300MHz RF module

bananasiong · Dec 15, 2006

Hi,
I've bought a pair of RF module, 300MHz. The transmitter looked simple, there is no even trimmer capacitor, and the coil is printer on the PCB. There is a relay on the receiver, and a LED as an indicator.
Once the tag switch of the transmitter is pressed, the circuit is closed, then relay at the receiver site is switched.
Now, I want to use them for transmitting digital signal. From a PIC to another PIC, I don't think the NC or NO on the relay is a good idea since it can bounce. I think I'll modify the circuit.
This is my question. If I send 8 bit digital pulse for example '10010011', how does the receiver know that which is the starting bit? Do I need a LED driver to convert serial to parallel data, then the paralleled connected to the receiver PIC.

Thanks

audioguru · Dec 15, 2006

The transmitter needs to have a modulator added. How are you going to add one?
Now the transmitter just turns on and off. It takes time for it to turn on and off. If you use a logic "1" to turn it on then the bit rate will be extremely slow.

bananasiong · Dec 15, 2006

I use PIC to turn it on and off with a transistor, can I?

audioguru · Dec 15, 2006

There might be a way to modulate it at a fast rate without turning it on and off.
Attach its schematic.

bananasiong · Dec 15, 2006

Then I have to draw it, because the schematic doesn't come together, only the layout. But the transmitter is really simple. A 2 input NAND gate IC (only 2 being used), a RFC, a few capacitors and a few resistors. I can see that the tag switch turns the whole circuit on and off.

ljcox · Dec 15, 2006

bananasiong said:
Then I have to draw it, because the schematic doesn't come together, only the layout. But the transmitter is really simple. A 2 input NAND gate IC (only 2 being used), a RFC, a few capacitors and a few resistors. I can see that the tag switch turns the whole circuit on and off.

If it uses a 2 input NAND gate, you may be able to modulate it by controlling one of the inputs. Post the circuit so we can help.

As for the receiver knowing how to determine the start and stop, there are many ways to do this. For example, look at the RS232 protocol.

Alternatively you can use pulse length modulation. eg. turn the transmitter on for say 100 us to send a 0 or turn it on for 200 us to send a 1. And keep it off for say 100 us between bits. This is a self clocking arrangement.

All the receiver has to do is to start timing when it receives a signal and then decide whether the signal is still there after 150 us. If not then it was a 0 if so then it was a 1. Then store the bit and await the next bit. You also need to count the bits.

You would also need to ensure that the signal is present for a minimum period (say 50 us) in order to avoid problems with noise bursts.

bananasiong · Dec 17, 2006

Hi,
This is the transmitter circuit.The RF coil is on the PCB. I couldn't find the schematic of the receiver yet, maybe I have to draw it according to the layout. Can I replace the 9018 npn transistor with other transistor? And, can I know any method of modulation?

Thanks

ljcox · Dec 17, 2006

bananasiong said:
Hi,
This is the transmitter circuit.The RF coil is on the PCB. I couldn't find the schematic of the receiver yet, maybe I have to draw it according to the layout. Can I replace the 9018 npn transistor with other transistor? And, can I know any method of modulation?

Thanks

Remove one NAND gate input and this will be the modulation input.

I'm not an expert on RF transistors, perhaps someone else can advise you.

audioguru · Dec 17, 2006

I think the transistor is the 300MHz oscillator and it is turned on and off at an audio frequency by the classic Cmos oscillator made from two Nand gates.

The Cmos oscillator can be stopped by diconnecting the input of one gate from the circuit and grounding it, then started with a resistor that pulls the input high.

bananasiong · Dec 17, 2006

This transistor can be replaced with other?
How do you know the NAND produce audio frequency?
Do you mean, one input from the NAND gate connected to the ground, the other input pulled up by a resistor, then the microcontroller input signal from here, right?
For data transmission, how does the receiver know that which is the first bit and which is the end bit? Or the receiver circuit needed?

