Electronics Q

[F-Red]

"Dave Liquorice"

Er but the 3909 has been obsolete for at least 3 years, you can still get them though. $14 plus postage. You could buy a new Fng alarm for that. !

[Frank Erskine]

I have a load of LM3909s. In fact a chip just fell to the floor from a "bookshelf" in the living room - it was a 3909 :-)

[F-Red]

Frank Erskine

Good for you, now for a little gold star, tell me were you can still buy them in the UK and at what price.

[Andy Luckman (AJL Electronics)]

Standard building block bistable circuit will do what you want. Two transistors, two capacitors and four? resistors.

[The Natural Philosopher]

Hoew about a unijunction with the LED in its collector?

[Andrew Gabriel]

It would be hard (maybe impossible) to get the on-off ratio

50%. Also, the with the output being saw-tooth rather than square wave in a conventional unijunction oscillator, the LED would fade off each time. BTW, there's no 'collector' in a unijunction transistor -- I presume you mean the drain? (I think it's about 30 years since I last used a UJT, and that was to generate a timebase for an oscilloscope I was building from an old telly;-)

[Bruce Tanner]

I've just come across this which appears to do what you want, although the flash rate might be a bit quick?

[Mike]

And that was probably 5-10 years after they stopped making them :-)

[Andrew Gabriel]

Oh have they? How sad. ...and I still remember the part number I used - 2N2646

[Mike]

A quick search on this number reveals two companies still make them - Comset and BOCA. Never heard of either but at $2 a piece there's obviously enough profit in it for some garage semicon plant to churn them out :-)

[Gerd Busker]

Ah, the metal TO-18 version. How many would you like? RS still stock them and they are sold individually :)

G.

[F-Red]

[e-mail] (Andrew Gabriel)

Just to be pedantic its E,B1,B2 on a UJT,Its an electronic switch, no gain so no collector or drain :-) It would work, but to be effective would need anther transistor, which would make the component count as high as an ordinary flip/flop.

[The Natural Philosopher]

I thought that a very short pulse every few seconds was what was required.

Don't use teh gate, use the (drain? No that's a FET surely...)

[The Natural Philosopher]

I thought that the anode went into heavy conduction durng the pulse discharge phase.