433 MHz Data Sheet Supply Voltage

dennis

Member
I am planning to use the RT4 -433 AM transmitter from RF solutions (standard one that used to be sold by Maplin). The data sheet covers several modules so I have highlighted in yellow the one I have.

The supply voltage is stated as 2 to 14 V. That´s fine but what I need to know is if the transmit power is the same over the whole supply voltage range (the data sheet seems to state the power only at 5V) . If it is the same then I can just use the main regulated Picaxe supply at 5V. If power increases with voltage then I may try using a 12V supply.
 

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russbow

Senior Member
Dennis, I use a pair of these. As you say < £10 from Maolin.

I use 3xAA cells for the Tx tied to an 08m and 3xAA on the Rx feeding an 18x.
I have consistent coverage around the garden and inside the house Good for a few hundred meters up the street.

I am sure Tx power would increase with voltage. but you get better returns paying attention to the antenna. If you trawl the RF solutions site, you will find a PDF on the topic but homebrew at 70cm is akin to precision plumbing!
With a simple 1/4 wave and a groundplane, I have been so satisfied with the units that I wont look elsewhere.

As a bye, have you found out what the test pin is for on the Rx ?

R.
 

manuka

Senior Member
In my experiences with similar ISM devices,TX output power indeed is higher as the supply voltage is increased. Of course a 12 V supply can be a minor hassle to organise when an attached PICAXE has just 4.5V (3 xAA)! These Maplin TX are rated at just 1mW on a 5 V supply, so best suit very short ranges. It never fails to amaze me that such simple & cheap UHF items can work so well however! I certainly encourage folks to start with the likes of these.

When the going gets tough something more powerful may appeal however. What is your application? What range do you need ? The total cost of ownership (TCO) for such RF projects needs to be factored in of course- more expensive devices with "sleep" features etc may save you battery drain for starters.

Many cheapie 433 MHz receivers are pretty deaf (I note the companion to this is a mere -90~95 dBm ), so better performance may arise with a more sensitive narrow bandwidth (& possible FM) setup. This is especially an issue if your locality has lots of RF noise &/or existing 433 MHz signals. Stan

EXTRA: Every 6dB gain doubles range of course, & I'll certainly agree that a decent antenna helps too, with ~6dB gain "cotanga" Yagis worth initial trialsl
 
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dennis

Member
Thanks Russbow - I have had reasonable coverage with no ground plane so will try again with groundplane and measured length of antenna.

I dont know what the test pin on the RX is for.

Stan,
Thanks for your help I am just tidying up my electricity meter reader and re-breadboarding to use mains frequency as a pulse counter. The receiver end currently does not work in some parts of the house. The transmit module power is from an AC wallwart (9V ac) regulated to 5V DC for the PIcaxe so I can fairly easily increase the transmitter supply to something higher. I will try both voltages tomorrow and let you know if there is much difference
 

westaust55

Moderator
That 9Vac wall wart has a peak voltage of (9 / 0.7071 =) 12.7V

So using a largish (220 or 470uF 25V might be okay) electro cap between the rectifier and the front of the 78xx series voltage regulator you could then use an 8 or 9V dc regulator (7808 or 7809 or similar in low power types).
The electro cap will charge to the peak voltage - taking into account some volt drop across the rectifier.
 

dennis

Member
It has been an interesting day playing with the RX and TX modules.

The reference point was with the transmitter at 5V on a breadboard with a short 69mm wire antenna. The RX was a similar set up but powered by 3AA cells.

I moved the receiver to an area of marginal reception (3 block walls away upstairs) and then tried optimising the transmitter.

Increasing the voltage to 8V (thanks for the method Westaus55) made a small but definite improvement. Using a biscuit tin lid as a ground plane and a full length 690mm antenna also helped a little. Other experiments were indeterminate so this "hanging basket" set up became the standard transmitter as I tried to improve the RX.

A biscuit tin ground plane made reception worse and after a lot of fiddling, constrained by a short antenna I ended up without any improvement. Strangely touching the antenna helped and adding a bit of foil (150mm square) to the antenna tip also made a slight improvement.

Overall it was a little disappointing until the receiver was assembled into its working position, crammed into a cardboard tube with short antenna and battery box and stepper motor in close proximity. In this mode reception was very much improved and it works throughout the house.
 

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manuka

Senior Member
Bravo- many 433 MHz RX are quite supply voltage picky. I've found most need to be 5 ±½V, so 3 x fresh AAs will probably be needed.
 
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