I want to play around with using a signal generator and thought I'd make one using this simple schematic:



C2 is supposed to be a .22uF non-electrolytic. I don't have anything close to that, other than a .22uF electrolytic. Looking at the 3909 datasheet

http://www.datasheetcatalog.com/data.../LM3909N.shtml

makes me think maybe that would be ok, with neg. term towards the probe. Can anyone confirm that?

Thanks.

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I would suggest probably not, but 0.22uF is a standard common value for non-electrolytics, and is relatively rare for an electrolytic - you should be able to source a 0.22uF very easily.

More of a problem is the long obselete LM3909 - do you already have one?.

Personally I would suggest making a simple two transistor astable multivibrator, that's the usual way of making a signal injector.

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If you can get the LM3903 then good for you but it's not been made for the last ten years.

It shoudln't make any difference whether C2 is polarised or not, just connect the + inbetween R1 and R2.

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Just use 2 x 100nF connected in parallel - its 0.2uF but close enough

You could also use a 220nF capacitor which would be the exact one you're after.

All are usually readily available from scrap PCBs and electronic stores.

 

I do already have the 3909, which is why I thought I'd make this really simple unit--and see how it works.

I did check all my scrap PCBs and was kind of surprised to NOT find a .22 non-electrolytic, but DID find the .22 electrolytic.

Had thought about parallelling two .1's, and was headed there next if the electrolytic was thought a bad idea.

Thanks for all the advice!

 

There's nothing wrong with paralleling two 100nF capacitors - the probably have a tolerance of 10% anyway.

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They will probably have a tighter tolerance than a 0.22uf Electrolytic capacitor

 

Futulec still have it for $2.45 each.

http://futurlec.com/cgi-bin/search/search.cgi

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