interfacing rf sender/receiver module

[variable]

Hi Folks,

a noob question, I have a 18x starter pack, and bought a pair of rf module(transmitter/receiver)

Transmitter:
can I hook the sender to any digital output pin and using the serout command?
also my transmitter is 3v, 10ma, it is right that I put a 200ohm resistor infront of the transmitter?

Receiver:
can I hook the receiver to any digital input pin and using the serin command?

Many thanks!

James

 

[womai]

Hello James,

I'm assuming your TX/RX is the low-cost 433 MHz type?

Yes to first and third question.

As to the second, where do you want to place the 200 Ohm resistor? Do you want to use it to drop the supply voltage? Apart from this not being a very reliable method, are you sure the 3V supply is not the minimum supply rather than the maximum? Most of thos 433 MHz transmitters can actually run with up to 12V supply, and the higher the voltage, the higher their output power.

Wolfgang

 

[variable]

Hi there,

thanks for the speedy response, the datasheet says 3v, no min/max specified, so lets assume max is 3v, and power source is 5v, is 200ohm 0.6w resistor is used correctly? I have soldered that resistor infront of the vcc pin of the sender module, however when I test the voltage using a multimeter with the vcc pin and ground pin of the sender module, it still says 5v...

sorry I am very new to circuits...

Regards

James

 

[manuka]

Variable: Let me guess- Jaycar's "Keymark" units? If so, then their info is hazy (mentioned to them ) as the TX runs at 3V minimum & is perfectly happy at 5V. It's the RX that's picky, with supply needs close to 5V. Both run a treat however on 3 x fresh AAs (~4.5V). In fact I must have run every 433 MHz device around to good effect on such a 3 x AA supply, so if your units are instead Surplustronics offerings I see no problems.

Check numerous mentions of both these 433 pairs & others at => www.picaxe.orconhosting.net.nz Stan.

 

[variable]

Thanks Stan,

that link helps alot. Cheers！

Just a side question, if there is a device wants max 3V 10ma, and source is 5v, is that right that I use a 200ohm resistor? just want to confirm I did the calculation right...

Cheers

James

 

[womai]

You are assuming that the device consumes exactly 10mA. But very often such numbers only give some maximum supply current. It may also depend on whether the unit is active or not. The moment the current changes, the voltage drop will change 1:1. So overall, such a scheme is not suitable to regulate down the supply voltage. E.g. if your unit goes to sleep mode and consumes very little current, its supply voltage will rise to 5V!

Two better approaches :

(1) instead of the resistor, use one or more diodes (in conducting direction). A normal silicon diode will drop approximately 0.6 to 0.7V, and that voltage drop is relatively (not completely) constant independent of the current flowing. To drop 5V down to 3V you will need 3 diodes in series. If you need smaller drops, use a Schottky diode (0.3 to 0.4V drop).

(2) use a linear, low-dropout voltage regulator (low dropout because the difference between input voltage (5V) and desired output voltage (3V) is fairly small and each regukator needs some minimum difference in order to work (the venerable 7805 needs 2 - 3 Volts, but LDO regulators can live with 1V and less).

Wolfgang

 

[variable]

Oh, thanks wolfgang, I didn't realise that the rf module goes to sleep mode(as I said I am new to this kind of things), so this will explain why I still get 5v for the rf module when the resistor is in place. Cheerio!

James

 

[manuka]

Wolfgang has nailed this nicely, although at diverse currents series diodes too have their quirks-thermal runaway etc. What brands are your TX/RF pairs?

 

[variable]

Hi Stan,

I got those modules from surplustronics. cheers

do you have a msn contact? can I add you to my contact list? I don't wanna spam the forum with noobie questions...

Regards

James

 

[womai]

Actually noobie questions are one of the main reasons for this forum to exist. Much better having many people look and supply answers than bugging always the same person (even such a helpful person as manuka) So I suggest - keep posting here! (there are no dumb questions, only dumb answers!)

 

[manuka]

Variable - shoot me a personal message (PM) using the Forum option.