4026 IC Problem

Would appreciate help with the attached circuit which was put together as a learning experience.

The 4026 seems to be connected as per specification, with outputs A to G forced high on start up to turn off the common anode display. However, output G hangs low with the g element in the display held active. I tried tying unused inputs to ground without any influence on the problem

Design Explorer and the Simulation Log do not reveal any difficulties. I looked at decomposing the 4026 symbol (separately) but there appeared to be no relevence there. An Archive search on 4026 did not produce any useful information.

I wonder if there is an error in the underlying 4026 template. Could someone help out please?


I just ran the dsn file, the segments are all inverted.

Segment G is on when the number displayed should be 0, segments a-f should be on and G should be off.

Cycle through the numbers and you will see this occurs for all numbers that should be displayed.

Try using a common cathode display.

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Thanks for the response Mycroft. Not sure I follow your reasoning. The display pins a-g are correctly arranged downwards and match those A-G on the IC.

At simulation start-up, the 4026 is installed to force all outputs A-G high to turn off the a-g display segments, otherwise all segments will be on as the 4026 sinks the display current. With only the output G low the the initial display and the counting sequences are false, as you found. The problem seems to be with that output G. If it were high initially all should be well.

I know the common cathode displays are a better choice but don't want to just walk away from this difficulty. It is meant to be a learning experience.

Do you have another suggestion?

It was very early inthe morning when I wrote that response. let me try again.

When you reset the 4026, a 0 (zero) is displayed. Tat is the outputs to segments ABCDEF are high and segment G is low.

The 4026 is designed to work with a common cathode (ground) display.

If you use a common anode display, as you have in your dsn file, segments ABCEDF will be off and segment G will be on.

It is the same as if you put a LED in backwards.

If you 'count up" with your circuit, you will see each number is "reversed", the segments that shold be on are off.

If you must use a common anode display, you will need to put an inverter (7414) into each of the segment lines.

Thanks again Mycroft.

The datasheet shows both CA & CC displays can be driven and in both cases a high input 3 causes outputs A-G to be active. In my inexperience I had assumed that the 4026 would automatically sink or source current according to the nature of the display, which in the case of the CA would have a blank display on start-up. Erronous assumptions that are now clear.

Both CA and CC displays have now been simulated successfully

Lessons learnt from this experience; study datasheets in depth and fully understand component operation