I've made a few circuits with 74 series TTL, counters, BCD and 7 segment drivers, things like that. They've worked out well. I've just spent a few hours making a similar circuit with CMOS chips, and things aren't going well at all.

I found that the output of my CD4511BC BCD-to-7 Segment Latch/Decoder/Driver output Vdd to the 7 segment when it was supposed to, but during those moments when it was supposed to output 0 to an LED, the voltage dropped only about a volt.

So I looked at the datasheet and found a typical applications page. It shows a schematic of the chip used with a common anode LED, with a transistor in between!

TL;DR --- with CMOS I have to use transistors on the output to drive a 7 segment? In TTL I just did it directly.

Thank you

 

You need to post your circuit. You may be using the CD4511 incorrectly. The common TTL decoder/driver is SN7447. This device sinks current and hence is used with common anode seven-segment displays. The output of the CD4511 can source 25mA and is used with common cathode seven-segment displays directly without the need for an external driver transistor.

 

Ok now i understand.

 

Compare the pinouts between the 7447 and the 4511. I was very surprised when I did.