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RF transmitter chip -- power up and transmit cleanly
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RF transmitter chip -- power up and transmit cleanly

by bythewater on Fri Oct 08, 2010 1:49 pm

I'm using the following transmitter / receiver chipset:

http://www.rentron.com/Files/TWSAKIT.pdf

It can transmit multiple bits however on this project I'm simply using it to transmit a signal -- one bit if you will -- to turn a light on or off. (I figured I would learn how to use it on a simple project even though it's overkill, so I would then be prepared for a more complicated requirement later.)

I want the transmitter to run off a 9V battery with some kind of dc-dc converter down to the 5V required by the circuits (voltage regulator? please advise). This will only be in transmit mode occasionally, a few seconds a week. Transmit would be initiated by a momentary pushbutton that sends a signal to the receiver which would toggle the light on, or toggle it off.

So I would like to have the 9V battery basically last for its entire shelf life, I don't want some 10mA draw to make me replace the battery every month. So either I need to know how to set this up so that it draws negligible current when not in use, or, I need to know a simple way the momentary push button to cleanly power up the entire circuit: the voltage converter and then activate the 'transmit enable' pin on the transmitter.

Thanks!

bythewater
 
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Re: RF transmitter chip -- power up and transmit cleanly

by zener on Fri Oct 08, 2010 5:55 pm

I assume you are using an encoder and decoder also, as shown in the schematic?

Anyway, it looks like the way it works is you give it power (with the pushbutton) and it starts transmitting. It will transmit an 8 bit code (I think) and will transmit it over and over about 100 times a second, or more (guessing again).

A switching regulator will be more efficient than a linear but since you are just pressing the button once in a while it probably doesn't make too much difference, so I would just use a 78L05 probably and see how long it lasts with that. Probably a long time. You could also just use a couple of AA''s and no regulator. The transmitter looks like it has a very wide operating voltage range. It is the receiver that needs 5V. Two AA's will probably last longer than a 9V with a regulator (and be cheaper).

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Re: RF transmitter chip -- power up and transmit cleanly

by bythewater on Sat Oct 09, 2010 9:07 am

Thanks Zener! I didn't somehow pick up the fact that it repeadetly transmits. I think the solves my initial concern that in the first few milliseconds after button push, along with switch bounce, it might not transmit...but if it just keeps transmitting, then it doesn't matter, eventually my receiver gets the signal.

So slightly separate question: if I run the receiver on A/C since it always has to be listening and since it's not inconvenient to get A/C, what's the easiest 5V power option? I don't see 5V wall warts (surprising) so do people generally use a 9V wall wart and a linear regulator if they don't care about efficiency? Or do they use a switching converter? I"m just looking for the best overall solution. Seems sort of silly that I should have to add half a dozen components to get regulated 5V. Thanks.

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Re: RF transmitter chip -- power up and transmit cleanly

by zener on Sat Oct 09, 2010 1:56 pm

You need to be aware that there are: AC output, DC output (unregulated) and DC output (regulated). The latter are generally switching type so they are lighter also, but may have some noise on the output, and may have a minimum load spec. But you just have to check the specs. You could definitely use the 9V supply they sell here (at only 7.00 it is a good deal) and just throw a 78L05 or 7805 on it (don't forget the input and output caps). 5V regulated wall warts can be a little hard to find but they are around somewhere.

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Re: RF transmitter chip -- power up and transmit cleanly

by richms on Sun Oct 10, 2010 6:02 am

There are plenty of 5v wallwarts, iphone chargers etc.

But there are plenty of crap ones, look on ebay at what you get for a dollar, and you will see some scary scary things.

No feedback on them, no overload protection, just one diode in them because a bridge would cost too much etc.

motorola razr chargers have a mini usb plug on them and the actual motorola ones are well made.

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Re: RF transmitter chip -- power up and transmit cleanly

by cstratton on Sun Oct 10, 2010 12:49 pm

Several years ago, nice very compact switching regulated 5V wall warts came on the market for less than 5 bucks. The first one I got in a digilent fpga kit was very nice in that it plugged vertically into a power strip and didn't exceed the allowable spacing for one plug.

Actually saw a whole rack of them on 99c closeout at walmart once, they were intended for some discontinued kids fake laptop thingy.

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Re: RF transmitter chip -- power up and transmit cleanly

by adafruit on Sun Oct 10, 2010 2:05 pm

we have a 2A / 5V switching supply in the shop thats pretty good. it has a nice long cable :)

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Re: RF transmitter chip -- power up and transmit cleanly

by bythewater on Mon Oct 11, 2010 9:47 am

Thanks for all the replies, I'm not getting any email notifications of replies to my post even though everything seems to be setup for that, if anyone can tell me if notifications are working for them that would be appreciated!

As for my original post/question, for what it's worth to anyone researching this transmitter / receiver: I have the thing working, haven't done much range testing yet but it's working fine in my house between floors. The "VT" pin on the receiver means "valid transmission". I'm using this as the trigger for my receive end, I brought one of the transmit bits low on the transmit end, and so whenever I push transmit I get a 5v high on VT for about 500 ms. This is exactly what I need since I'm not sending data per se. There is probably a way to just use the tx/rx devices without the encoders and decoders but then there would likely have to be supporting analog circuitry and then I'm thinking you would get false positives from other transmitters.

I still have to test whether this arrangement is robust enough to handle a single pushbutton that 1) powers up the regulator and 2) fires Transmit Enable all at once. We'll see if that is reliable.

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Re: RF transmitter chip -- power up and transmit cleanly

by richms on Mon Oct 11, 2010 10:30 am

What chips are you actually using? have you looked at the datasheet for them?

Am asking because those chips are often used in keychain type remotes that run on a 12v battery. Are you sure you need the regulator?

And I am getting alerts via email, there is a tickbox when replying to get notifications. All I have ever done is make sure that was ticked and it has worked.

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Re: RF transmitter chip -- power up and transmit cleanly

by bythewater on Mon Oct 11, 2010 11:43 am

I'm using this:

http://www.rentron.com/remote_control/T ... t-pack.htm

Yes, the transmitter has a wide voltage range but the HT-12E encoder IC requires 5V, unless I"m missing something. I'd love to not have to use regulation on the transmit end.
CORRECTION: HAVE found the full datasheet for the HT-12E and it handles 2.4 - 12 V which is great.

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Re: RF transmitter chip -- power up and transmit cleanly

by zener on Mon Oct 11, 2010 1:20 pm

That schematic is a bit odd. The transmitter has power all the time, but the "transmit enable" is a pin on the encoder. I am guessing it is really not a transmit enable but an encoder enable. It would be interesting to know what chip is actually on that transmitter. Their datasheet leaves something to be desired.

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