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Simple AM transmitter Answered

I wish to make an extremely small (size and part count) AM transmitter using only a few transistors and passive components. I wish to transmit an audible tone on a set frequency.

I am thinking along the lines of a transistor flip-flop circuit oscillator, perhaps routed through another. It really needs to have less that five/six small components. DIP components are too big. Small electrolytic and other capacitors, transistors, and resistors are acceptable. Transmission distance needs to be at least 1/4 of a meter, but further is better. It needs to run on between 1.5 and 6 volts, low to middle'in milli-ampage im thinking watch batteries for a few hours at least. I have read the following article but as stated before it needs to be very small. https://www.instructables.com/id/Make-a-simple-AM-transmitter/

Can anyone help me out with a schematic, thoughts or advice?
Thanks in advance,

P.s. I can work out transistor loop timings and I understand how the Amplitude Modulation system works. Current thoughts/plans (if you can call them that) are an audible tone switching a secondary transistor loop from high (no resistor in series) to low (resistor in series) power and back. The secondry loop will be switching at the target frequency.



10 years ago

So what's wrong with the crystal oscillator you linked to - too big? You can find smaller crystals, and in terms of complexity it's going to be hard to beat a crystal, a resistor, a capacitor and a battery...

Well, I don't think there's anything wrong with it but if I can do it without using potentially hard to get/expensive components all the better. If I can manage it with transistors then its more repeatable and modifiable for others. Plus if a given frequency is in use then the unit is useless whereas the transistors need only have a resistor swapped out. I may be mistaken but I thought the oscillators tend to be that size... I've seen smaller crystals but not the oscillator units, though this is me remembering back to when I first saw the instructable so I could well be mistaken.Finally I'd like two transmitters or more operating on different frequencies to the same receiver which could switch between all the units, although that's rather hopeful and not directly what I want to do, more a far off vague intention. But yeah, thanks for your input, I will do as you mentioned if this transistor idea doesn't work (when they arrive and I test it out). I just feel a little out of my depth with this RF stuff, even if it is on a small small scale with AM radio... There's just so many unknowns (known unknowns mind... I have a gut instinct that there are a few unknown unknowns as well but time shall tell) Thanks, Andy

Actually, using multiple frequencies is where crystals really shine. Since the crystal itself is the frequency-determining element, you can just swap out a crystal with a different frequency. Plus you can use a second crystal with a matched frequency in the receiver to tune into the right transmitter.

That's essentially how RC controlled cars, planes, etc work. At a meeting of RC enthusiasts (e.g. RoboGames!), there may be dozens of radio-controlled devices, many of them governed by the same model of controller. So people just swap out the frequency crystal in the transmitter and receiver to make sure there's no overlap.

Ah so that's how they do it.. always wondered watching robot wars as a kid whether they send data addressed differently, (but what if they send at the same time?! I would always think). Crystals tend to be of few fixed values though, common values such as 10, 12 and 20 Mhz are the types that I have encountered (microchip programming). Unfortunately the Instructable regards a crystal oscillator which I think is different, I'm not one hundred percent sure as this information itself was acquired in the comments. It would lead me to believe that the crystal on its own would require more circuitry, so a mini sized crystal unfortunately wouldn't be as great a space saving as it first appears. For testing purposes the frequencies need really to be within Standard AM radio station frequencies allowing me first to pick it up on a radio so that I can work on the transmitter knowing if I send a signal I can get it through and work on the the receiver half knowing I am indeed tuning into a signal. Limiting me to about 3 value oscillators. (speeds here: http://en.wikipedia.org/wiki/Crystal_oscillator
& medium wave here: http://en.wikipedia.org/wiki/AM_radio). In the end I will go with what works and all though the transistor solution is appealing on a "change it a bit later/hackable/reconfigurable" kind of level in terms of practicality/simplicity level your no doubt correct. When it gets right down to it I don't suppose It'd matter one way or the other so long as it works and fits I but I have a feeling that the oscillator might be more bulky (as seen in the instructable) and of course I don't have any oscillators to hand). Anyway the parts are due Friday and a prototype should be running (or not) by Monday assuming I have (or can scrounge) enough passive components, so I'll report back next week.
Thanks for your thoughts,

Thanks, although (I know this is ridiculous but hey) I would really like it to be smaller even than that, the penultimate link has given me an idea...I could have the tone coming from an external source which would be a fall back position (reduced power and size). Transmission distance need only be very small so no worries on the power front (I think). Google alas has given only bigger and better than I really need, which is small simple and most of all low power small parts. What do you think of the transistor loops idea? Is it feasible in your opinion or am I missing something obvious with it? Thanks for your input, Andy

Well, if you are able to work with surface mount parts, that would make things smaller. Getting really miniaturized is hard without machine placement and soldering though.

Just to give you an idea of what may be involved, at the following link if you click on transmitters, you get a picture of how tiny the parts can be when making them really small.

Mm, they look better but again they seem over complicated for my needs, so a pre built system is probably going to be plan C. I think ill go ahead and breadboard the transistor idea when the parts come through and ill report back to the forums, I don't think its worth making an Instructable over but ill see whether it even works. I've never worked with surface mount and my soldering iron is a little too blunt to work at that level (not to mention my hands are very unsteady) but I've go an old board from a floppy drive with SM components so I could practice with that if the need arises, i.e. this transistor idea not working out. Thanks for your time, Andy

No problem. I personally could not work with surface mount either, for the reasons you give, plus my eyesight is fading. Good luck.