help with 3v rf modules

[Vandal S]

signal+module aurel tx-saw mid

hello
I bought some rf modules from aurel tx-saw-mid-3v and rx-mid-3v, am with ook.
The problem is that I haven't paid attention and they are 3V modules. So I have to make a separate source for them, not the same as for the microcontroller for example.
The datasheet is very small, and lacks information so the question is: can I connect the max232 with 5V out to data in of the transmitter?
and if the receiver has at data out 3v, will the microcontroller detect the correct levels?
thank you

to minimise, the question is: can a 3V 433Mhz Rf transmitter have on DATA IN 5V or does it have to be Vcc, which is 3V?

 

[vfone]

rx mid 3v

I think you can, because MAX232 has 1.4V treshold on the inputs.

 

[Vandal S]

tx-saw mid 3v

yes, I have found that TTL signal high is from 2 to 5 ( low 0 to 0.8).
that's one question; but the other one, more important, will the transmitter "burn" if it has at Vcc 3V and at DATA IN 5V?

 

[vfone]

what is 3v

Generally 3V IC’s has an Absolute Maximum Rating of +5V on the inputs.
So, is possible to be right on the burning limit.
If there are not too many input pins, maybe you can just place a resistive divider to drop the voltage swing.

 

[Vandal S]

tx-mid-3v datasheet

thanks for the replay, but I'm an absolute beginner so I have to ask: where will the ground (gnd) connection be.. connected? it's only one wire with data that comes in the rf transmitter.

 

[vfone]

max232 rf

The gnd goes to the grounds of the circuits MAX232 and RF chipset, which I suppose are tight together.
Chose the values of the resistors in the way to don’t overpass the load current of the circuit (check the datasheet).

 

[Vandal S]

small data transmitter module 3v

I'm sorry to bother you again.. but what values do I have to choose? (should it be KOhms or just Ohms with the 0.6 ratio) I'm not very familiar with this things. Here is a printscreen of the transmitter datasheet

 

[vfone]

rx-mid-3v

Have to check also MAX232 datasheet (output current, which drives the divider).
I would go (without too many calculations) to use few Kohms resistors.
If you are not sure, just use a scope and measure the level when the design is done, and adjust the resistors in concordance.

 

[Vandal S]

tx-saw mid/5v ttl

still me with the module problems.
How can I be sure that they are functioning? I connected to a function generator and a source but without any caps. The problem is that I wasn't paying much attention and the function generator was generating -1.5V -- +1.5V instead of 0V -- 3V; also I saw on the scope that there were many spikes, reaching even 4V (and maximum voltaje allowed is 3.5V); and there were many gnd pins and I connected them all, not just one. also, enable and vdc from the 3V voltage source.
I also put small 16.4cm antennas, the rx and tx were at 2-3m apart, but nothing. just noise.
after many hours I put them on the same test board, with vdc and gnd comming from the same source. I connected just one gnd pin (there are 3 on rx) After a few tries the out data was 800mV and constant, I don't know how. After other tries this was the best result, in the attachement. which is not that great, and it could be from other reasons, they were only a few cm apart.

what should I do? are they broken? or I don't know how to test them? (it's very possible)

 

[vfone]

I think is dead. The Minimum Absolute Rating is -0.3V.
Replace the circuit, don't place the divider, power on, measure using the scope, place the divider, measure again with the scope, tune the divider for proper level. I don't see what is so complicated to attenuate a signal.

 

[Vandal S]

are they both "dead"? I thought that only the transmitter is (it's the one that was getting -1.5V) but I was expecting to see something at the receiver, everybody on the internet is complaining about noise so if at least the receiver would work, would it be random 1's and 0's from the noise, right?
[the divider is the last worry now, I just want to see some bits transmitted; I'm working with variable source, so 3V is not a problem now]

 

[Darko_swl]

Try to look for 3v3 interface chip:
**broken link removed**