Calculating Coils for AM Radio Frequency

[mearvk]

How does one know how many coils one needs to pick up a given frequency? AM frequency requires how many coils?

I have a VC w/ inductor setup running into an AM IC but I'm not getting anything out of it. Pretty sure I have it set up correctly, alas no music!

 

[Bobbywhy]

How does one know how many coils one needs to pick up a given frequency? AM frequency requires how many coils?

I have a VC w/ inductor setup running into an AM IC but I'm not getting anything out of it. Pretty sure I have it set up correctly, alas no music!

Will you please explain what is a VC w/ inductor setup?
What is an AM IC?
How/where are the coils in the circuit?
Can you post a schematic diagram or a website reference?

Thank you,
Bobbywhy

 

[mearvk]

VC = variable capacitor = http://www.stormwise.com/amvc384b.JPG

AM IC = integrated circuit for AM frequencies = http://u.jimdo.com/www17/o/s9d9200af5bf0f371/img/i1696eb04a9f4fffe/1322251469/std/image.png

Similar circuit to the one I'm using: http://users.belgacom.net/hamradio/schemas/mgradio_zn414_mk484_ta7642.gif

Inductor (coils) : http://t1.gstatic.com/images?q=tbn:ANd9GcT4Poge1wDd44aOfbXQv-mt9cPJYWBNrZS_gri6obGVrViXbPL5Hs7MLTpp

 

[Bobbywhy]

mearvk, Thank you for posting all those references. A short time using Google I found these two examples of how to connect up the external tuning capacitor and the coil. Also, notice the power and headphone connections are included...everything you need! Good luck!

http://www.circuitdiagram.org/simple-am-radio-circuit-ta7642-ic.html
http://www.angelfire.com/electronic2/index1/mk484.html

 

[NascentOxygen]

How does one know how many coils one needs to pick up a given frequency? AM frequency requires how many coils?

If you live near to an AM station you´ll probably find that you don´t need the tuning circuit at all. But it´s a big help in separating stations when your aerial picks up more than one strong station.

There are formulae for designing the tuning coils. But it´s not all that critical, because adjustment of the tuning capacitor can compensate for being a bit off the mark with the inductance. You could always wrench the tuning circuit, complete with ferrite rod, out of an old transistor radio and use that for your tuning, at least to get your construction going.

I´m sure that a google search will turn up plenty of construction projects that provide details of the coil dimensions and wire gauge for AM broadcast frequencies. If you will be chasing distant or overseas stations during late evening listening, then the bigger and more efficient the coil, the better your chances.

Good luck with your first radio. None will ever sound better! http://img808.imageshack.us/img808/28/biggrinw.gif

 

[mearvk]

I asked elsewhere but let me ask again. The circuit diagram I posted has the inductor with the VC as one unit. However it's not clear how exactly should the wiring go.

My current setup is to have the voltage go into one of the feet (separated from the chassis and moving capacitor piece) and then the other side of the circuit (going to ground) goes from the chassis to ground.

My current (no pun intended) circuit looks like:

4.5v Vdd --> left inductor leg thru the inductor to right inductor leg --> VC foot (via capacitance) to VC chassis ground --> into IC per diagram

One problem is that the VC + inductor drains the voltage big time. I have 4.5v going in and about 1v at the input to the IC (after passing thru the VC + inductor). The IC says it requires between 1.2 and 1.6 volts to operate. I'm not getting anything at all in the way of AM radio so either it's a voltage issue or my circuit is wrong or both.

Help me out guys.

Here's the page I referenced: http://www.crystalradio.net/beginners2/caphookup.shtml

 

[Bobbywhy]

mearvk, What do you mean when you posted "One problem is that the VC + inductor drains the voltage big time."? Where is your voltmeter connected? What happens, and when? Wiring up your three-pin AM radio IC if not done carefully, will destroy it.

What kind of audio interface device are you trying to use, ceramic headphones or high-impedance headphones? (If the former then click on the -4 kit.) If the latter, then you can go to the below website
http://www.angelfire.com/electronic2/index1/mk484.html
and click on
MK484-Kit-5.pdf

Here you will find the physical pin-out diagram for your IC, and a complete schematic diagram of your radio. Connect the two leads from your Variable Capacitor to the points indicated. Follow the schematic diagram exactly, and then verify it again before connecting up your power source.

Good Luck,
Bobbywhy

 

[mearvk]

I'm interested in the VC + inductor wiring per my previous post. If it's too much to expect that I can ask a question and get an answer on a forum designed to do exactly that then I will move on.

Good day.

