3.3volts Regulated Supply reqd!

[UroBoros]

max232n 3.3 out

Hai

To drive a GPS module I want to generate 3.3Volts DC. The Supply available is 5Volts. (Or 12Volts, if must).

The datasheet of GPS module says to provide 3.0 to 3.65 volts DC.

Which is the cheep and easy method without risking the GPS module!

How to do it with LM317?

Or what will be the risk if we drop 7805 output by two or three 4007 diodes in series .

Please suggest

 

[namqn]

rs232 to ttl/cmos converter schematic max232n

Find a low-dropout (LDO) voltage regulator. There are definitely some cheap ones around you. The LDO voltage regulator will happily take your 5V supply and give you a clean 3.3V output.

Cheers,

 

[creative_35]

3.3volts from 7805

Howmuch current needed for GPS module?

 

[IanP]

convert 4.7 volts to 3.3

If you have 5V then the easiest way of generating ≈3.5V will be to use 2 silicone diodes (1N4001 or similar) followed by a 1kΩ-4.7kΩ dummy load (this will force voltage drop accross the diodes) ..

317 will not work of 5V as it needs at least 3V drop accross itself ..

Regards,
IanP

 

[UroBoros]

max232 3.3 volts

Hai

Thanks for the replies.

GPS module needs 37mA at 3.3 volts ( typical) according to datasheet.

Is there any special protection or advantage if I go for LM317 after bringing 12volts into the PCb.

What you feel? Is this 4001 dropping method suffiecient for my purpose?


Another Issue


I am about to test GPS data by hyperterminal.

how much distence can max 232 and RS232 levels can travel sefely.

Will 5 meter be safe? More length at lower baud rates?

My GPS module has a small passive antena. So GPS module and Antena placed outdoors ,converts to rs232 level there itself using MAX232 and send to PC or PIC micro for processing.Will this 5 meter length ok for my purpose?

Active antena costly. this unit comes with an antena conneccted with 5 cm cable.

I plans to convert to 232 level there itself.

with regards

picstudent

 

[IanP]

I think 2 diodes should work well .. I wouldn't bother running additional 12V and step it down to 3.3 with 317 (317L) - unless you really want to ..

GPS receiver can be directly connected to a PC via 2 wires (TX-gps to RX-PC and GND-to-GND) .. If you add MAX232 (or similar driver) then the signal will be reversed - keep that in mind ..
If you use shielded data cable you can run signal from GPS-receiver over more than 5m ..

Regards,
IanP

 

[UroBoros]

GPS receiver can be directly connected to a PC via 2 wires (TX-gps to RX-PC and GND-to-GND) .. If you add MAX232 (or similar driver) then the signal will be reversed - keep that in mind ..

Regards,
IanP

I didint get this Point

How GPS 3.3 volt level to RS232 of PC

And how the signal reversed?
Please clarify
picstudent

 

[IanP]

Direct connection stands only for 5V units - 3.3V modules will need something to convert from 3V to at least 5V .. see picture below ..

5V units output signal which is "in phase" with the RS-232 signal - 0V (equivalent to -3 to -25) and 5V (equivalent to +3 to 25), but because contemporary computers' COM ports "understand" 0-5V levels you don't need any level translator or driver ..
With 3V modules I am not sure - you will have to test it - but if you build driver consisting of 2 transistors - can be MOSFETS (picture below) or BJTs - you will have both options ..

Regards,
IanP

 

[UroBoros]

Hai

Can you provide a circuit with BJTs ?

Which is the mosfets used here?

picstudent

 

[IanP]

As MOSFETS you can use 2N7000 (or similar - **broken link removed** ), if you go for BJTs (2N2222, 2n3904, BC547 ..) - add 4.7kΩ-10kΩ as base resistors - the concept remains the same ..

Regards,
IanP

 

[creative_35]

If your requirement is less than 50ma, why you can try a load resister with zenerdiode? [/img]

 

[UroBoros]

Hai

Here is the circuit I am planning plus the datasheet.

Please suggest level shifting from 3.3 Volt to 5 volt is a must for MAX 232 to sense

picstudent

 

[creative_35]

hi pic student,
I think the load resister in your supply voltage 2.2 ohm will kill your regulator chip. use a 100 ohm 1/w resister instead. from 100 ohms resister you will get a current of 50 ma. for level convertion it's recomended to use appropriate 232. for example MAX3232E will work in 3.3v.
the diode IN4148 also not recomented in the serial data path.
here is an example circuit of interface GPS to 232.
and MAX selection guide.

 

[IanP]

- power section: I would add 2 x 100nF between 7805's 1-2 and 3-2 and another one in parallel with D12 .. Make R9 ≈ 6.8Ω ..

My gut feeling is that, as with other modules, the phase of the TxD signal from pin 8 is already ready, as far as its phase is concerned, to be fed (with a buffer) to PC's serial port ..
When you look at the MAX232 it is like an inverter gate between T1IN and T1OUT (just different levels), so if you run the signal from the GPS module through it you will reverse the phase of this signal ..
Also, for the time being, you don't need t connect the RxD signal from PC to the GPS module ..
Then, why not to try that simple double-transistor 3.3V-to-5V level converter?
Of course, you can use the MAX232 driver as per your drawing, but you will need to add 2 x 3.3V signal inverters (one for TxD and one for RxD) between the GPS module and the MAX232 ..

Regards,
IanP

 

[creative_35]

- as far as its phase is concerned, to be fed (with a buffer) to PC's serial port ..
When you look at the MAX232 it is like an inverter gate between T1IN and T1OUT (just different levels), so if you run the signal from the GPS module through it you will reverse the phase of this signal ..

Regards,
IanP

I think Phase can be changed in the software too instead of adding an extra inverter right?

 

[UroBoros]

Hai

Doing the level converter.

But this section of datasheet suggests that this MAX232N can take my GPS out level as valid data?(3.3Volts)

picstudent

 

[IanP]

I agree that there is no problem with levels between the GPS module and a TTL/CMOS circuit such as the MAX232, but what about the phase of the signal?
In the GPS receiver's data sheet there is no single word on whether you can reverse the phase of the TxD and RxD signals or not ..
If, what I suspect, the output from the GPS receiver is 0V in the idle state than it has to be inverted to 5V, should you wish to connected it to a PC via the MAX232 ..

Regards,
IanP

 

[electronicus]

Use LM1086IT-3,3, here is the Data Sheet.

 

[IanP]

Take a look at the attachement below ..
It is a description of another NMEA device .. In its case "serial output" will be connected with external MAX232 driver, but the other output, "Inv. Serial", can be connected directly to a PC serial port ..
So, connect power to your GPS module and check what is the output level (TxD pin) when it is idle ..

Regards,
IanP

 

[UroBoros]

If, what I suspect, the output from the GPS receiver is 0V in the idle state than it has to be inverted to 5V, should you wish to connected it to a PC via the MAX232 ..

Regards,
IanP

Hai

Thanks for the ideas.

I have tested the GPS module by powering it. It seems Ok for our purpose.

Now I have not connected anything to the RX and TX pins of GPS module.


RX of GPS is in 3.2volts (steady)
TX of GPS is varying between 1.2 and 3.2 volts.(Seems it is sending out some data).
But it in not holding on to any particular level.I think by default the module is configured to out data continiously.
Now Is it Ok for direct MAX232 Connectivity like in my schematic?
the TXD and RXd lines seems to hold on to HIGH level idle condition.
I am using the RXD pin with a purpose to configure the module to send out data which is of my interest. Will it cause any malfunction ?

With regards
picstudent