Diode question

[col]

I am hopefully about to build a circuit I've been itching to do for weeks and realised I don't have the correct diodes. The ones on the schematic are BAT65s which apparrently are silicon shottky rectifiers. The alterantive on the data sheet is MBR0530. The silicon diodes I do have are lots of different 1N400x type, 1N1418 and 1N914s as well as some mystery blue ones with no numbers on them. I suspect that the 1N400x diodes would be closest but would appreciate any help in this.

Col

[Paul Perry (Frostwave)]

Can't be 100% sure without seeignthe circuit - or even knowing what it is!! - but if it is a stompbox, the BAT65s are there because the designer wanted diodes with a low foward voltage drop, maybe it is a rectifier in an envelope filter, or diodes in a voltage doubling power supply.
Usually, an 'ordinary' diode will work, just not as well. Sometimes you can sub a germanium, at least it will have a lower voltage drop than a silicon diode.
And almost certainly, ANY other schottky will do. But what I suggest is, build the circuit with ordinary diodes & I expect it will work & then you can replace the diodes with BAT65s or similar later.

[col]

It's this;

http://www.thereminworld.com/EPEArticle.asp

thanks

Col

[Ge_Whiz]

You can use any germanium diodes in that circuit - I did. I don't think ordinary silicon diodes will work (they're working as a full-wave RF rectifier) but any Schottkys would also do.

[Paul Perry (Frostwave)]

Ge Whiz, any comments on how well the simple theremin worked for you?
Was there much RF interference?

[Ge_Whiz]

It worked very well, even built on veroboard. However, part of the reason is that the frequency of the RF stages is low (~100 kHz), and so the playing range is pretty small (only about 20 cm span), making it almost useless for real musical scales, but great for sound effects. It's a little difficult to set up (as are all theremins) as you have to match the oscillator frequencies very closely, and even the position of an inductor wire will upset the tuning. I didn't follow the circuit 100%, as I built it with a preamp stage rather than an inbuilt amp. The output waveform from mine is a severely clipped sine wave, so it doesn't sound anything like a traditional theremin, and I don't know if this is a design feature or a biasing problem in my build. Still, I don't care.

One indicator of the flexibility of this design was that I set out to build it some years ago between Christmas and New Year, relying only on parts from my junk stock. I had to substitute transistors and diodes, 'pad out' RF capacitor values with caps of a few pF, and even wind my own inductors on bits of ferrite rod from scratch. It still worked first time, and was audible by the time I had matched the oscillator frequencies. Great fun.

No, no RF interference whatsoever - probably, again, because of the low RF frequencies used.

[George Giblet]

Ordinary diodes should work to some degree.  You may have to reduce R9 and R10 to half their values to compensate.  As other mentioned probably easier to get hold of a GE or Schotky.

[col]

Thanks for the advice. I'll probably wait t'ill I can get some schotky diodes next week. Out of interest this is the stripboard layout I intend using. I checked it after I'd done it and it all seems OK;

http://i159.photobucket.com/albums/t159/col07768/theremin.jpg

[col]

Mr Whiz! Is there any chance you could post the voltages around the transistors and diodes on yours? I built it but just get hiss!

Col

[Ge_Whiz]

Er, yeah, not sure when but I'll try. Have you got any means of checking the frequency of the two oscillators? Digital frequency meter best, dual-beam 'scope next best, single-beam last. A quick and dirty method is to grip gently, and then tightly, each inductor to try to pull the two oscillators together in frequency. This might just create a 'whoosh' or 'whump' as the frequencies briefly coincide. If fingers fail, try a steel screwdriver or similar lump of metal. Like I said, synchronizing this gadget is not easy.

If you haven't heard from me re. voltages inside a week (unfortunately, I'm away all weekend), PM me a reminder.