LM3914 analog display and resistance measurement
U2A opamp LF353 is used here as a constant current
source, R10-D13-D14 is for protection in case you
measure voltage in the ohms range. U1 4052 helps
digitally select four different currents, the
currents pass thru the unknown resistor to be
measured and an voltage developed across the
resistor is measured.
U2B is a buffer which passes on the voltage measured
to U5A for inversion of polarity as current source
is a negative current (current sink). U5B amplifies
to the level required for the LM3914 display
circuit. U7 555 is used as a de-bouncing for switch
SW1 so as to advance counter 4029 to change the
range of resistance measurement.
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display and resistance measurement - del20008
Use any FET input dual opamp, TLO72 will work but
LM358 will cause error as it is transistor input..
D1, D12 and R8 are to ensure that the FET can be
turned off, as the opamp swings from +/-3.5V only,
with some FET it needs to be tweaked. LF353 is a
Wide Bandwidth Dual JFET Input Operational
'474 CD' is 47 with 4 zeros pF, 470000 pF,
remove 3 zeros 470 nF, shift decimal 3 places
0.47uF. "pl" is plastic, low leakage multilayer.
Use MFR 1% for all Resistors, 33E means 33 ohms, 22K
means 22 kilo ohms, 1M is 1 megohm. 10T Trimpot
means like 10 turn bourns.
"analog ground" and "digital ground" must be linked
at power supply only, avoid loops, let grounds
radiate from a ground plane.
Unused inputs of logic and opamps pull up or down to
avoid oscillations and noise. connect supply of all
chips if not mentioned.