Just a thought...
You might have noticed I used a Solar Garden light in the session on QRM/EMC, well it occurred to me to check the design in the UK. Slightly different but essentially the same simple circuit. Which oscillates around 1 MHz!
What? Yes it does. The question I was going to ask is this; "How do you get a L.E.D. to light when you only have a 1.2 Volt battery?"
When you did the section on transistors did someone tell you about 'inductive loads'? Or why you need a 'protection diode' across a relay? The same applies to a r.f. circuit with a coil in the collector circuit. Tuned or not it will swing above the supply rail voltage to double the collector voltage rating. So if you connect a transistor to a 12 Volt rail, remember the Vce rating must be 24 Volts or greater.
Remembering that a coil stores the energy in the magnetic field. So when you turn off the current the coil tries to maintain the field. Generating a 'back emf' that in some cases can be kiloVolts! Just arrange a relay as a 'buzzer' with the contacts making and breaking the circuit. Don't touch it! Depending upon the coil inductance the voltage can reach a very high voltage.
So the simple circuit these days is a single integrated circuit. A coil, the solar panel and the L.E.D. With the i.c. doing the charging and switching on the L.E.D.
[I just a fruitless search for a circuit that I know I downloaded months ago! Now thanks to Bert's blog I have one. https://bertsblogs.com/garden-solar-light-circuits/]
He explains the circuit as well. As he and I was going to here is the simple fact. The LED is a 'super-bright' white LED which needs well above the typical 1.8 Volts to turn on. L.E.D.s are much better these days from the 60s and 70s versions which were always red and quite dim in comparison to the filament bulbs/globes of the day.
The one thing I noticed as a difference between the ones I had bought in South Africa and the ones in the UK, the batteries are NiCd in SA (cheap). The ones in the UK are Nimh. Nickel-cadmium versus Nickel metal hydride. Small pricing difference but on thousands of models makes a big difference.
So are they going to interfere with your h.f. reception? Probably not unless you have a few hundred in the garden.
They don't last long. Water ingress usually rusts the connections and screws. Treating the plastic with sealant is just not worth it. Replacing the NiCd battery with a shop bought Nimh helps. But then sealing with marine silicone will keep maybe a few years longer.
73 John ZS6WL
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