# timer

Discussion in 'Electronic Basics' started by Frank, Nov 26, 2004.

1. ### FrankGuest

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.
Frank

2. ### John FieldsGuest

---
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.

3. ### CBarn24050Guest

Subject: timer
Learn to post your circuits in spice format, it's a whole lot quicker and can
be read on any text program.

4. ### JamieGuest

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

5. ### FrankGuest

Thanks, that sounds like what I need. I'll look into it.
Frank