Bowmar LED Display Unit

The "Bomar" 10-digit LED panel could display numerical values and some special symbols. In its time, it was the most advanced display panel used in consumer electronics products.

In this photograph, you can see a pair of SN75492 chips, and a pair of SN75491 chips. The SN75491 is a segment driver chip with four circuits (quad). The SN75492 is a digit driver chip with six circuits (hex). Since this calculator has ten digits, two SN75492 chips are used, which means two driver circuits that will remain unused.

LED Display Driver Circuit

Each digit consists of seven segments, and a floating point, hence eight segment drivers are required in total. The SN75492 chip had four drivers hence two chips were required.

In the circuit diagram above you can see that IC4 and IC5 are the segment drivers. The control signal for each segment (a through to h) comes from the TMS0107 chip.

IC2 and IC3 are the digit drivers. They receive control signals from the "d" port consisting of d1 through to d11 lines, which perform the scanning of the display. Interestingly, the same lines scan the keyboard matrix as well; hence, the display blanks for a moment when entering a number.

SN75491 MOS-to-LED Segment Driver

The SN75491 is an LED segment driver chip that interfaces MOS voltage levels to common-cathode LED displays. This chip has the capability to provide high output current, which makes it ideal for applications where time multiplexed methods drive multiple LEDs. Two chips are required for 7-segment LED display.

• 50 mA sink or Source Capability
• Supply voltage 10 V
• MOS Compatible
• Low standby power
• Four Darlington circuits

SN75492 MOS-to-LED Digit Driver

The SN7492 is a digit driver that converts MOS signals to higher currents to drive LED displays in a digit scan fashion.

• 250 mA sink capability
• Supply voltage 10 V
• MOS Compatible
• Low standby power
• Six Darlington circuits

Magnifying Display

A magnifying effect created by encapsulating the display within a convex shaped plastic, was an ingenious way of making the digits appear bigger and brighter.

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