the previous blog entry.

The green/ed on-time can be changed any time--in any mode. However, after doing so the light will immediately enter normal traffic light mode.

You'll notice the 5-pin header around the center of the main board. That's for the ICSP connection to a PICkit 2 or 3. I foresee the kids wanting some changes to how the light works--a max green/red on-time of 90 seconds perhaps, or different non numeric characters when the amber/red lights are flashing. The ICSP connection will make firmware updates a breeze. I've actually used it already. The firmware I initially uploaded to the MCU (whilst on the breadboard) had a tiny bug--upon power-up the bulbs aren't all off and so--since the circuit defaults to a flashing amber upon turn-on--either or both the red and green lights are also on. Fail! After inserting a call to the function turning all bulbs off, I used the PICkit 2's Programmer-To-Go feature by uploading the firmware to the PICkit 2 and then plugging it into the board's ICSP headers and pressing the PICkit's red button to upload to the MCU. Note the board has to be powered up when uploading.

While breadboarding the circuit I discovered that the resistors pulling the bases of the PNPs to the 12VDC supply were actually unnecessary. The LED display works fine without them. But I retained them anyway just to make sure the transistor bases aren't floating.

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