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I tried two circuits below, none of them seems to be working (I used 74LS04 and 74LS00), the output doesn't change when I press the switch.

I found that the voltage at point "A" doesn't drop below 0.8V (stays around 2V) when the switch is pressed. My guess the reason is that: when the switch is pressed, the capacitor starts to discharge, but there is a also a current flow out of the logic gate to ground thru R2, caused a voltage drop across R2, so that the voltage at A can not reach logic "0".

Any other ways to make it work? Thanks much!

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    \$\begingroup\$ Please give the values for R1, R2 and the cap. \$\endgroup\$ – Vladimir Cravero Jan 26 '17 at 16:21
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    \$\begingroup\$ R1=80K, R2=20K, C = 1uF (the first chart) \$\endgroup\$ – starx Jan 26 '17 at 16:56
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    \$\begingroup\$ The 74LSxx parts are bipolar TTL logic. The inputs source current, and you must draw about 0.4 mA from the input for the input to be seen as a Low. 20K for R2 is much too high a value. \$\endgroup\$ – Peter Bennett Jan 26 '17 at 17:12
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    \$\begingroup\$ As peter says, those values are pretty crazy. I'd do something like 10k for R1, 100ohm for R2 and 100nF for the cap. 1u is fine but 100n is enough. \$\endgroup\$ – Vladimir Cravero Jan 26 '17 at 21:56
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Refering to the first circuit diagram:

If \$V_A\$ stays too high when button is pressed, \$R_2\$ obviously is too large.

So you have to decrease \$R_2\$.

If that in turn decreases your time constant \$\tau=(R_1 + R_2)C\$ too much for the circuit to work properly (i.e. debounce properly) you have too increase \$R_1\$ and/or \$C\$ in addition.

BTW: You wouldn't have this problem if you used CMOS logic (74HC04 or better 74HC14) which has very high impedance inputs (i.e. practically no current is coming from the input).

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  • \$\begingroup\$ R2 can be removed. \$\endgroup\$ – Vladimir Cravero Jan 26 '17 at 15:09
  • \$\begingroup\$ @Vladimir Cravero: Probably, yes. That'd be the extreme of decreasing R2. \$\endgroup\$ – Curd Jan 26 '17 at 15:14
  • \$\begingroup\$ I meant that for real. There's no particular reason for R2 to be there, unless you wanna limit the current through the switch. \$\endgroup\$ – Vladimir Cravero Jan 26 '17 at 15:16
  • \$\begingroup\$ I'm not dissenting in any way. \$\endgroup\$ – Curd Jan 26 '17 at 15:20
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    \$\begingroup\$ The reason for R2 is the longevity of the switch. Shorting out a charged capacitor is not, in general, good for the lifetime of a mechanical switch. For some switch constructions, even a single discharge of a cap can affect switch performance. \$\endgroup\$ – WhatRoughBeast Jan 26 '17 at 17:39

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