Seven-segment decoder using both direct and inverse outputs of a BCD counter

"Binary clock" had time to come into vogue, and get out of it, and again the translation of a binary-decimal code into a more convenient for reading by a person, positional or seven-segment, became relevant. The author chose the latter.

The solution lying on the surface is to convert the code into a positional one, then a diode array into a seven-segment one. This is sometimes done, but there is another option: a decoder consisting of four inverters, which allow, in addition to direct signals from the counter outputs, to receive inverse, as well as signal conditioners for each of the indicator segments:
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But here the decoder is designed to connect to this counter , which already has both direct and inverse outputs, which is why additional inverters are not needed in the decoder. Outputs Q0 - Q3 and Q0 '- Q3' of the counter are connected, respectively, to the inputs B0 - B3 and B0 '- B3' of the decoder. The signal conditioner for each of the segments consists of:

• one or more diode "And"
• diode "OR" (if required)
• transistor switch that controls the LED segment (the indicator must be with a common cathode).

Due to the two diode chain shown below, the indicator supply voltage is about one volt lower than the logic supply voltage.



To the keys you must add resistors in the base and load circuits. The author used the indicator with the following cap:



In this form, the decoder can display only 0 - 9 characters on the indicator. But the counter used together with it can be switched by a switch from binary-decimal to binary mode. By adding additional diodes to the decoder, you can also display the characters A - F. Or a hyphen, L, C, G, E and a space if you prefer them.