Thanks

Hero999 · Dec 17, 2006

That module is nothing but a simple 'FM bug' RF oscillator tuned to 300MHz with a NAND gate oscillator modulating it. Here was me thinking that those expensive modules were generally well-designed and worth the money.

Look the NOT gate oscillator audioguru posted and look at the NAND gate oscillator on the transmitter schematic. Notice how they are essentially the same except yours uses NANDs while his uses NOTs?

There are two ways of making a NOT from an NAND aren't there? You can either tie the two inputs together or tie one output to +V, if you do the latter using a pull-up resistor it will work as a NOT gate but if you ground the other input it won't work. This is exactly what you do with your circuit you cut the link between two of the inputs on either of the gates, and use the disconnected input as an enable pin.

audioguru · Dec 17, 2006

bananasiong said:
This transistor can be replaced with other?

There are thusands of NPN RF transistors.

How do you know the NAND produce audio frequency?

Because I am a smarty-pants geek! It is about 7kHz but I looked on a graph without calculating it.

Do you mean, .....

Like my sketch. The on-off switch can be a transistor. The resistor and switch can be the output of a microcontroller.

For data transmission, how does the receiver know that which is the first bit and which is the end bit? Or the receiver circuit needed?

You need Manchester encoding at the transmitter. Use a gap between each transmitted word or use start bits or something.

bananasiong · Dec 17, 2006

This is the only RF module I can find.

if you ground the other input it won't work.

Oh ya, always high.

bananasiong · Dec 17, 2006

Like my sketch. The on-off switch can be a transistor. The resistor and switch can be the output of a microcontroller.

Oh I see, what's the speed for 8 bit signal? if I use Manchester coding, usually what method is used for decoding it at the receiver to the original signal?

Thanks

Nigel Goodwin · Dec 17, 2006

bananasiong said:
Oh I see, what's the speed for 8 bit signal? if I use Manchester coding, usually what method is used for decoding it at the receiver to the original signal?

Fairly obviously, you use Manchester DECODING!.

Sceadwian · Dec 17, 2006

Manchester decoding isn't as simple though, at least I couldn't find a simple sollution after a couple minutes of googling. Perhaps you'd care to share a simple clock/data decoding circuit from manchester Data? I'm curious as I'm going to want to use Manchester encoding at some point. Encoding is simple, no idea how to decode though.

bananasiong · Dec 17, 2006

Yes, that's my problem too.
For example, the data is 10010011 (start from the left), so the coded data should be: 1001011001011010. How does the microcontroller read this? How does the microcontroller know that which one is the start bit? Will the coded data be read like this:0100101100101101? (The 1st bit is 0)

Thanks

Sceadwian · Dec 17, 2006

It doesn't, that seems to be the trouble with decoding manchester. A Preamble packet should solve that though. Known data allows you to figure out where the bit transistion occurs.

Nigel Goodwin · Dec 18, 2006

Sceadwian said:
Manchester decoding isn't as simple though, at least I couldn't find a simple sollution after a couple minutes of googling. Perhaps you'd care to share a simple clock/data decoding circuit from manchester Data? I'm curious as I'm going to want to use Manchester encoding at some point. Encoding is simple, no idea how to decode though.

Why google?, why not just click on the link in my signature and look at my PIC tutorials?.

One of them implements Manchester coding/decoding - it's not my actual routines though, I got them off the net (and modifed them) - and as you say, it takes a LOT of finding.

bananasiong · Dec 20, 2006

Hi,
If I replace the oscillator transistor with 2N3663 or 2N5770, is there any problem? The purpose of doing this is because the transistor from the hobby kit is C9018 which is not available here.
And for the receiver, another transistor c945p, which I found that is just a general purpose npn transistor, so I can use 2N3904?
There is also a IC, kia567p. I think it is a phase lock loop and can be replaced by ne567n?
For the rfc at both transmitter and receiver, can I use 41uH instead of 1uH? I couldn't find it too.

Sorry, I can't find the schematic for the receiver yet, maybe I can't expect much without the schematic.

Thanks

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