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How do I wire a tuning capacitor and antenna for a basic radio?

  • Hi! I recently watched this video: http://www.youtube.com/watch?v=4E6uwEiOGsQ which got me interested in starting some circuit work myself. I'm doing advanced electromagnetism this year in uni and although it's not a large part of the course, I think it will still help me gain an interest for the subject. So I went to a store for this stuff and spent $120 on a bunch of components that I no doubt do not need at this stage - but that's okay! I got a breadboard, electrolytic capacitors, ceramic capacitors, resistors, a variable capacitor, potentiometers, a speaker, a [free because the guy felt sorry for me - not knowing what I was doing] extendable antennae and some other junk. I attempted to build the radio in the video but as entirely expected it didn't work. I am confident I got most of it right but I know some of it is wrong because I simply couldn't figure our how to connect the antennae and variable capacitor. I will be getting a soldering iron tomorrow so I can solder wires to components (such as the speaker which at the moment has wires held there by blutac), so please let me know what to solder where... Basically here my tuning capacitor: http://imgur.com/4VRjs it is one of these (for clarity): http://www.shopforelectronics.co.nz/product_info.php?products_id=33772 And here is my antenna: http://imgur.com/GN1Xs I have absolutely no idea whether that antennae will work and if not, which type I need so that would be helpful to know. I'm not sure what type of wire is required to connect it also. And I'm not sure how to wire up the variable capacitor. If it had only 2 prongs I'd wire a positive and negative and chuck it in the circuit but the third one has thrown me off. Finally, I have all of the same resistors and capacitors the man uses in his video (plus a bunch of others) except my speaker is 4 Ohms instead of 100, will that make a difference? TL;DR I mainly want to know how to wire up a tuning capacitor (The one in the photo) and how to attach an antenna to a breadboard circuit (ie what type of antenna and wire I need). Thanks

  • Answer:

    Take a look at the improved video here: http://www.youtube.com/watch?src_vid=4E6uwEiOGsQ&feature=iv&annotation_id=annotation_678601&v=3Zzmi_Kno4w . It has a schematic right after the parts list at the end. Holler if you need help with the symbols. The antenna is the point-down triangle. You may find that a thirty-foot wire works better than your little telescoping antenna, but it should do for strong signals. You'll only need one side of your two-gang tuning capacitor. He specifies 2-200 pF so you won't get as much tuning range as he did. 2-200 seems an awfully wide range to me, may be a typo in there.

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you have an expensive hobby there, i had spent more than $1000 just for curiosity your image 1 and 2 is the same, it is a variable capacitor 60pF to 160pF the image 3 is just an antenna, you don't need it, just hold the wire with your finger, your body can be acted as antenna what missing is the coil, the coil is parallel with the variable capacitor for tuning to a certain frequency i am not sure you have AM & FM radio in your country, assuming the tuning is for AM stations capacitor 60pF-160pF, average C = 110E-9 farad AM 530kHz-1700kHz, average f = 1 115 000 Hz f = 1 / [ 2pi * sqrt(LC) ] L = 1 / [ (2pi * f)^2 * C ] L = 1 / [ (2pi * 1 115 000)^2 * 110E-9 ] L = 1.85E-7 H or 1.85 uH you need a coil of 1.85 micro-henry the connection is LC parallel to ground (chassis) ant o--- coil ---o grd ant o--- capacitor ---o grd |_ input to RF amplifier the video shown will not work because of interference, all the wiring should be enclosed by a metal chassis, after the tuning, it go through RF then detector then audio then speaker

Tony Q

Have a look at http://www.techlib.com/electronics/crystal.html where it shows a basic crystal radio wiring and a number of amplifiers, most of which use a crystal earpiece as an output, You will need an audio transformer to change the impedance of your 4 ohm speaker to 100 ohms.

GibsonEssGee

This video has some errors in it as the presenter gets the pin number in of the chip backwards and is confusing pins 6 & 7. He is telling you that pin 7 is output and pin 6 is power. THAT's BACKWARDS !!!!! He is connecting the wires to the correct locations but telling you the incorrect pin numbers. If you know the correct pin ordering of an IC and follow this guys verbal instructions you will hook up the output and Vcc BACKWARDS. (You might have blown the chip) Now you know why its best to work from a schematic diagram. Here is a 741 radio diagram http://www.aaroncake.net/circuits/radio.asp?showcomments=all http://www.circuitdb.com/circuits/id/31 EDIT If you want to know how to wire the cap you need a datasheet of the part that will show you the connections on the component. Since you using a dual variable cap pick one of the caps and connect a wire to either plate. As for the antenna wire no special type is required but a long wire is best to give you the strongest signal. To keep cost low you can use light gauge wire. Chances are that the middle connection is the common point between the two caps. Connect a wire between that point and either of the remaining conections.

MarkG

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