AM Transmitter & Receiver - Page 1
Forum section: electronics / projects

author: AJ4OM
author_rank: Newbie
timestamp: October 27, 2012, 09:35:03 pm
content: I'm looking for some good circuits for a broadcast band transmitter and receiver that we are building in my wireless communications class. The goal is for us to transmit and receive a voice signal over a distance of about 50' inside our building. If anyone has any good circuits that don't call for obscure or difficult to find components, I would greatly appreciate seeing them.

author: IanB
author_rank: Super Contributor
timestamp: October 27, 2012, 09:40:04 pm
content: Check out this chap's channel:https://www.youtube.com/user/AndyDaviesByTheSeaSome nice references and examples of simple builds there. You may find some ideas.

author: Thor-Arne
author_rank: Unknown
timestamp: October 27, 2012, 11:12:56 pm
content: Quote from: IanB on October 27, 2012, 09:40:04 pmOh, now I want to build a radio.

author: poptones
author_rank: Frequent Contributor
timestamp: October 28, 2012, 02:33:47 am
content: I love stuff like this so much I felt inspired to concoct something. You can't get more ghetto or more "classic" or more "8 bit" or whatever you like than this. How about a transmitter that uses BOTH a 555 and an LM386? Throw in two 2N3904s and you got a transmitter. That's a total of about 18 cents in semiconductor costs. The only part you won't find at radio shack is the tuning cap and you can use one out of an old am transistor radio just as well.I call it "smokey" because it's powered by magic blue smokeThe 555 is the local oscillator. The first transistor buffers and drives the output stage, which is directly biased and modulated by the lm386. The LM386 "self biases" at 1/2vcc, so we can exploit that in the output stage.I don't think the 555 likes 12V so you might want to keep it to a 9v battery. Even at 9v you should be able to get 50 feet with a meter or two of antenna and a proper tuning.

author: AJ4OM
author_rank: Newbie
timestamp: October 28, 2012, 02:57:53 am
content: That looks like a great option for the transmitter. Thanks for drawing that up. Now all I need is the receive and I'll be in business.

author: poptones
author_rank: Frequent Contributor
timestamp: October 28, 2012, 07:24:47 am
content: Here's a great article on the ta7642 which is pretty much a complete radio in a 3 pin to-92 package. You can get two of them on ebay for about 5 bucks.http://theradioboard.com/rb/viewtopic.php?t=2791Connect it to another lm386 with gain set at 200, and you got yourself a radio.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: October 28, 2012, 11:07:38 am
content: http://home.alphalink.com.au/~parkerp/gateway/nonov99.htmis a simple regenerative AM broadcast receiver. Add an LM386 for better speaker volume. Being regenerative it's not so good for really strong signals, but you can back off the regen control if that happens.As for an AM transmitter, use a transistor VFO (or 1 MHz crystal oscillator if you can get crystals and there are no local stations around there), then one or two transistors to amplify. Also make up an LM386 audio amp (or just use audio from an FM radio or simlar - via the earphone socket). Apply that (via a capacitor) to a point on the final RF amplifier (maybe the collector) to get modulation. You'll end up with something like this:http://www.gameangler.eu/delboy/m0dad/construcion/poppet_top_band_am_transmitter.htmI've built simlar rigs (about 1/2 watt output) on 1.8 MHz. Even with a 9 metre long antenna have had reports from 10km away. Or go beside a bay for 50km or more range. Here's a video:

author: poptones
author_rank: Frequent Contributor
timestamp: October 28, 2012, 09:12:28 pm
content: My transmitter is simpler.And damn, I thought I was the first to think of using an LM386 to automatically bias a modulator stage...I managed to track down a reasonable source for those little to-92 radios. Got 25 of'em on the way. Fun!

author: RCMR
author_rank: Frequent Contributor
timestamp: October 28, 2012, 11:20:26 pm
content: Quote from: poptones on October 28, 2012, 09:12:28 pmCare to share?

author: poptones
author_rank: Frequent Contributor
timestamp: October 29, 2012, 12:36:44 am
content: My experience is they will take a couple of weeks to arrive, but if they're still needed then, sure. I bought these from futurelec. They're pretty good but you really need to order a quantity to make up for the four dollar shipping.I bought a couple of those dds modules off the bay. When it arrives I'm going to build a QRP transmitter and use the receiver IC to make a small receiver. I took a preparatory FCC technician class license cold the other day and got a 77 which is enough to pass, I think I am going to schedule a formal exam soon and get my license. I've been "around" radio since I was a teen, worked contract engineering for stations (although I'm technically not allowed because I didn't have a first class license), seems like I should finally get some accreditation.

author: RCMR
author_rank: Frequent Contributor
timestamp: October 29, 2012, 01:28:33 am
content: I recall using the ZN414 many years ago.Although it's billed as a complete TRF receiver, it actually makes a pretty fine IF amp (at 455KHz) and detector for AM radios.If you use a dual-gate mosfet for the mixer then you can take the AGC signal and use that to change the bias on the second gate of the mixer mosfet and that will give *much* improved dynamic range. Just output the mixer into a 455KHz ceramic filter and feed the output of that filter into the 414, with the output of that going into a 386. I added a squelch gate as well so ended up with a very cheap and pretty useful AM receiver for communications purposes.I also built a full-duplex voice-link radio system using these chips. The two transceivers used crystals that were 455KHz apart so the transmitter oscillator doubled as the local oscillator. Worked very well, considering the low component count.