Build a DMM – LM3914 Analog display
This a part of a complete Benchtop DMM that you can build to learn the internals of a Digital MultiMeter. This is oriented towards learning the measurement aspects and the protection of input circuits.
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. 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 Amplifier.