Ok, just trying to encourage good habits.? You might want to consider putting a silk screen outline of the part (you draw it) on the board, or add two spdt style (middle with two ends) jumper pads hooked up to reverse the polarity if both are shorted in one position, and not reverse the polarity if both are shorted in the other position.? This, of course, goes to the diode and will allow you to use normal LEDS as well as the reversed one.? It does add an extra step, but allows more of a "just make it like the drawing" assembly, which is very frequently a good thing.
The same technique can be used to control the state of address pins, write enable pins, and the like *if* the pins can tolerate a direct connection to vcc.? If not (TTL would be an example where you'd want a 1K series resistor, old design practice here), then put in the appropriate resistors. You could also just wire pullups to the pin and use an SPST jumper to ground the pin when needed.
The jumpers look like resistor pads, but closer together. They're designed to be shorted together with a solder blob.? I'd rather use them than 0 ohm resistors because the resistors are an extra step.? You use the 0 ohm resistors if you want to run traces between the pads.? Some manufacturers use 0 ohm resistors for configuration since they use pick and place machines to put parts on the board.? Since you're doing things by hand, that's not needed.
The closest I can come to a polarity switcher is the set of jumpers above.
Harvey
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On 11/14/2020 1:18 PM, Dave wrote:
Harvey,
? In my case the circuit is so simple no harm can be done but I do get your point. From the batteries to a jack with a female plug. Inside the box the jack is hardwired to the pcb. The only time it has to be swapped is on assembly and is covered after that. What got me in the beginning is that the IR led's I use have the anode and cathode reversed so the flat on the LED is not the cathode or the negative like 99% of the other led's out there. I always test them on the breadboard but being my mind always thinks the negative is the short lead, I have solder the red and black wires onto the led incorrectly. Then as assembly continues it ends up backwards. It is not a big deal though as now I have ordered some Dupont female pins and housings to solve that issue if it ever comes up again. For the most part I likely won't ever make the mistake again but was curious if a polarity switcher was possible a device because they seem to make everything.
Thanks,
Dave
On 11/14/2020 11:59 AM, Harvey White wrote:
I would not use reversing a connector for the fix of an LED installed backwards.? I'd be very tempted to check them first. Most of what I use has an index mark on it indicating the cathode.
If you have a board where you can plug in the power backwards, then you have to think about how to plug in the power every time. Thinking is a quantity noticeably absent as the hour for late night debugging goes past 12 midnight.
One group of college students was in a robot competition, powering their project with a high power lithium ion battery. They didn't key the connector (for whatever reason), nor did they install any sort of polarity protection, not even a fuse and a reverse biased diode.
Three hours from home on a Sunday and before the competition, they plugged in the battery backwards and blew every power module in the robot.? Even an early morning run for spare parts made no difference to the outcome.
And that is why George always wear hat when visiting Tooki-Tooki bird friends.
One of the tweezer probes with banana plugs and one of the parts checkers (transistor/diode/LED/capacitor) can be used to identify an LED before you put it in.? Since they run off 5 volts (regulated from a 9 volt battery), you have enough voltage for blue and white LEDS.? You can also get them with a PC board pattern that allows directly putting SMT chips on the tester.
Harvey
On 11/14/2020 11:34 AM, Dave wrote:
Bertho,
? Funny story. :) But in my case I was hoping and also using my JST connectors, there was a way to reverse polarity. Of course putting together the project correct would be ideal, but in the past not all LED's are properly configured as far as polarity. Once the project is soldered up I have (only once or twice) had the polarity backwards. The easy fix would be to unplug a connector and re-plug it in opposite for the power feed. So far everything I have done has been soldered so it makes it a little harder to fix but not really that hard. But when I switch to plugs using JST connectors I was just curious if there was a trick for future use just in case. Like a few jumpers that could be swapped around. In the end I will just pay attention and color code with wire colors etc.
Dave
On 11/13/2020 10:35 PM, Bertho wrote:
One trick is if you have a three pin connector is to use the center pin for
positive and the two outside pins connected to ground.? Then polarity does
not matter.
Another common thing on multipin strip connectors is to remove a male pin
and insert a dummy plug in the matching female location.? If reversed, it
will not plug in.
That reminds me.? A very long time ago we were manufacturing a product with
a 12 pin in line connector with a missing pin.? It turns out that a square
toothpick fit perfectly as a plug for the female connector: Just push it
all the way in and cut it off flush.
We had an angry customer calling about our terrible quality: He said there
was even a piece of wood stuck in the connector and it took him a long time
to remove it.? Then even worse he said, when he finally plugged it in there
was smoke coming from the PCB and it did not work.? Of course he had plugged
it in backwards.
Bertho
