I am teaching myself digital electronics by working through the exercises in the second half of “Student Manual for The Art of Electronics.” I have encountered a problem that has left me totally befuddled: in section 19-4 (see attached image), I need to wire some circuitry to control output to a data display. The data displays have latches, so that in order to actually display the value being input at any point in time, pin 5 needs to be low. The input to pin 5 of the left two data displays comes from pin 8 of the HCT10 (see highlighted part of attached image), which is a 3-input NAND gate. And here’s where the weirdness comes in: in my circuit, I’ve measured the three inputs (to pins 9, 10, and 11) to all be high, which means the output from pin 8 of the HCT10 should be low. When I measure pin 8, it is low—UNTIL I connect pin 8 of the HCT10 to pin 5 of the data display. Then it goes HIGH! Clearly the output from the HCT10 should not depend on where I connect it, and yet that is definitely what is happening, and it’s preventing the data display from working as it is supposed to. I’m guessing there’s something I’m not understanding about what’s going on inside the guts of the HCT10. Any help would be much appreciated. Thanks.

 

I suspect you have the LED's incorrectly connected, check the pin configuration.

 

I double-checked (and triple-checked), and I definitely have the pins wired correctly. I even switched out the HCT10 and installed a new one, thinking maybe the original was faulty. The problem still persists.

 

Hi,
That connection should not pull the HCT10 output pin High.

Could you do the following test.

Disconnect pin #8 from the LED's.
Ensure that pins 9,10,11 are High.
Pin 8, should be Low
Connect a 2k2 test resistor from pin 8 to the +5V, check that pin 8 remains low.
Next, connect a 2k2 from pin #5 of the LED's to 0V, measure the voltage drop across the 2k2.

Lets know what you see.

 

You were right: it was wired incorrectly. The wiring I myself laid down was correct, but I failed to notice that the hex displays were on a stand whose pins were connected with their own little red wire. Once I removed that, it worked just fine. Sorry to trouble, and thank you very much for the help!