 

[davenn]

I'm interested in the VC + inductor wiring per my previous post. If it's too much to expect that I can ask a question and get an answer on a forum designed to do exactly that then I will move on.

Good day.

dont be so touchy,
people are trying to help you, we can only work off the info you give and if that is erroneous then its difficult for us to know what exactly is going on

you have already got all the info you need in the links you provided above
and just to clarify... the variable cap is in parallel with the inductor

and for your other question how many inductors do you need ... the circuit shows only one inductor ... that's all you need.
If you are draining the battery quickly then obviously there is an error in your wiring
you said your circuit is similar to the circuit you posted... where did YOUR circuit come from ? can we see it and make sure there is no problem with it ?
why do something different when you have a good circuit to follow ?

cheers
Dave

 

[vk6kro]

The circuit posted earlier was this one:

So, the ZN414 would be running on about 2.1 volts or less (because of the 3 silicon diodes across the power source) and the 100 k in series with the inductor would be more likely to drop the voltage than the inductor would.

Just as a test, the inductor should have a resistance of less than 100 ohms, and the points on the variable capacitor where you are making a connection should have a resistance of many megohms between them.
So, if you have a multimeter you could check these things.

The variable part of the capacitor should be connected to the left side of the circuit where the capacitor meets the 100 K and the 0.022 uF capacitor. It should not be connected to ground in this circuit.

The 10 K in the power line should limit the current to less than 0.45 mA if your battery is 4.5 volts, so the inductor shouldn't be having much effect on the battery life.

 

[davenn]

the problem is, he's not using that specific circuit, rather one similar
so as I said in my previous post, till he posts a circuit diag of "HIS" version, we don't really know what is going on :)

Dave

 

[mearvk]

The problem is that it's on a piece of paper. I'll scan it sometime this week and post it.

 

[mearvk]

Paper diagram as promised.

 

[mearvk]

Paper diagram as promised. I'm trying to use the top one but instead of using the transistor I've been trying to feed the IC output into a LM386.

 

[davenn]

Paper diagram as promised. I'm trying to use the top one but instead of using the transistor I've been trying to feed the IC output into a LM386.

ok, thanks for that :)
it would probably be an awesome idea to get the unit running in its designed condition before doing any changes to it. :)
I would strongly suggest that you use the single transistor output as shown and see how you go. Then once its working play with the LM386 variation. because the thing is without seeing how you have wired in the LM386 we still don't know if youo have wired it up correctly... actually without seeing your construction we don't even know if your have wired up the first stages correctly according to the circuit diag. ??
one wire in the wrong place and it isn't going to work

I am trying to figure out the transistor output in that first diagram it doesn't look right.
the collector would normally go to + volts and the emitter to - volts
Am sure vk6kro can comment on that and why they have done it that way :) definitely not the way I would have configured the output. ... it would have been more like the way as shown in the circuit vk6kro has shown a few posts back

so finally for this post ... how about a sharp focussed pic of your construction please
so we can compare it to the circuit :)

cheers
Dave

 

[vk6kro]

It is a PNP transistor, so the polarity is right, but the headphones should be in the line between the collector of that transistor and ground. Even this is bad practice (DC should not run through headphones) but at least it can work like that.

There should also be a capacitor between the IC and the base of the transistor. The 10 μF that is across the battery would be OK for this. The + side of the capacitor would go to the base of the transistor, in this case.

I did a simulation of the second audio circuit and I suggest you leave that one alone. It takes a lot of changes to get it working at all. Low voltage, direct coupled circuits are tricky for anyone.

A LM386 sounds like a good idea, but it will need normal voltages, more than 6 volts preferably, not 1.5 volts.

 

[NascentOxygen]

Paper diagram as promised. I'm trying to use the top one but instead of using the transistor I've been trying to feed the IC output into a LM386.

How about concentrating on just the IC stage? Do you own a crystal earpiece? If so, go to the end of this article http://www.mds975.co.uk/Content/trfradios.html and work your way back up a little to the diagram (third from the end of the page) showing the ZN414 (equivalent to your TA7642) in a circuit by itself and driving a crystal earpiece. Copy their earpiece arrangement for your use. (BTW, that's a 47kΩ resistor.)

Once you know the detector stage is working and the tuning range is correct, you can explore some audio amplification.

Your initial quandary was how to connect the tuning circuit. You said yours is a combined L-C arrangement, so I assume it has just two terminals. Your schematic shows the L-C block needing just two connections, so I can't see where there is room for confusion. I think you need to elaborate on this, as getting the tank circuit right is crucial.

Good luck.