Electrical – 7432 and 7408 ICs not working properly


I am making a simple BCD to 7 segment decoder but for some weird reason, the ICs I am using, 7432 and 7408 are not working properly.

My 7408 ICs (all 5 of them) are always producing high outputs, regardless of the inputs. While my 7432 ICs (again, all 5 of them) are acting like AND gates, i.e. only producing high output when both inputs are also high. I am using a 9V battery to power the circuit. Pin 14 of the ICs are connected to positive and pin 7 are to connected to negative.

Are my ICs broken or have I done something wrong in my circuit that I failed to notice?

Best Answer

74 series ICs need a 5V supply. For the original 74 series logic, the typical absolute maximum VCC voltage is 7V, above that you kill the chips.

If you want to experiment with discrete logic and use 9V power, 4000 series logic is the simplest. Use a 4081 for the AND gate and a 4071 for the OR gate.

Unlike the 7400 series the 4000 series logic works from 3V to 18V, however unlike the 7400 series chip you need to connect all unused inputs to something.