bananasiong

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

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audioguru

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.

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Uncle $crooge

bananasiong

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

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audioguru

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

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Uncle $crooge

bananasiong

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.

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ljcox

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.

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Len

bananasiong

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

Attached Images

300MHz tx.GIF (4.6 KB, 81 views)

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ljcox

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.

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Len

audioguru

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.

Attached Images

300MHz transmitter.PNG (16.1 KB, 40 views)

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Uncle $crooge

bananasiong

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

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Hero999

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.

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audioguru

There are thusands of NPN RF transistors.

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

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

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

Attached Images

Gated Cmos oscillator.PNG (4.6 KB, 35 views)

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Uncle $crooge

bananasiong

This is the only RF module I can find.
Oh ya, always high.

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bananasiong

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

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Nigel Goodwin

Fairly obviously, you use Manchester DECODING!.

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