Re: coutdown 555 timer values?

John Fields wrote: 
Then, of course, there's always:

VCC>------+----+
| | V+ | [RT] | | | [10K] [R7] +----|-\ | | | | >--+-->180s
+----|----|+/ | | V+ | | | [R6] | [10K] | +----|-\ | | | | >--+-->150s
+----|----|+/ | | V+ | | | [R5] | [10K] | +----|-\ | | | | >--+-->120s
+----|----|+/ | | V+ | | | [R4] | [10K] | +----|-\ | | | | >--+-->90s
+----|----|+/ | | V+ | | | [R3] | [10K] | +----|-\ | | | | >--+-->60s
+----|----|+/ | | V+ | | | [R2] | [10K] | +----|-\ | | | | >--+-->30s
+----|----|+/ | |
| +----D
[R1] | G--+
| [CT] S |
| | | |
GND>------+----+----+ |
_ |
R/T>---------------------+

In this circuit we use six comparators with references derived
ratiometrically from Vcc, which will make the circuit insensitive to
longish-term variations in Vcc.  The circuit works by applying a short
positive pulse to the gate of the MOSFET, which discharges CT and
forces all of the inverting (-) inputs to the comparators lower than
the reference voltages on their non-inverting (+) inputs.  This will
cause all of the outputs to go high.  Then, when CT charges it will
rise through  each of the reference voltages on the + inputs
sequentially, turning each of the  comparators ON sequentially,
forcing, and holding, their outputs low, one at a time.

In order to determine the widths of the various outputs we can use 

                   Vcc
     T = RC ln ----------- = kRC
                Vcc - Vth

to determine what the voltages will be on the - inputs of the
comparators at the times we're interested in.



I know the thread took a left turn, but I'll just point out that one may want to use a constant current source to charge CT. Then, all the resistors can be identical. If I is constant, dV/dt is also constant. One can get quad packages of comparators...

--
Regards,
 Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
                                 Thomas Henry Huxley, 1877
.

Relevant Pages

  • Re: coutdown 555 timer values?
    ... In this circuit we use six comparators with references derived ... longish-term variations in Vcc. ... the reference voltages on their non-inverting inputs. ...
    (sci.electronics.basics)
  • Re: coutdown 555 timer values?
    ... >> The tricky part is shortening up that first 555 pulse in astable mode ... >will begin the first timing cycle at about 1/3rd of the total voltage, ... which will make the circuit insensitive to ... >> forces all of the inverting inputs to the comparators lower than ...
    (sci.electronics.basics)
  • Re: coutdown 555 timer values?
    ... >>>counter and using 555 to generate a 30 sec pulse seems like a good idea. ... In this circuit we use six comparators with references derived ... John Fields ...
    (sci.electronics.basics)
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