rf ... getting close to understanding

[daviddoria]

see this link for circuit:
https://www.electro-tech-online.com/threads/headaches-and-the-pic18f452.6780/

i dont understand how the tank circuit uses the pulse from the 555. it seems to me that you set the frequency of the 555... and you set the frequency of the tank circuit... so i dont understand how you have 2 frequencies at the same time..?

if i get this answered, i'll fill in values and send it back to you guys and see if it makes sense

thanks
david

 

[Sebi]

There is no two frequencies: the tank resonance freq. and 555 freq. must be same.

 

[daviddoria]

really..............

then why even need the 555? i thought the tank resonated by itself?

 

[Sebi]

The tank cct is passive.Need some energy for oscillating.

 

[daviddoria]

and that energy needs to be provided at the same frequency as the tank oscillates? the energy cant just be constant?

 

[k7elp60]

starting to understand RF

and that energy needs to be provided at the same frequency as the tank oscillates? the energy cant just be constant?

The excitation frequency to the tank does not need to be at the same frequency the tank is tuned to. If it is at a sub-harmonic the circuit wil double the input frequency. If it is at 1/3 the tank frequency the tank circuit will triple the frequency, and so on.

 

[daviddoria]

rather than match exactly the resonant frequency of the tank circuit with the 555 frequency, am i not supposed to use a feedback type thing? so it supplies more power exactly when its supposed to? how would i go about doing this? or is it necessary?

 

[Nigel Goodwin]

To be honest, that design is an absolute load of rubbish - neither the transmitter or receiver make much sense, I wouldn't suggest bothering with it!.

 

[daviddoria]

nigel, you seem to be very confident of this.

what i am trying to do is simply send the state of a switch via RF. do you have something about this level of difficulty that will work?

thanks
david

 

[Nigel Goodwin]

nigel, you seem to be very confident of this.

I am!, it's a (supposedly) RF design, using nothing but audio type components - plus a couple of tuned circuits. It's probably one of the most horrible circuits I've ever seen!.

what i am trying to do is simply send the state of a switch via RF. do you have something about this level of difficulty that will work?

How far do you need it to reach?, is it 'line of sight'?, and does it have to be RF, or would IR do?.

For RF your best (and easiest) way is probably to use a pair of UHF TX/RX modules along with a pair of Holtek encoder/decoder IC's. They also are 'licence free', the design you are looking at is probably illegal in most countries - and if not, it should be!.

I don't wish to appear rude, but anyone considering building that 555/741 design shouldn't be building any RF circuits at all!.

 

[daviddoria]

i am in my first year of electrical engineering, so i have no idea what i'm doing yet, but in a couple years i hope to! for the mean time... i essentially want to trigger something through a door.. so line of sight and IR are out of the question. but it only needs to go like 20 ft. or so....

i was under the impression that the RX/TX modules with en/decoder ICs are more for sending signals... where all i need to do is say "yes, the transmitter is transmitting" or "no, there is no signal"

you still think this method is best? if so, could you give me a couple of part numbers and i'll try to look up what to do, and if that fails i'll ask again on here !

thanks
david

 

[Nigel Goodwin]

i am in my first year of electrical engineering, so i have no idea what i'm doing yet, but in a couple years i hope to! for the mean time... i essentially want to trigger something through a door.. so line of sight and IR are out of the question. but it only needs to go like 20 ft. or so....

Yes, 20ft is no problem for IR - but doors are 8)

i was under the impression that the RX/TX modules with en/decoder ICs are more for sending signals... where all i need to do is say "yes, the transmitter is transmitting" or "no, there is no signal"

It's far better to send a coded signal, so you have "correct signal" or "no correct signal" - it will be far more relaible.

you still think this method is best? if so, could you give me a couple of part numbers and i'll try to look up what to do, and if that fails i'll ask again on here !

You don't have your location filled in, so I've no idea what country you might be in - but in the UK Maplin at https://www.maplin.co.uk sell a matched pair of transmitter and receiver for £19.99 and stock the Holtek encoder/decoder chips as well.

One option you could think about is a cheap car alarm, you may be able to find one with a radio remote control - you could then strip the receiver part out to use, and use the transmitter as it was.

 

[daviddoria]

i am in the US. i considered the stripping of an already made TX/RX device, but 1) i couldn't find anything less than like $50, and 2) i didn't think i would get much new knowledge out of doing that.

are there specific en/decoder chips that go with specific tx/rx sets? or can they be used interchangably? where would i find some more info on the basics of doing this?

thanks
david

 

[daviddoria]

sorry for the double post

 

[Nigel Goodwin]

are there specific en/decoder chips that go with specific tx/rx sets? or can they be used interchangably? where would i find some more info on the basics of doing this?

Have a look at https://www.holtek.com who sell encoder/decoder chips for use with transmitters and receivers.

 

[John Sorensen]

I haven't tried this yet, but I'm thinking it should work. Make one of those FM transmitter circuits (usu. one or two transistors, some passive components, usu. titled FM Transmitter 1, 2, etc...). They work pretty good and can transmit 20 or more feet easily. Put a micro on it to transmit, at a very slow rate, a signal like you would see on a RS232 line: start bit, data, stop bit. Either have two frequencies, like a modem would, or just a frequency that equals a 0 and lack of a signal is a 1. On the recieving end, hack a $5 Wal-Mart radio (cheaper to buy the radio than build a reciever yourself) with a band pass filter (or two) into a micro, to recieve and interpret the incoming data.

I built one of those circuits I mentioned above, that had a mic on it, and I paralleled in a signal from a micro (square wave, though I also did it with the capacitor voltage on a 555, which was more sinusoidal though it made no difference). Even when the input from the mic was undiscernable, you could still make out the beep-beep-beeps from the micro.

The design should be very low power, if you put the micro in a sleep mode when it's not transmitting.

j.

 

[daviddoria]

i'm not familiar with "a micro". a microprocessor? that sounds a bit complex for this eh?

 

[jem]

Try www.qualitykits.com, and go to their RF section. I am sure you can find something suitable for your use there. Plus, it is inexpensive as well.

Good luck.

Jem