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I am trying to use a 555 IC to create a low-high-low pulse that will trigger only once and still go low even if the button continues to be pressed. This is the circuit I have now but the pulse stays high if I continue pressing the button:

Circuit I have now

Edit: Wrong circuit diagram

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  • \$\begingroup\$ You seem to have a contradiction in "that will trigger only once and still go low even if the button continues to be pressed". You can have one or the other but not both. Maybe you mean a low-high-low pulse? Draw a timing diagram using the {} code button. \$\endgroup\$
    – Transistor
    Apr 7, 2021 at 21:31
  • \$\begingroup\$ Yes, I did mean low-high-low. Fixed that. \$\endgroup\$
    – dom_g
    Apr 7, 2021 at 21:45
  • \$\begingroup\$ electronics.stackexchange.com/questions/180716/… \$\endgroup\$ Apr 7, 2021 at 22:31

2 Answers 2

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enter image description here

Figure 1. Adding C1 prevents hold-on problems. Image source: Homemade Circuits.

How it works:

  • Before PB is pressed R1 charges the left side of C1 to V+ and R2 charges the right side. Both will be at +9 V.
  • When PB is pressed the left side of C1 is pulled to GND. The right side is pulled down to GND as well but immediately starts to charge up to +9 V via R2 even if PB is held.
  • D1 and D2 prevent pin 2 ever seeing V > 9 V or V < 0 V.

See the linked article for more.

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  • \$\begingroup\$ This is exactly what I was looking for! Thank you! \$\endgroup\$
    – dom_g
    Apr 7, 2021 at 22:14
  • \$\begingroup\$ Actually, I'm just realizing this is for an active low pin. Is it a completely different circuit to get a HIGH-LOW-HIGH pulse? \$\endgroup\$
    – dom_g
    Apr 7, 2021 at 22:50
  • \$\begingroup\$ Never mind, I just used a transistor to flip the signal. Thanks again! \$\endgroup\$
    – dom_g
    Apr 7, 2021 at 23:05
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74xx123 (74LS123, 74HC123, etc.).

Note that the pulse time vs. C and R values are different between the TTL parts and the CMOS parts, so read the datasheet carefully.

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