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Attempting to make a Keyfob transmitter...

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adamthole

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I got this idea a while ago to make for my Dad. I was going to design the transmitter and receiver and give them to him on Father's Day. The basic idea was to have a 4 button keyfob transmitter. 2 of the buttons would open various garage door openers and the other 2 would be left empty and could be hooked up to whatever he wanted later. This would require one transmitter and 4 receivers. I had no problem with the receiver, bu the transmitter was a little rough, and never got done.

I didn't get it done because 1) The board I used was too thick for the transmitter case to fit everything in properly and 2) all of my components were very squished together on a single sided board. I figured next time I would go with the 2 sider and a thinner board.

I user the serpac ca-4 case. This case is rather large, but it was the smallest I could find. Does anyone know where I can get a case that is smaller? Preferrably one that runs off of 1 12 volt battery, instead 2 button cells. Either would work I suppose.

I was using the HT12D/HT12E combination to do this with. Again, no problem with the receiver but the transmitter was a pain because of all the extra circuitry I had to use. Does anyone know of a chip that does basically the same thing, but is only on when it is receiving a signal and then off when it isn't receiving one? The HT12D, if a pin is active it will stay active until it is told to go inactive. This means that on the transmitter I had to add a capacitor to send out the signal slightly longer than when they pressed the button, and 4 diodes that would enable the transmission. I could have just kept that low, but then the battery would run out very fast.

So what I am looking for is a chip that is possibly smaller, possibly SMT, and doesn't have this enable pin stuff and only works when it is being transmitted too.

I hope I have been clear enough that I can get some help. If you don't understand something, let me know.

Thanks!

I have attatched the schematic of the transmitter.

(Note: While my sche says 3V it was actually running on 6)
 

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A simple 1-transistor transmitter can fit in a 1 inch by 1 inch by 1 cm high box.

and what is that IC you are using?
 
The IC is an HT12D. The thing with the 4 pins and the circle is the transmitter module I am using. Which reminds me, that I would like to replace that with my own transmitter. The current transmitter is Laipac 315 MHz. Anyone know how to make one similar to that?
 
Hi there adam. I think I asked you a couple questions about the encoders and decoders a while ago.

I don't see why you need those diodes and capacitor? I used the HT12E and HT12D and did not need those, and mine worked fine. I connected the TE pin to the pushbuttons just like you did, but without the diodes in series.

Not sure what you mean by the output pin on the HT12D staying low?

You are using the right voltage source for the HT12D I hope. I believe it's around 4.5 to 5 volts, but im not sure as I don't have the datasheets on my computer right now.

Also, make sure that your PSU does not have any kind of ripple in it. When I took a look at the signal coming out of the receiver on my o-scope, it was really noisy when I used a wallwart supply, and it made the decoder do strange things. Best to use 3 AA batteries if don't have a smooth PSU.

Hope this helps :lol:
 
Yeah, I've been playing around with RF for a while now. I've made many applications with the HT12D/HT12E combo. Everything is fine their. BTW, the HT12E has operating voltages of 2.4V to 12V so I am good their also.

The schematic I included works. I was just trying to show how unnecessarily complicated the HT12D makes things with this application.

Zach, how many of the outputs were you using? If you don't use the diodes, if I were to press S1, it would do the same thing as S2, S3, and S4. All 4 of the inputs would go low, causing the 4 outputs on the decoder to go low. You could not use the diodes if you used 2 switches, or if your switch was double pole double throw. Unless I am missing something?
 
Ohhh, i see why you have the diodes. I forgot that you have TE activated only when you touch a switch. When I was testing out my encoder/decoder, I had TE permenantly grounded and forgot about that on your schematic. No, you're right, my bad :lol:

My idea would work just fine, but like you said, it would run down the batter because the transmitter would always be on.
 
I need datasheets

I'm doing a project at the moment which invloves using encoders and decoders. Sice u r also using the HT12E and HT12D, would u have the data sheets for these, plz
 
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