Radio Junkies - Building a basic AM Radio

[ch33zw1z]

This project just kinda dropped on me with a week to do. Never built a radio or really worked on one. Basic troubleshooting skills on my side.

Using this article: https://www.wikihow.com/Create-a-Simple-AM-Radio

The piece I'm stuck on: variable capacitor

I purchased https://www.amazon.com/dp/B081YRTZ7M?psc=1&ref=ppx_yo2ov_dt_b_product_details

There's no instructions on how to wire this up. Bottom side has the tuning rod with no wheel / screw. Top side has four small nuts labeled c1, c2, c3, c4.

Anyone know where I should be attaching wires on this variable capacitor?

 

[William Gaatjes]

This project just kinda dropped on me with a week to do. Never built a radio or really worked on one. Basic troubleshooting skills on my side.

Using this article: https://www.wikihow.com/Create-a-Simple-AM-Radio

The piece I'm stuck on: variable capacitor

I purchased https://www.amazon.com/dp/B081YRTZ7M?psc=1&ref=ppx_yo2ov_dt_b_product_details

There's no instructions on how to wire this up. Bottom side has the tuning rod with no wheel / screw. Top side has four small nuts labeled c1, c2, c3, c4.

Anyone know where I should be attaching wires on this variable capacitor?

When viewing the amazon website, i noticed there are several types.
Can you make a photo of the top side and the bottom side of the capacitor please ?

There are definitely solderpins attached, but i would like to see what you have before i can give advice.
Perhaps a productnumber and a manufacturer can be found on the variable capacitor or do you have any datasheet or paper with some information that was shipped along with the item ?

 

[ch33zw1z]

I have two different kinds

 

[William Gaatjes]

I have two different kinds

It has been over 30 years since i used such a device. I am more familiar with PLL devices and varicap diodes.
So i had to call a friend who loves radios, als AM radios. Handy to know people who are near a century old. Yes, i am also going into that age range already.
As it turns out, the tunable capacitor has as mentioned in the description 2 capacitors.
one of 60pF and one of 140pF.
The 60pF capacitor is used for the local oscillator and the 140pF capacitor for the input LC filter.
According to the website, you need preferably 200pF but 160pF will do. Up to 500pF is oke.
The way i see it, you can combine the two capcitors by connecting them in parallel.
Then you have 1 capacitor of 200pF.
Since i cannot measure it, i have to guess it, but i assume the middle pin is the common pin of both capacitors
So connect both outer pins together.
I made a diagram quickly, bare with me. I know it looks awful.
That is if you use the large one.

if you used the small one with 2 pins, just solder a wire to those pins and see if it works.

I do not know if you have a multimeter capable of measuring capacitance in that range ?
Than you can see the capacitance change as you turn the knob.

I looked at the schematic and seeing how the 741 operational amplifier is connected. I am amazed it actually works.

 

[ch33zw1z]

It has been over 30 years since i used such a device. I am more familiar with PLL devices and varicap diodes.
So i had to call a friend who loves radios, als AM radios. Handy to know people who are near a century old. Yes, i am also going into that age range already.
As it turns out, the tunable capacitor has as mentioned in the description 2 capacitors.
one of 60pF and one of 140pF.
The 60pF capacitor is used for the local oscillator and the 140pF capacitor for the input LC filter.
According to the website, you need preferably 200pF but 160pF will do. Up to 500pF is oke.
The way i see it, you can combine the two capcitors by connecting them in parallel.
Then you have 1 capacitor of 200pF.
Since i cannot measure it, i have to guess it, but i assume the middle pin is the common pin of both capacitors
So connect both outer pins together.
I made a diagram quickly, bare with me. I know it looks awful.
That is if you use the large one.

if you used the small one with 2 pins, just solder a wire to those pins and see if it works.

I do not know if you have a multimeter capable of measuring capacitance in that range ?
Than you can see the capacitance change as you turn the knob.

I looked at the schematic and seeing how the 741 operational amplifier is connected. I am amazed it actually works.

View attachment 97518

Thank you very much! I will give it a shot soon and let you know.

 

[Paperdoc]

My radio circuit knowledge is rudimentary and rusty. From the simple circuit instructions you posted, I am guessing that this design uses a single input tunable input stage with a fixed inductor (the coil made on the cardboard roll) and a parallel small adjustable capacitor to select the specific broadcast frequency in the range of a standard AM radio - 500 to 1500 KHz. The selected signal then is fed to the Op Amp which contains a diode to rectify that AC signal to a half-wave DC signal whose amplitude is changing - that is the basis of Amplitude-Modulated radio. That, in turn, is amplified in the Op Amp in a circuit that only amplifies signal components below 5 KHz (typically for AM radio) and feeds that audio to the speaker.

