Been working on the LW beacon transmitter over the last week and have added a two tone generator to make the signal more noticeable.
Getting multiple tones didn’t involve too much hacking around with the existing circuit. By adding another 555 timer then using it to control the tone output I already had, it produces a loud (if slightly annoying) siren noise 🙂 (see video below).
In an effort to dial down just how annoying the tone is, there’s a variable resistor fitted that controls the speed of transition between tones.
Mixers Taking A Back Seat (For Now)
After messing around with a mixer to modulate the carrier a little neater, I backtracked and decided the diode feeding the audio to the oscillator was just about good enough.
There’s still a little bit of frequency shifting as the tone level increases but its not major and I’d have to sort something out if it was transmitting in a narrow band mode rather then what is nearer amplitude modulation.
Might revisit the mixer problem at a later date once I’ve worked through all the other things I’ve got going on.
Here is the complete beacon circuit as it is now. The new section added to create the two tone sound is marked in red.
Part 3 will be all about given the beacon a better range by increasing its output. Have played around with a basic RF amp using small signal transistors but the max output just isn’t cutting it.
I have much bigger power transistors on hand that would be a massive overkill if run at full tilt but turning things down a bit to a workable RF output might do the trick (provided I can get them to work with little input).
The other point to note is the frequency that the transmitter operated on is very dependent on the voltage supplied to the circuit. This change be a problem is running it off a battery as the transmitter drifts as the battery drains.
Might well need a regulator to ensure things stay on track and as the current needed is going to be low (even with a RF amp) a 7812 would do. This would have to be fed with about 15 volt to make it regulate properly, if memory serves me right.