Schematics of delabs

This is the third circuit that you will need to build a Simple Benchtop DMM with no Microcontroller. Here the U1D and U1B 4093 acts like a de-bouncing circuit for the push keys. The 4029 counts up scrolling to binary values 00, 01, 10, 11 for the four ranges.

DMM range and AC-DC mode Logic

The binary value of 4029 is decoded to decimal by 4028 in order to light four LEDs which indicates the range or mode on the front panel. When any of these pushbuttons are pressed and held, the nand schmitt 4093 clocks to scroll the range continuously. The binary output of these counters control CMOS switches 4052 which are analog multiplexers.

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This is a unregulated supply for low power circuits. You may be able to regulate the outputs with zeners or small regulators like 78L05.

Circuits using only  discrete devices

The transformer can be hand wound in a mini ferrite pot core. you can use 2N2222 or any other fast transistor. The transformer should have 1KV isolation. The dot polarity of TR1 should be properly observed, else it may fail to oscillate or give output.

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74HCU04 is a chip that was made for this purpose, HCT may not work for such a circuit. C1 and C2 can go to upto 33pF and R2 can be increased to make R2 * C2 = t.

Time constant much less than the period T of the crystal T = 1/F . This is to remove higher frequency components in the Oscillator.

More on Piezoelectric Crystal Oscillator

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This is a simple charger circuit which will work for a light load like a DMM, the Battery can be a sealed maintenance free battery of 9V-2AH or better. The circuit will work best if the Unit is powered on many times daily on regular use, else battery will drain down.

Power supply with battery backup - DMM

The 555 Astable is used to generate a AC signal from which a negative voltage is generated, A 79L05 which is a low power TO92 equivalent of 7905 a negative -5 volts regulator is used as -5 volts load is less. A TO220 7805 is used for the +5V supply.

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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.

Build a DMM - LM3914 Analog display

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.

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