In commercial radios the "front end" design is different so that the radio can "pull in" weak signals. The design is called superheterodyne. It uses a tuned input circuit to select the originating signal from the antenna PLUS a local oscillator circuit and a mixer to combine those two AC signals. The local oscillator circuit has its own second inductor and tunable capacitor, and it is calibrated so that its frequency is 455 KHz lower than the input frequency selected. The result of the mixed two signals goes through a narrow-band filter circuit and then is passed through a series of (typically three) narrow-band amplifiers tuned to that 455 KHz frequency band. These can have very high gain without amplifying a lot of noise because of the narrow passband of the amplifiers. THEN the highly-boosted signal goes to the Diode and later stages. To do this, the two tunable capacitors used in the tuner input and local oscillator normally are driven by a single shaft of the tuner so the the DIFFERENCE is always 455 KHz. That is why the items OP bought have two tunable capacitors of different ranges driven by a single shaft.

This type of unit is NOT needed for the simple circuit that OP is trying to build. That circuit calls for only one tunable capacitor for the input stage. BUT it certainly CAN be used as William Gaatjes advises above. Connecting as he suggests turns those two capacitors into one unit of approximately the right range, so it should work. It will be limited to only nearby AM stations with strong signals requiring little aplification, but it will work.

 

[William Gaatjes]

My radio circuit knowledge is rudimentary and rusty. From the simple circuit instructions you posted, I am guessing that this design uses a single input tunable input stage with a fixed inductor (the coil made on the cardboard roll) and a parallel small adjustable capacitor to select the specific broadcast frequency in the range of a standard AM radio - 500 to 1500 KHz. The selected signal then is fed to the Op Amp which contains a diode to rectify that AC signal to a half-wave DC signal whose amplitude is changing - that is the basis of Amplitude-Modulated radio. That, in turn, is amplified in the Op Amp in a circuit that only amplifies signal components below 5 KHz (typically for AM radio) and feeds that audio to the speaker.

In commercial radios the "front end" design is different so that the radio can "pull in" weak signals. The design is called superheterodyne. It uses a tuned input circuit to select the originating signal from the antenna PLUS a local oscillator circuit and a mixer to combine those two AC signals. The local oscillator circuit has its own second inductor and tunable capacitor, and it is calibrated to that its frequency is 455 KHz lower than the input frequency selected. The result of the mixed two signals goes through a narrow-band filter circuit and then is passed through a series of (typically three) narrow-band amplifiers tuned to that 455 KHz frequency band. These can have very high a gain without amplifying a lot of noise because of the narrow passband of the amplifiers. THEN the highly-boosted signal goes to the Diode and later stages. To do this, the two tunable capacitors used in the tuner input and local oscillator normally are driven by a single shaft of the tuner so the the DIFFERENCE is always 455 KHz. That is why the items OP bought have two tunable capacitors of different ranges driven by a single shaft.

This type of unit is NOT needed for the simple circuit that OP is trying to build. That circuit calls for only one tunable capacitor for the input stage. BUT it certainly CAN be used as William Gaatjes advises above. Connecting as he suggests turns those two capacitors into one unit of approximately the right range, so it should work.

Exactly the same words i heard yesterday. Indeed right you are, the 455kHz frequency difference and all components around with it.

 

[Paperdoc]

This all reminds me of the first two build-it-yourself radio kits I got as a child. In each case it required a good antenna for a decent signal from a nearby station, and a good connection to Ground to ensure a complete circuit from antenna to Ground. The first was a true old Crystal Radio kit with a variable inductor and some small capacitance simply from the spacing of circuit components comprising the tuner circuit (to select the frequency). It had a small metal spring and a crystal of quartz (I think) to function as the diode "detector". You had to fiddle with how the spring tip touched the crystal surface to get a poor connection that acted as a diode that allows current to pass in only one direction. That current was fed to a simple earphone for sound. All of the electrical power for that sound was from the signal picked up by the antenna, so it had to be a strong signal.

The second came a bit later in a different kit. It had a variable inductor again, but the signal selected was fed to a single-transistor amplifier powered by an AA battery. The transistor acted as diode detector and amplifier that fed an earphone, again. So it could receive marginally weaker signals than a simple crystal radio, but they still had to be pretty strong. As a child I used these to listen secretly (via the earphone) to evening radio programs after being put to bed.

