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I am trying to create a manual demonstration circuit for a 74HC595 shift register. Se my circuit below. The problem I am having is that I am only getting a +0.7V signal to the data, clock, and latch pins of the 74HC595 when one of the buttons are pushed. The S4 RESET switch works fine (normally high at 5V, press switch to bring down to 0V), however S1, S2, and S3 do not accomplish the reverse scenario (normally LOW, press switch for HIGH) correctly. You will note that I am also trying to debounce the switching with a capacity or resistor. I suspect this debouncing set up is creating my problem.

Does anybody have any advice on how to create a debounced normally LOW, momentarily HIGH switch that switches between 0V and 5V?

74HC595 Manual Demo Circuit

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  • \$\begingroup\$ Your gate debouncing looks almost straight out of a very old document. Except for one thing. The Schmitt trigger inverter/buffer such as the 74HC14, which makes it normally low. You may need to add that. You probably need the hysteresis. I haven't attempted to compute resistor and capacitor values, though. But on the referenced document, and where they are using HC parts, they came up with \$82\:\textrm{k}\Omega\$ and \$18\:\textrm{k}\Omega\$ to go with the \$100\:\textrm{nF}\$ capacitor. Close. But there could be something more to do there, as well. Can you show how you got those values? \$\endgroup\$ – jonk Jul 3 '17 at 6:04
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0.7V is the base emitter voltage of the transistors.

To verify: Remove the transistors from the circuit.

Fix: Add a resistor (1k to 10k) to the transistor bases. Better yet: Connect the transistors (with resistor) directly to the switches.

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  • \$\begingroup\$ That was it. Connecting the transistor base to a 1k resistor which is then connected directly to the switch works perfectly. Thanks! \$\endgroup\$ – Kamprath Jul 3 '17 at 8:14

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