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ICL7135 to Printer port Interface - del20002

A PC based Analog Voltmeter. The Printer Port or Parallel Port Interface is used to acquire high resolution analog data using ICL7135. This is a method of getting analog data into your PC. This is only of Educational Value. Applications include Data Loggers and PC based Process Monitors, chart recorders. (USB, Wireless Interfaces and Tablet Computers are in vogue today) 

The ICL7135 details can be seen here 41/2 A/D with BCD ICL7135 . This Mixed device was far ahead of its times, a product of Intersil, 

The four Digit Drive outputs and four BCD outputs of the Multiplexed Display Driver of 7135, along with A-D status output which is also four, are routed to four input pins on printer port via three 74HCT373 Digital Switches. This is because we have less Input pins on Port and 4 are shared by making one 74HCT373 transparent and others High Impedance when PC takes a reading. So the three sets of four outputs of 7135 can be read sequentially.

Digital Voltmeter - Printer

PNG-Schematic     PDF-Circuit     DSN-Source
ICL7135 to Printer port Interface - del20002
To view circuit click the Link of PNG or PDF, The PNG Image can be Drag Scrolled with Mouse.

74HCT139 U9A which is controlled by the PC program can select U3, U4 or U5 for sending data to 4 pins on printer port.  Data sent on the eight output pins of printer port  can be latched by U10B into U6. This eight outputs of U6 can drive relays or lamps.
Analog and Digital Voltmeter using ICL7107

The Design source files in Orcad Capture 9 format is here  A example VB program with source to test above circuit with PC is here, Visual Basic Digital Voltmeter. This program may not be error free but works.

When you build above circuit and voltmeter on PC. Use it only with 5V battery  projects. Any high voltage goes to printer port due to wrong connection you may lose a motherboard or worse. Better  isolate all data lines to port with opto couplers if you want to build this for regular use