Voodoo 74HC161 counter

pmlonline · 2006-01-26T16:53:57+00:00

Hi,I'm not that knowledgeable in TTL but know enough to get myself introuble. ;-) I built a very simply TTL circuit that is not behaving asit should IMHO.I have one 54LS00J NAND chip, one 74HC161 hi-speed CMOS counter, andsome phototransistors. I am simply trying to tie the phototransistor tothe counters clock input. I am doing this the common way by placing aresistor (47K) from Vcc to collector and grounding the emitter and thenusing the phototransistors collector as the output, which goes to thecounters clock input (pin 2). I understand 47K is not common, but itworks best with my phototransistor since the phototransistor goes fromover 2 mega ohms in darkness to several kilo ohms in light. So as atest, I simply move the phototransistors from light to dark, then backto light and so forth. It is a gentle transition and I see no noise. Ofcourse it is not an instant transition either. The counters clock inputlooks fine as it goes from nearly zero to 5.22 volts. For some reasonthe counter won't respond. Below you'll see that I've tried addingvarious size capacitors to eliminate noise.So then I use two of the four NAND gates to create a J/K flip-flop justto see if the counter is working. So now I have two phototransistorstied to a J/K flip-flop. Both phototransistors are in the dark and Imove one of them into the light and then back into the dark then do thesame to the other phototransistors. This makes the flip-flops R inputgo from H to L to H and then the S input goes from H to L to H. Thatcauses the flip-flop to change. I tie either one of the flip-flopsoutput to the counters clock and it works! So fine, I figure thecounter just don't like my phototransistor. I have no idea why becausethe voltages look fine. I don't want the phototransistor to go to aflip-flop, that was just a test. So instead of feeding twophototransistors to a flip-flop, I feed one single phototransistor toone NAND gate and then feed the NAND gate output to the counters clock.So then I verified that...