need peak detector circuit for slow biased signal

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i need to detect the negative swing of the output of pressure sensor.
some info about the signal:

the pressure sensor outputs 2.5VDC at atmospheric (its "gauge"
pressure)

vacuum goes from 2.5VDC to 0V, positive pressure goes from 2.5VDC to
5V. the range of the sensor is +/- 15 PSI.

i need to detect the peak vacuum voltage, which is somewhere between
2.5VDC and 0V.

i am measuring the pressure of a very slow pump, which operates at
about 60 rpm, so I get about 1 peak per second.

im thinking an LM339, output connected to cathode of diode, anode of
which goes to a cap, which is connected to ground on one side and a
momentary "reset" switch that charges up the cap to 2.5V whenever you
press it (between measurements)

negative input of 339 goes to the cap voltage
positive input of 339 goes to the pressure sensor output

seems to work good in circuit maker

add resistors on output of 339 for pull-up (not really needed i
suppose since it only DISCHARGES the cap) , and to limit discharge
current into 339 when it trys to pull the cap down

perhaps a 10Meg resistor on cap to 2.5V to pull it up slowly...might
not be needed either since you have "Reset" switch which does it
instantly

sound good?

 

[MooseFET]

i need to detect the negative swing of the output of pressure sensor.
some info about the signal:

the pressure sensor outputs 2.5VDC at atmospheric (its "gauge"
pressure)

vacuum goes from 2.5VDC to 0V, positive pressure goes from 2.5VDC to
5V. the range of the sensor is +/- 15 PSI.

i need to detect the peak vacuum voltage, which is somewhere between
2.5VDC and 0V.

i am measuring the pressure of a very slow pump, which operates at
about 60 rpm, so I get about 1 peak per second.

im thinking an LM339, output connected to cathode of diode, anode of
which goes to a cap, which is connected to ground on one side and a
momentary "reset" switch that charges up the cap to 2.5V whenever you
press it (between measurements)

negative input of 339 goes to the cap voltage
positive input of 339 goes to the pressure sensor output

seems to work good in circuit maker

add resistors on output of 339 for pull-up (not really needed i
suppose since it only DISCHARGES the cap) , and to limit discharge
current into 339 when it trys to pull the cap down

perhaps a 10Meg resistor on cap to 2.5V to pull it up slowly...might
not be needed either since you have "Reset" switch which does it
instantly

sound good?

You have 3 sections of the LM339 left over

I assume you are doing this:

LM339
IN -----!+\
! >---/\/\---+------ Vout
---!-/ !
! !
-----------------+
!
---
---
!
GND

May I suggest:

LM339 N-MOS
IN -----!+\ !!---Vcc
! >---/\/\---+------!!
---!-/ ! !!--+---Vout
! ! !
-----------------!----------+
! !
--- \
--- /
! \
GND !
GND

The MOSFET means that the capacitor won't see the bias current of the
LM339 or the load. The LM339 will servo out the Vs(off) of the
MOSFET.

 

[Robert Monsen]

sound good?

Yes.