After finding a few unfinished radio projects that had been more than a bit mashed by being slung into a box, it gave me the incentive to rebuild a few of them from scratch. As I’ve not had much practice with the old soldering iron lately, starting out with something easy and working my way up seems like the best course of action.
Getting together a simple LW transmitter isn’t that much work, provided you don’t want too much power output. Even a basic oscillator will generate a strong enough signal to be picked up by nearby SW receivers.
Building The Oscillator
This circuit is by no means new and has been documented many times, but it does give advantages when getting together a simple oscillator. By only using a 555 timer and some other basic components, it keeps the finished circuit small and uncomplicated.
Choosing the right 555 timer is important as some operate at higher frequencies than others. Lower power 555 timers (like a TLC555CP) are more suited as the core of this oscillator because they will function happily right up to about 2 MHz.
I,m looking to build something that doesn’t need to reach those sort of frequencies, so I,m using a NE555, which has a maximum top frequency limit of 500 KHz.
R1, R2 And C2 in the circuit diagram above are the components that control the frequency output of the oscillator. Even though a combination of these components will change the output, I’m only using C2. A smaller capacitor INCREASES the frequency, with the 220pF used in this circuit resulting in a transmission about 220 – 230 KHz.
1 X NE555 I.C
2 X 4K7 Resistors
1 X 100 nF Capacitors
1 X 100 uF Capacitor
1 X 220 pF Capacitor
5 to 15 volt power source (a PP9 battery will do but you,ll have much more joy with a stable mains powered supply source).
Your choice of base board (Veroboard, copper plated board etc)
Note : If your using copper plated board your going to need at least a 100 Watt soldering iron to work effectively.
Type and tolerance of the resistors and capacitors isn’t that crucial for this circuit and I’ve used the first ones I could lay my hands on. Please note that the big 100uF Capacitor must be rated to at least the supply voltage of the beacon and its best to go for something around 25 Volt.
Using The Nesting Method
The circuit above is built using “nesting” and although it may not look all that pretty, it functions perfectly and takes away the need to work with veroboard or make a PCB yourself. One other advantage is how easy it is to quickly change (or add to) the circuit as the build progresses.
I like to keep circuits simple when using nesting as it can be difficult to keep track with bigger circuits. That said, I do know constructors that will build full blown transceivers this way and still find their way around the project easily (I need lots more practice).
The pins on the 555 timer don’t take much wiggling until they break so I’ve used a small tie wrap to secure the wire antenna to the big 100uF capacitor.
Adding The Tone Generator
All we’re looking for here is something to modulate the carrier so we have a better chance of finding the signal than if we only transmitted an empty carrier . Luckily the very same circuit used to create the “core” of this beacon can be used again to add the audio component.
By building yet another 555 circuit that operates at a vastly reduced frequency, you’ll have a nice strong audio source to modulate the carrier with.
You can also get a little carried away (if you like) and use a couple of tone generators to give a very noticeable siren effect rather than just a simple one tone modulated carrier (Coming up in part 2).
This is where something like the NE556 dual timer would come in handy to keep things tidy and reduce component count (which I would have used on this project if I had/found any).
1 X NE555 I.C
2 X 4K7 Resistors
1 X 10K Variable Resistor
2 X 100 nF Capacitors
1 X 1N4148 Diode
The Beacon In Action
The range on this beacon is pretty limited (20-30 feet) as your taking the output of an oscillator without a RF amplifier (which I’ll be adding in part 2).
Switch on the beacon, tune a nearby radio to around 200 KHz until you find the transmission and by adjusting the variable resistor (VR1) you can alter the tone.
Improvements To Be Made In Part 2
There are a few things wrong with this circuit and although they don’t stop it from functioning, its hardly a transmitter that has practical applications (yet).
1. Add More Range…
Even a simple final output stage will greatly increase the range of the beacon and will be slapping on a RF amp to give a few 100 mW.
2. Change the Modulation Method…
At the moment the tone is added to the RF signal by simply applying it to the output. This works OK as we have a constant and strong audio signal to play with but isn’t the best why of doing it.
Any load on the output changes the whole way the circuit works (including output frequency) and the diode (D1) is there to try and minimize this. Adding a proper way of modulating the carrier before the final RF amplifier makes the overall operation much more stable.
3. Make the Beacon 2 Tone…
A constant tone at the same sound level is easier to make out in noise than an empty carrier but an alternating tone beats them all hands down. Part 2 will see the tone generator turned into a multi tone output with (wait for it!) another 555 timer 🙂