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transistor astable multivibrator

Hi, when i make one of these using say 2n3904's and a couple of 10uf caps with a cycle time of about 1hz (r=100k approx) i find that the extinguishing led (the other will be lighting) never quite goes out and it spoils the alternating light effect. What I'd like is a completely extinguishing led when the led goes off as the other lights. I could use a 555 timer and I have but I'd like to do it with transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any advice greatly appreciated.

tnx

Reply to
John
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Can you post a schematic? According to your description, there's something wrong with your design, which we can't diagnose if we don't know what you have now.

I've never had a problem with multivibrator transistors not cutting off completely. If you've got a lame circuit from the get-go, then darlingtons will only make it worse.

Thanks, Rich

Reply to
Rich Grise

I'm just guessing, as I don't know what circuit you are using, but do the two LEDs have resistors in parallel with them? Something like 10k ohm? If not, add them and see what happens.

Jon

Reply to
Jon Kirwan

As others have said, we're kind of groping in the dark here, not knowing details about your circuit. If you're using the classic text-book astable MV, the only reason I can think of right now is that the 10µF timing caps are leaky and are pulling some current through the LEDs even when they are supposed to be off. Any chance that they are electrolytics and you have them connected with the polarity reversed?

Reply to
pimpom

They are polarized electrolytics but they are not reversed. I could try tantalum beads.

Reply to
John

No they haven't. Thanks for that info.

Reply to
John

Ok about the darlingtons. I'll post the circuit later - bed time here in the UK.

Reply to
John

Tantalum will not make a difference except that they are more sensitive for reverse voltage. Below you find the classic a-stable. Looking in my crystal ball I see you may have forgotten the series resistors of the LEDs.

+-------+----+-------+- Vcc 5Vdc | | | | .-. .-. .-. .-. | | | | | | | | | | | | | | | | '-' '-' '-' '-' | | |100k | 270 | | | | V LED | | V LED - | | - | #| | | | +--#|---)----+ 10uF | | #| | | |# | | +----)---|#--+ | | | |# | | | | | \\| | | |/ |-----+ +-----| 2N3904 | | | | +--------------------+---GND created by Andy´s ASCII-Circuit v1.24.140803 Beta
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petrus bitbyter

Reply to
petrus bitbyter

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Thanks petrus, I was wondering where the LED's were in the circuit.

George H.

Reply to
George Herold

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