I am reminded also of a unique radio design I read about but never built. It had TWO tuners for different functions. The first was a simple standard AM radio with a single-stage amplifier, like one above. The other tuner was to get free electrical power "out of the air". You tuned that to a very strong local AM station and the circuit merely rectified the signal to half-wave DC that charged a large capacitor, and that was the power source for the actual radio's amplifier.

Old MAD magazine joke, I believe. What happens when a HiFi enthusiast overloads the equipment to get HUGE sound? Ampli-Fire.

 

[William Gaatjes]

This all reminds me of the first two build-it-yourself radio kits I got as a child. In each case it required a good antenna for a decent signal from a nearby station, and a good connection to Ground to ensure a complete circuit from antenna to Ground. The first was a true old Crystal Radio kit with a variable inductor and some small capacitance simply from the spacing of circuit components comprising the tuner circuit (to select the frequency). It had a small metal spring and a crystal of quartz (I think) to function as the diode "detector". You had to fiddle with how the spring tip touched the crystal surface to get a poor connection that acted as a diode that allows current to pass in only one direction. That current was fed to a simple earphone for sound. All of the electrical power for that sound was from the signal picked up by the antenna, so it had to be a strong signal.

The second came a bit later in a different kit. It had a variable inductor again, but the signal selected was fed to a single-transistor amplifier powered by an AA battery. The transistor acted as diode detector and amplifier that fed an earphone, again. So it could receive marginally weaker signals than a simple crystal radio, but they still had to be pretty strong. As a child I used these to listen secretly (via the earphone) to evening radio programs after being put to bed.

I am reminded also of a unique radio design I read about but never built. It had TWO tuners for different functions. The first was a simple standard AM radio with a single-stage amplifier, like one above. The other tuner was to get free electrical power "out of the air". You tuned that to a very strong local AM station and the circuit merely rectified the signal to half-wave DC that charged a large capacitor, and that was the power source for the actual radio's amplifier.

Old MAD magazine joke, I believe. What happens when a HiFi enthusiast overloads the equipment to get HUGE sound? Ampli-Fire.

Crystal detectors, AKA cat whisker crystal detectors.
From the same friend, i got a cat whisker detector about 20 years ago.

Also known as a crystal detector. And a small piece of mineral. It is either galena or Pyrite. I have to have a closer look to see what it is.
I mention pyrite in the schematics below but i was not sure at the time and had to make a choice between galena or pyrite.
To show that it is a real diode. I made a small setup. I posted it once on a similar (famous) gathering but there i was hated and being called a "Wise-Ass".
I had to look up the definition and i certainly am nothing like that : Presenting myself as a very annoying know it all. 😢
Soon i found out that these people were nothing more than jealous and lazy and they love to make others work for them.

And when you have passion for something, a hobby interested or something other, and you enjoy it and love to share the love and passion for nature and electronics...
These unfortunate simple minded people will do everything in there power to take away the joy you feel. Mostly because i would not either build or repair what they desired...
Or explain everything on demand.
What too much booze and perhaps drugs can lead too. It is sad... 😢
Their favorite way is to prevent people like me from a good night of sleep, night after night for months. Waking people like me up, while those people like me are in their deepest sleep. Which made my sickness worse... 😢

Studying hard as a young lad to have a good life later in life. Only to be gazed upon with jealousy.
Of course, working with the mind while in the office or allowed to work at home because of being ill but still able to work. 😢
That is not working in the eyes of the jealous.

Anyway, back to subject :
It is fun to see how many materials exhibit semiconductor behavior and diode behavior.
There is quite a list here i just found out :

Crystal Detector: Cat's Whisker Radio Detector » Electronics Notes

Details of the cat's whisker radio detector or crystal detector - how this detector worked, the different types and the history of its development.

www.electronics-notes.com

And some years ago i made a setup to show the diode behavior of a cat whisker detector.
The image is with Dutch words. But the symbols are easy to understand.







One can see the forward conducting current (black multimeter) and forward conducting voltage (red multimeter).



One can see the reversed biased diode in effect. The non conducting voltage(red multimeter) and leakage current (black multimeter).



