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.



Elegant solution. "Mikey likes it!"

Ed
.

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)
  • Re: what to do with unused gates on 4070, LM339N, AM26LS31C, AM26LS32 ?
    ... AM26LS31C differential line driver sender and a AM26LS31 differential line ... So for the 4070 you tie both inputs to ground or VCC, but for the comparators and differential line receiver you tie the inputs so that there's a healthy difference. ...
    (sci.electronics.design)
  • Re: coutdown 555 timer values?
    ... which will make the circuit insensitive to ... >> forces all of the inverting inputs to the comparators lower than ... >> rise through each of the reference voltages on the + inputs ... John Fields ...
    (sci.electronics.basics)
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