DMM range and AC-DC mode, logic control -
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
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
To view circuit below, Click the Link of PNG or PDF
and view the Circuit, PNG can be Scrolled with Mouse
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
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.