Author Topic: Inductor value needed for Match Box size radio based on Transistors BC 548 & 495  (Read 9145 times)

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Offline JimRemington

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A similar design is shown here http://www.techlib.com/electronics/reflex.htm (see section titled "Two Transistor Reflex Radio"), in which the first stage amplifies both RF and audio. I had forgotten about that design!
 

Online Ian.M

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The closest example schematic for a  single transistor, two diode charge pump detector reflex regenerative HF/AF amp stage direct coupled a ferrite rod antennae that I have been able to find is the Ladybird TRF radio by George Dobbs, (G3RJV)

http://www.mds975.co.uk/Content/george_dobbs_trf_radio.html

If you want to understand it enough to draw the actual schematic of your radio, read the book, but remember, it was written for Germanium PNP transistors, so the positive rail is at the bottom going to the transistor emitters.   To rearrange for modern Silicon NPN transistors, the bottom becomes the negative rail, still going to the emitters, all diodes must be reversed, and usually some attention to the base bias resistor networks is required, due to their higher Vbe and HFE.  The feedback capacitor C3 can actually be two insulated wires twisted together as a gimmick capacitor (http://sm0vpo.altervista.org/begin/gimmik-0.htm) or even just a carefully positioned wire rather than a discrete trimmer or fixed capacitor, so its presence may not be obvious.

Edit: Jim's just found a NPN silicon version of a non-regenerative reflex stage with a two diode charge pump detector. You will have to see which is closer to your set's circuit.

The first stage circuit in your schematic is a total mess, and the second stage has B and C swapped.
 

Offline gaurav279

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The circuit diagram has several obvious errors.

1) one of leads to the "495" transistor is not connected
2) the C and B of the BC548 are switched
Please check everything again.

Also, try using google image search for "two transistor radio schematic" -- there are a couple of similar results.
This seems very close and indeed is labeled "match box size radio": http://bestengineeringprojects.com/electronics-projects/smallest-radio-circuit-using-two-transistors/

Dear JimRemington,
Thanks for the reply.You indeed made my day.This was the circuit I have been searching since a long long time.The circuit on bestengineeringprojects.com that you have pointed is exactly the one that I have drawn from referring my Tiny non working Matchbox AM Radio.
By observing the same I have corrected the circuit in which the Base lead of Transistor 495 is going to the junction of 0.01uF capacitor and 33K resistor.But the Value of L3 Inductor Coil is still not mentioned and the author has advised to make it using 56 SWG over IFT Former... but he hasn't mentioned the length or number of Turns of this 56 SWG Wire to be used or the definite Inductance Value here.

Can you pls advise me now on what Value of a Fixed Inductor (a Resistor Type Inductor) should I use in this Circuit to make it Alive again.

Thanks !!!

Gaurav
 

Offline JimRemington

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As many people have already suggested, including the author of the circuit I posted, the value is not critical. Try a 1 mH inductor.
 

Online PA0PBZ

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But the Value of L3 Inductor Coil is still not mentioned and the author has advised to make it using 56 SWG over IFT Former... but he hasn't mentioned the length or number of Turns of this 56 SWG Wire to be used or the definite Inductance Value here.

It is mentioned in the article: "Counting of turns is not necessary. Wind the wire completed over former pulley."
Or in other words, just fill it with wire. We already told you a number of times that it is not critical.
Keyboard error: Press F1 to continue.
 


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