Theremin

I have been wanting to build a Theremin for ages, so I have started.

There is a YouTube video HERE so you get the idea of what is going on.

To start I need some oscillators.  I have chosen this for an oscillator for no other reason than I like it and have been wanting to design it into something for a while.

Its a 'lambda diode' oscillator, which is basically using the 'lambda diode' in its negative resistance region.  I need two of these, one reference oscillator and the other that will be detuned by your hand.

The output of these oscillators go to a mixer.  To prevent the two oscillators 'pulling' each other, the mixer is this....

With the two oscillators built and connected to the mixer, thats the basic Theremin done.  One oscillator requires a small tuning capacitor across its main tank capacitor to get both oscillators running at the same frequuency.  The other oscillator has an aerial connected to the top end of its L-C tank, this is where your hand waves about.

Next I need to build the volume control bit and something to drive a speaker...

This is the pitch oscillators and mixer with a speaker amplifier.  The volume oscillator and associated bits aren't shown in this schematic.

I'll put up pictures of the prototype shortly and a full nice schematic.

Lightning detector

I wanted to build a lighning detector with a display that would show lightning activity and strength.  I wanted the thing to run from batteries, draw less than 1mA, detect lightning and have some sort of display.  I Googled lightning detectors and found some commercial units and some construction projects.  This page www.techlib.com/electronics/lightning.html is great and gave some good ideas.

But I didn't want to rip off someone elses stuff and the design constraints I had put in place were pretty tight so I started from scratch...

Detecting lightning requires 'listening' for the RF energy produced during a thunder storm.  Any AM radio can detect this, so build an AM radio, connect it to something that will register the loud pops and crackles from the lightning and your done.

Display
I decided the display power requirements were 2V to 5V at no more than 500uA.  I wanted an LED display that would show how often the lightning was occurring.
LED's are realy efficient now and putting 250uA through some white ones I had on my desk gave a satisfactory light output.  The LED's would need multiplexing since I could only light them one at once.  Again going with what was at hand I used a PIC16LF819, which when clocked at 31kHz only draws about 7uA.
   
Radio
I wanted a simple AM radio front end that would use a ferrite bar antenna so it would fit nicely in a box.  I have collected hundreds of circuit ideas that would suffice so I tried out one I had on a scrap of paper...
 

      This worked quite well but then I got obsessed with building an AM radio that tuned the LW and MW bands.  The radio I needed would tune to somewhere below 500kHz, peak detect the received signal and buffer it for sampling by the PIC.  The test circuit is shown below, its a simple reflex receiver followed by an amplifier and peak detector.  It uses a moving coil meter as the display.


The coil is a LW radio coil wound on a 12cm ferrite rod.  The meter is a 1mA FSD moving coil meter.  The detector draws under 500uA from a 5V supply.
      

Wireless thermostat

So we have one of these Salus RT500RF Wireless thermostats..

So far its working ok.  The thermostat is quite long winded to set up.  If you don't want a timezoned background temperature the least you can get away with is programming 10 time zones to the same temperature, aint nobody got time for that.

I think we can do better...
I have cobbled together a PIC16F1455 with an RFM12B and should soon be able to switch the boiler on and off from my PC.

Cobbled together USB Radio...

Not very neat and not a good photo, but it receives and transmits on selectable frequencies around 868MHz.

This continues in my 'home automation' post...