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DMM range and AC-DC mode, logic control - del20011


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.

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.

The DPM or DVM gives a readout of an Analog Value or process. The Analog reading we obtain from a Circuit Measurement Jig represents some real world parameter.

DMM range and
                        AC-DC mode, logic control
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DMM range and AC-DC mode, logic control - del20011

In a multi-parameter instrument like DMM, the measurement jig functions and the analog signal routing is done by ganged range selection switches.

The range selection was first done by Ganged Rotary switches or Interlocked Modular Push Switches. These are Electrical Switches but they may need to have Gold Plating in the contacts. They also have to be in Dust free or Hermetically sealed units. Especially so when you are measuring Nano-Amps and Micro-Volts. The contact resistance, thermoelectric effect, corrosion, dust, humidity make Electromechanical switches for Analog a Stringent Quality defined. Component.

Then they started using miniature signal relays for small signal switching. Later came the Boom in telecommunications and Reed Relays were invented by some nice people. Dust free and very fast, these amazing devices work even in Near-RF signal switching.

Later came the CMOS Fet Switches, these revolutionized analog switching. A low cost CMOS analog switch is the CD4051, CD4052, CD 4053 set, at least in my good old days. I even made them popular among process control instrumentation designers. They have a small disadvantage of contact resistance(fet on resistance). There are now analog and RF fet switches with low resistance too.

Data sheets and info useful to study and build this circuit.