As we see in this graph :




You know what would be great fun ?
Let us say we have all decided to pretend we need to survive for a fun survival week and have to build rudimentary transmitters and receivers.
Keep it with rudimentary receivers first as that is easier. To receive assignments.
A fun thing to know, is that it is even possible to use a flame as a lousy diode. It works, not perfect but nevertheless, interesting .
Maybe anybody reading this can test this out, if an AM radio will work with a flame based diode detector ?
One would have to use two (the same) metal wires with a match or perhaps it needs to be a tube gas flows from that is burning.
Of course, one can experiment with different metals in the flame to improve the diode behavior.








Disclaimer :
I took the diode characteristic from this site, it was easier than to draw one of my own.
For those interested, there is a section about diodes on this site :

Introduction to Diodes

Introductio to Semiconductor Diodes, silicon diode construction. How diodes work and diode characteristics

learnabout-electronics.org

 

[Paperdoc]

I'm starting to feel guilty in helping to hijack this thread into a whole different area of nostalgia.

 

[William Gaatjes]

I'm starting to feel guilty in helping to hijack this thread into a whole different area of nostalgia.

Why ? Building an AM radio and improving upon it.
Being able to build a basic AM radio from perhaps almost anything one can find. That is like MacGyver man !
For enthusiasts and youngsters great fun. The : "Whoooo" factor, the "Eureka" moment.

When a person over time gains a lot of knowledge , everything that is obvious seems plain.
While actually it is a step up to greater things to come aware of.

And of course, the basic history from the radio from then long ago to these days . Nothing to feel guilty about.
Anybody who reads this with interest in basic AM reception and beyond (for example FSK, SSB, DSB, FM, QAM), is happy.

 

[ch33zw1z]

I'm starting to feel guilty in helping to hijack this thread into a whole different area of nostalgia.

Nah, you guys are fine. I never had these opportunities, so now I’m old and my kiddos getting projects. While I basically refuse to DO it for him (he’s old enough, if he puts his mind to it) I’m still curious and can better help if I know more about it. I’ve got a build-a-kit also that requires soldering, which I’ve never done but my older son has. So I’m grateful for the walk down memory lane, as it’s new information for me.

 

[William Gaatjes]

Ch33zw1z , how is the project going ?
If you got the radio working, there is fun experiment to do with eager youngsters.

I thought, it would be fun if you do so desire, to try to augment the tunable capacitor with a syringe (or bottle) based tunable capacitor.
Then you can adjust the radio reception channel a little bit by using a syringe or a bottle and changing the amount of water inside that syringe or bottle.

I once made a similar a device to learn stuff doing fundamental research : What makes a capacitor a capacitor.
Only then i used a bottle instead of a syringe. But both can be used.
I am not going to write a huge story about how capacitors function. But below is a more detailed explanation that will suffice.
But i will present that with a capacitor, the value is also based on the the dielectric constant (AKA permittivity) of the material between the capacitor plates.
The capacitor plates are in this example the copper rectangles with electrical wires attached to it.
I used copper with a sticky tape layer in the example to fixate the copper to the syringe but you can also make the two plates from aluminium foil but you have to use isolation tape to firmly secure the wires.
With a bottle it is the same : Just use two large rectangles of electrically conductive material that shall not touch each other.
See images at the bottom section of this post for explanation and to give you a visual idea.

For more background information see this post about :

Just a brainfart about igniting fuel, use microwaves instead of spark plugs.

Just a brainfart. But would it not be a good idea to replace sparks plugs with RF emitters. Use microwaves to change the energystate of the atoms in the pressurized fuel + oxygen mix . That could be an idea. The shape of the antenna, or use multiple antenna's would perhaps be a useful method to...

forums.anandtech.com

Small excerpt :
"
I remembered back then how i did experiments as a young lad to build a tunable capacitor by using a plastic bottle with two sheets of aluminum foil and measuring the capacity of the setup.
Based on the dielectric constant of around 80 for water. More water means more capacity. The capacitance increased with the amount of water in the bottle. And decreased when the amount of water was reduced.
Which fits perfectly with the calculation formula to determine capacitance of a typical parallel plate capacitor.
Fun experiment for eager youngsters, even today.
"


For background information about capacitors and how to roughly calculate the capacitance :

The Dielectric Material Used in Capacitors

Learn more about polarization and the dielectric material used in capacitors here.

resources.pcb.cadence.com

Simple formula :

A list of various dielectric constants (AKA permittivity) belonging to different materials :

Liquids - Dielectric Constants

Dielectric constants or permittivities of some fluids or liquids.

www.engineeringtoolbox.com

Abbreviations :
AKA = Also Known As.

