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Posted by Frank on November 27, 2004, 12:28 am
Please log in for more thread options I have a simple circuit for a time delay. Small problem is that it does not switch entirely as I want. With a transistor I fill an elco of 100uF and then switch off that transistor. The voltage on the elco is used to switch on a second transistor. As long as the voltage is above about 1V the transistor stays open. But, between 1 and 1,5V (around that, dunno) the switched current goes down and the LED on the second transistor fades out. I want the led to switch off quickly. How could I do that? It should be something like: if voltage on elco goes below 1,5V switch immediatly to 0V. Any website a could inspect? Hope this is clear enough, drawing schemas in text is very tiresome. Thanks for your ideas. Frank | |||||||||||||
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Posted by John Fields on November 26, 2004, 7:20 pm
Please log in for more thread options --- It's also tiresome trying to figure out what you mean. For example, you don't say whether the output of the electrolytic is supposed to supply the operating current for the LED or whether you're only using it for timing. Then you say that the LED stays on with 1V on the cap, but that it starts to dim when the voltage on the cap is between 1 and 1.5V, which doesn't seem to make much sense. Since you're the one asking questions it seems to me that you're the one who should be putting forth the effort to at least make yourself understood for the convenience of those whom you are asking for help. -- John Fields | |||||||||||||
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Posted by Jamie on November 26, 2004, 10:03 pm
Please log in for more thread options Look at Schmit triggers and voltage comparators.
you need a circuit that can switch on at a higher point than what it takes to switch off. you can also acheive this by putting a bias feed back transistor to help drive the base as soon as current starts to conduct. there are lots of options but i would look into the use of a simply Op-amp. with that you can simply use a loop back resistor to the + input that would counter act the fixed voltage point. look up voltage comparator's Frank wrote: > Hello,
> I have a simple circuit for a time delay. Small problem is that it does not > switch entirely as I want. > With a transistor I fill an elco of 100uF and then switch off that > transistor. The voltage on the elco is used to switch on a second > transistor. As long as the voltage is above about 1V the transistor stays > open. But, between 1 and 1,5V (around that, dunno) the switched current goes > down and the LED on the second transistor fades out. I want the led to > switch off quickly. How could I do that? It should be something like: if > voltage on elco goes below 1,5V switch immediatly to 0V. Any website a could > inspect? > Hope this is clear enough, drawing schemas in text is very tiresome. > Thanks for your ideas. > Frank > > | |||||||||||||
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Posted by Frank on November 27, 2004, 3:28 pm
Please log in for more thread options Thanks, that sounds like what I need. I'll look into it.
Frank > Look at Schmit triggers and voltage comparators.
> you need a circuit that can switch on at a higher > point than what it takes to switch off. > you can also acheive this by putting a bias > feed back transistor to help drive the base > as soon as current starts to conduct. > there are lots of options but i would look > into the use of a simply Op-amp. > with that you can simply use a loop back > resistor to the + input that would counter > act the fixed voltage point. > look up voltage comparator's > > > > > Frank wrote: > > > Hello,
not
> > I have a simple circuit for a time delay. Small problem is that it does > > switch entirely as I want.
stays
> > With a transistor I fill an elco of 100uF and then switch off that > > transistor. The voltage on the elco is used to switch on a second > > transistor. As long as the voltage is above about 1V the transistor > > open. But, between 1 and 1,5V (around that, dunno) the switched current
goes
> > down and the LED on the second transistor fades out. I want the led to
could
> > switch off quickly. How could I do that? It should be something like: if > > voltage on elco goes below 1,5V switch immediatly to 0V. Any website a > > inspect?
> > Hope this is clear enough, drawing schemas in text is very tiresome. > > Thanks for your ideas. > > Frank > > > > >
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Posted by CBarn24050 on November 27, 2004, 4:45 am
Please log in for more thread options >Subject: timer
>From: "Frank" frank@frank.com >Date: 26/11/2004 20:28 GMT Standard Time > >Hello, >I have a simple circuit for a time delay. Small problem is that it does not >switch entirely as I want. >Hope this is clear enough, drawing schemas in text is very tiresome.
>Thanks for your ideas. >Frank > Learn to post your circuits in spice format, it's a whole lot quicker and can be read on any text program. | |||||||||||||
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>I have a simple circuit for a time delay. Small problem is that it does not
>switch entirely as I want.
>With a transistor I fill an elco of 100uF and then switch off that
>transistor. The voltage on the elco is used to switch on a second
>transistor. As long as the voltage is above about 1V the transistor stays
>open. But, between 1 and 1,5V (around that, dunno) the switched current goes
>down and the LED on the second transistor fades out. I want the led to
>switch off quickly. How could I do that? It should be something like: if
>voltage on elco goes below 1,5V switch immediatly to 0V. Any website a could
>inspect?
>Hope this is clear enough, drawing schemas in text is very tiresome.
>Thanks for your ideas.