GPS NMEA, Failover Circuit upgrade attempt.

[Sadlercomfort]

Ash, I see you've put considerable time into this topic. My apologies for being very late to the party. Especially since it appears you didn't need any help.

Thanks Chris! It's been great to write some PICAXE code for a project like this and help out a friend!


Nice diagram are we still moving the two LEDS from the relay to the PICAXE Constantine? I noticed these posts are a day old and somehow missed them.

 

[Sunnysky]

which clock do you need to be precise? The ToD digital sequence or the 1pps clock or 10MHz?

 

[davenn]

It is now Thank you for pointing this out.

soooooooooooo much better

dunno what you are using for drawing ?
I use MSPaint ... gives a much better final drawing without all the added grid dots and gaps etc
have done so since windows was first released with it

that pic above isn't mine, but it is the quality of using Paint

here's one of my examples of a pretty complex cct .....




cheers
Dave

 

[HellasTechn]

dunno what you are using for drawing ?

I use Express SCH. That is not a problem for now.

which clock do you need to be precise? The ToD digital sequence or the 1pps clock or 10MHz?

Clock to precise ?

are we still moving the two LEDS from the relay to the PICAXE Constantine?

I think we better leave them on the relay. Considering that each chip can sink or source up to 90ma of current, by adding two extra LED's we increase total ammaunt al least by 20ma. there is no point in pushing it to its limits by adding loads we can avoid.
Besides we need it to be as stable and reliable as it can be.

 

[Sunnysky]

Is this just to compensate for time of day clock drift without Internet access? Or some other purpose? The RTC can be made more stable if you want... perhaps easier to handle LOS conditions on GPS.

 

[HellasTechn]

Sir Sunnysky,
This is not what it is about.

If you read post #1 of this topic you will get an idea and if you read the link below you will get the whole pcture.
https://www.electronicspoint.com/threads/help-with-data-switch.259040/

Long story short is that using a picaxe we connect two identical GPS that send RS232 NMEA data to a PC.
The picaxe based on NMEA sentence GSA that contains DOP information has to determine which GPS is more accurate and connect it with the output. Ofcource it is more than that...

 

[CDRIVE]

Constantine, if your LED's are Red your current limiting resistors should be 330Ω.

Chris

 

[CDRIVE]

In my last post I was referring to the LED's directly driven by the Picaxe. You may want to recalculate the limiting resistor for the two LEDs powered by the 12V source too. Running LEDs at their max rated current will shorten their life span.

On another note 1N914's are switching Diodes. They're not typically used for inductive flyback suppression. The 1N400x series are most popular for flyback suppression as well as power rectifiers. Typically the 1N4002 is the Diode most often used.

Chris

 

[HellasTechn]

Constantine, if your LED's are Red your current limiting resistors should be 330Ω.

Not for 5 volt supply

LED's consume max20ma at 3 volts so at 5volts an 100 ohm resistor is fine.
right ?

Again i may use an 120 or 150 ohm resistor to keep it running at lower current and prolong its lifespan.

On another note 1N914's are switching Diodes. They're not typically used for inductive flyback suppression. The 1N400x series are most popular for flyback suppression as well as power rectifiers. Typically the 1N4002 is the Diode most often used.
Chris

That i didn't knew. I can change that

 

[HellasTechn]

You may want to recalculate the limiting resistor for the two LEDs powered by the 12V source too.

Yes you are correct about that. i sould use a 390 or 470 Ohm resistor for theese.

 

[CDRIVE]

Not for 5 volt supply

LED's consume max20ma at 3 volts so at 5volts an 100 ohm resistor is fine.
right ?

No... recalculate for Red LEDs. To insure that the Picaxe I/O pin currents are less than.. "I=" <= 20mA R should = ~ 330Ω.

EDIT:
It's never a good idea to run a component near its maximum parameter. The Picaxe manuals specify 330Ω resistors because they'll run the LED at about 10mA, which is 50% below the I/O pin's max current. The LED should still be quite bright.

Chris

 

[CDRIVE]

Here's an "R vs I" plot for a typical low current Red LED. V+ = 5.0V

Chris

 

[HellasTechn]

The Picaxe manuals specify 330Ω resistors because they'll run the LED at about 10mA, which is 50% below the I/O pin's max current. The LED should still be quite bright.

I see that you have a good point here. I will change the diagram with the 330Ohm resistors and after i build the circuit i will test it and if it is not bright enough i will try a smaller value.

Thanks !

 

[Sunnysky]

Sir Sunnysky,
This is not what it is about.

If you read post #1 of this topic you will get an idea and if you read the link below you will get the whole pcture.
https://www.electronicspoint.com/threads/help-with-data-switch.259040/

Long story short is that using a picaxe we connect two identical GPS that send RS232 NMEA data to a PC.
The picaxe based on NMEA sentence GSA that contains DOP information has to determine which GPS is more accurate and connect it with the output. Ofcource it is more than that...

Not sure which model you have, but they all have a Loss of Signal or LOS status signal internally, some external.
That would make it a lot easier if that was accessible.

 

[HellasTechn]

Not in my case. i have some old rockwell receivers that do not provide LOS info.
Besides the upgrade we are attempting is to make it not only understand a loss of siglal but also to be able to compare Dilution of precision between the two inputs and switch to the better one.
Plus i would like the system to be able to accept all GPS that transmitt NMEA data through rs232 4.800 (and use GSA).

 

[CDRIVE]

I see that you have a good point here. I will change the diagram with the 330Ohm resistors and after i build the circuit i will test it and if it is not bright enough i will try a smaller value.

Thanks !

If 330Ω doesn't provide enough LED brilliance for your liking a 220Ω limiting resistor will. It provides both max permissible current (~20mA) for both the LEDs and the Picaxe I/O pin.

Chris

 

[HellasTechn]

The diode will not conduct if the serial input is less than 5V. But if the serial swings higher than 5V the diode will conduct and clamp the voltage to 5V.. protecting the picaxe.

Regarding posts #22, #41 and #43

The protection diode is ment to protect picaxe against serial data voltage (from the GPS in our case) being higher than 5 volts ?

If that is the case then i will need to add that to the circuit also.

According to post #3 the gps output is TTL level and only goes up to 3,7 volts but just to be on the safe side right ?

 

[CDRIVE]

Actually, Picaxe I/0 pins contain two Diodes. One Diode is as Sadler pointed out and the other is connected with its Cathode = Pin and Anode = (Vss) GND. It's incorporated to protect the input from Negative voltages like you'd find from RS232 Serial Port data and control pin levels.
With regard to you saying you need to add something because of them... No you don't. Your 10KΩ and 22KΩ resistors are already dealing with those integral Diodes.

Chris

 

[CDRIVE]

Constantine, I forgot to mention that the Diode connected to the 7805 output pin is backward. For that matter, what's its purpose? I see no need for it.

While we're on the subject of schematics here's a tip. Always include component ID numbers IE; R1, C1, C2, D2, etc, when you draw a schematic. This way we can reference them by saying D2 is backward.

Chris

 

[HellasTechn]

the Diode connected to the 7805 output pin is backward. For that matter, what's its purpose?

Yes i understand. I am not sure why picaxe needs it but it was added for the reason stated in post #22