Example setup i made to replicate old research.
This is my own home made capacitor and inductance measurement instrument but with added automatic calibration and auto-zero at power up, which i added myself.
This device is based on a homemade instrument from VK3BHR, a radioamateur and such devies are often readily build available to buy or as DIY solder kit :
See link :

VK3BHR - Digital LC Meter Version 2

Updated software - March 2020. The revised software checks for the presence (or absence) of a 1K resistor from pin 11 of the PIC to ground. This determines how the LCD is addressed. An "improved" inductance / capacitance meter by Phil Rice VK3BHR Actually, it works much the same as the

sites.google.com

I suspect that this device when understood can easily be made with an arduino.
The arduino does then the heavy lifting when it comes to the calculations instead of the PIC16F84.
But you have to write the calculations yourself in the program. But that is quite a few steps further down the road.






No water :


Filled with water :

 

[ch33zw1z]

The project is still going. We pivoted to a kit that my kiddo had to solder instead. So my older son, who’s naturally talented with this stuff, taught both myself and younger son how to solder, and my younger son did most of the work. I’m adamant that we will complete the wikihow one, baby steps! My kid waited until almost no time left to do his project, driving me bonkers cuz I’m not a miracle worker here lol.

Good info above though, I’ll review with both of them. The younger one wil surely eye roll like woah.

 

[William Gaatjes]

The project is still going. We pivoted to a kit that my kiddo had to solder instead. So my older son, who’s naturally talented with this stuff, taught both myself and younger son how to solder, and my younger son did most of the work. I’m adamant that we will complete the wikihow one, baby steps! My kid waited until almost no time left to do his project, driving me bonkers cuz I’m not a miracle worker here lol.

Good info above though, I’ll review with both of them. The younger one wil surely eye roll like woah.

I bet your older son will love the site from the radioamateur.
Version 2 :

VK3BHR - Digital LC Meter Version 2

Updated software - March 2020. The revised software checks for the presence (or absence) of a 1K resistor from pin 11 of the PIC to ground. This determines how the LCD is addressed. An "improved" inductance / capacitance meter by Phil Rice VK3BHR Actually, it works much the same as the

sites.google.com

and version 1 :

VK3BHR - A Surprisingly Accurate Digital LC Meter

This design has been superceded by "V2", which has simpler hardware AND software calibration. You should build that one instead!

sites.google.com

And a lot more is there to find.


When in time your kids are ready for a project like this and would really want to build such a device :
See the schematics of my version of the LC meter. It is 11 years old now. I made this device in the year 2013.
What i do is essentially, is automatically and not by hand by shorting manually and then pressing a zero button. I short the measurement terminals automatically with a relay for a moment when the capacitor mode is selected. This happens when turning on to capacitor mode from the off position.
Thus, auto calibrate and auto zero is done by activating the CAL-L RELAY1 during the calibration. This instead of the manual shorting and pressing zero. I do this to auto calibrate and auto-zero. It needs some reverse engineering.
The inductor, needs to be an air coil for reasons i no longer remember, the decision was made by the original designer. Air coils never saturate since there is no core material but only air.
These are zero instructions from the text from VK3BHR :

"
The Inductance measuring function is automatically calibrated when you calibrate the capacitance function. All the testing required is to check that the meter can be "zeroed" with the terminals shorted together.
"

If you replace the pic MCU with an arduino, i think it is possible to rebuild the circuit but with a modern microcontroller like an MCU form ST, that is programmable in C language or just taken an arduino or an arduino derivative. I have not tested that but i am sure it is possible.
I checked the datasheets between a 16F84 PIC and a ATmega328P-AU MCU which is used in arduino. The PIC has an 8 bit timer module and an arduino based upon an ATmega328P-AU, has an 8 bit timer module and a 16 bit timer module available.
If you use the 16bit timer module, the programming will be easier because it can hold a value 256x greater.
You can use for example either an arduino or another microcontroller and programmed in true C language.
The PIC assembly source code is there on the site of VK3BHR.
Just text search for source code.

It can measure capacitors as low as in the single digit pf and even into the hundreds of femtofarads that is a capacitor with 10^-15 capacitance values. Picofarad is 10^-12 capacitance values.
And also inductance up to around 10mH.
Great stuff these people invented pure by using mathematics.
Ideal for radioamateurs which often use capacitors in the picoFarad range and inductors in the uH range.
Cheers. Have fun and good luck !

From the site :






A peek into the inside of my capacitance and inductance, measurement tool.