v 20080929 1
C 45500 43900 1 90 0 aop-std-1.sym
{
T 45350 44950 5 10 0 0 90 0 1
device=AOP-Standard
T 44750 44950 5 10 0 0 90 0 1
footprint=DIP8
T 45150 44950 5 10 0 0 90 0 1
symversion=0.1
}
C 46700 44000 1 90 0 resistor-1.sym
{
T 46300 44300 5 10 0 0 90 0 1
device=RESISTOR
T 46400 44400 5 10 1 1 180 0 1
comment=Probe
T 46300 44700 5 10 1 1 180 0 1
value=1R
}
C 46700 46800 1 90 0 resistor-2.sym
{
T 46350 47200 5 10 0 0 90 0 1
device=RESISTOR
T 47000 47300 5 10 1 1 0 0 1
comment=Load
}
T 46900 42300 9 10 1 0 0 0 13
can be any value you want,
as long as it produces appropriate voltage
and reasonably low power.

1R is handy with 1N4148 to measure
currents up to 0.5A, and produce only 1/4W heat.
Really magical value with trivial use.

Remember you can not use 1R - 1N4148
to measure corrents below 10mA
because most cheap OpAmps only have 5mV sensitivty.

Be clever, and tune this for YOUR project.
C 46500 42300 1 0 0 gnd-1.sym
C 43800 44100 1 270 0 pot-bourns.sym
{
T 44700 43300 5 10 0 0 270 0 1
device=VARIABLE_RESISTOR
T 44200 43500 5 10 1 1 270 0 1
refdes=R?
}
C 46100 45600 1 0 0 nmos-3.sym
{
T 46700 46100 5 10 0 0 0 0 1
device=NMOS_TRANSISTOR
T 47000 46000 5 10 1 1 0 0 1
comment=MOSFET N
}
C 46400 48200 1 0 0 vcc-1.sym
C 43100 44100 1 270 0 diode-1.sym
{
T 43700 43700 5 10 0 0 270 0 1
device=DIODE
}
T 39100 41700 9 10 1 0 0 0 9
I use a diode as cheap Vref
It is very sensible to temperature variations
and should not be used in high end projects.
A simple 1N4148 remains a very cheap Vref
for prototypes, and low end projects.

To test how sensible your circuit is to temperature variations,
just put your project over a radiator for a few minutes,
or put the iron solder 1cm below sensitive components.
C 43400 45000 1 90 0 resistor-1.sym
{
T 43000 45300 5 10 0 0 90 0 1
device=RESISTOR
T 43000 45400 5 10 1 1 180 0 1
value=10k
}
N 46600 48200 46600 47700 4
N 46600 46800 46600 46400 4
N 46600 45600 46600 44900 4
N 46600 44000 46600 42600 4
N 45300 43900 45300 42800 4
N 44900 43900 44900 43600 4
N 44900 43600 44400 43600 4
N 43300 43200 43300 42800 4
N 43300 42800 46600 42800 4
N 43900 43200 43900 42800 4
N 43300 45000 43300 44100 4
N 43900 44100 43900 44600 4
N 43900 44600 43300 44600 4
N 45100 44900 45100 45800 4
N 45100 45800 46100 45800 4
T 39100 46600 9 10 1 0 0 0 10
For the OpAmp, remember that it can only detect voltage variations above a given value.
Cheap devices can usually have around 4mV error.
High end devices (measurement - metric) can go down to 0.1mV
(or even lower if you can pay the price).

Also remember that this schematic is working with voltages close to 0V.
Thus, cheap OpAmp will require negative supply (see my other schematics).

You can either build up a cheap pump with an ICL7660,
or use a Rail-to-Rail (R2R) OpAmp.
N 46600 48000 46100 48000 4
N 46100 48000 46100 46200 4
N 46100 46200 43300 46200 4
N 43300 46200 43300 45900 4
T 43600 45000 9 10 1 0 0 0 4
NB: I may have
put the AO
upside down !!!
( invert + and - )
T 51300 48800 9 10 1 0 270 0 1
An original idea by Whygee