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Author Topic: Confused about a component  (Read 689 times)

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Offline extreme.alyTopic starter

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Confused about a component
« on: April 26, 2013, 02:59:47 PM »
Hello,

I'm trying to make this Walkie Talkie circuit but I don't know about a component in the schematics that I have.

Schematics: http://www.talkingelectronics.com/projects/200TrCcts/images/WalkieTalkieWithLM386.gif

Can anybody tell me what that 'variable transformer type thingy' is on the top-left, with text 1t,2t,3t around it?

Thank you

Offline jwatte

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Re: Confused about a component
« Reply #1 on: April 26, 2013, 03:07:30 PM »
That looks like a Variac (variable transformer) and I imagine the 1/2/3t are "turns" of the wire in the transformer.
I could be wrong, though. Seems like a esoteric component...
Also, when dealing with radio transmission, make sure you're operating within a band and power level where unregulated traffic is allowed in your country!

Offline extreme.alyTopic starter

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Re: Confused about a component
« Reply #2 on: April 26, 2013, 03:18:26 PM »
Well, on internet, I see very big components when I search for variac.

Is there any equivalent replacement for this part?

Offline jwatte

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Re: Confused about a component
« Reply #3 on: April 26, 2013, 05:30:46 PM »
I think you have to make it yourself. My guess is that you need to get a ferrite core and some transformer wire. The "variable" part means sliding the secondary winding more or less onto/off-of the ferrite rod, IIRC.

In general, is there a particular reason you need this particular circuit to work? If all you need is wireless walkie-talkie, there are probably easier ways to do that these days, including crystal-based tuners, or microcontrollers with something like nRF24L01+ based transmitter, etc. A microcontroller can do class D microphone input, or simply use the A/D converter, and PWM analog output for phone level quality very easily. Put 32 bytes from that circuit into a nRF24L01+ packet network, and you have a much more robust solution than the analog circuit you're linking to.


Offline waltr

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Re: Confused about a component
« Reply #4 on: April 26, 2013, 05:59:17 PM »
It is an RF transformer. 2 turn primary, 4 turn secondary tapped 1 turn up and wound on bobbin with an adjustable (the arrow) core of powered iron (the two vertical lines).
This couples the antenna to the circuit while providing the correct impedance match.
It also is part of the Local Oscillator tank circuit.
What frequency does this transceiver operate at?

That circuit will not be easy to get working without a lot of experience with RF circuit and fiddling.
If you're good at fiddling and have some test equipment, O'scope, then go for it.

Offline MikeK

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Re: Confused about a component
« Reply #5 on: April 28, 2013, 05:53:12 PM »
You can find plenty of "walkie talkie" circuits on the web that won't require a tunable transformer.

Offline jwatte

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Re: Confused about a component
« Reply #6 on: April 28, 2013, 10:35:51 PM »
Again: Why are you trying to build this particular circuit? If all you want to do is build a walkie-talkie, here's a simpler and cheaper design using only commonly available through-hole parts:



Eagle schematic file

(Note: This is sketched out; I didn't actually build this exact circuit right now)

You'll need to write some code to sample the microphone at 8 kHz or so, and transmit as 64 kbit of data, and then play back using OC1A PWM on the receiving end. The bonus is that this circuit is also full duplex!

(Note: This is sketched out; I didn't actually build this exact circuit right now)

You'll need to write some code to sample the microphone at 8 kHz or so, and transmit as 64 kbit of data, and then play back using OC1A PWM on the receiving end. The bonus is that this circuit is also full duplex!

For the nRF24L01+ board, you can for example use this module for $3.20 or if you want much longer distance, while still fully within regulatory limits in most countries, pay $18.50 for a module with proper antenna stage.

So, with a $3 microcontroller, a couple of $1 amplifiers, a $3 RF stage and a few dollars of discretes, the most expensive bits will likely be the microphone and speaker (although there are some amazingly cheap and crappy options there, too, if you look around :-)

« Last Edit: April 28, 2013, 10:39:37 PM by jwatte »

Offline paulstreats

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