I have a 555 one shot timer circuit in which the Trigger and Reset pins are tied together in attempt to reset the timer every time the timer is triggered so that the one shot really extends for the designed time after the last signal in a group of signals is received.

The problem I'm having is that the signals are very fast so the rest pin is not held low for long enough to actually reset the timer. Is there any hardware I can add to the circuit to artificially hold the signal low just long enough when the trigger is received to get it to actually reset? I've successfully gotten the reset to work when delaying 5ms on trigger to hold the reset pin low using firmware, but this slows down my signal processing significantly.

Is there anything I can add to the physical circuit which will hold that pin low just a tad longer?

  • \$\begingroup\$ What value are you using for the timing capacitor? \$\endgroup\$ – brhans May 12 '17 at 3:23
  • \$\begingroup\$ 470 uF. 30s one shot \$\endgroup\$ – Alex May 12 '17 at 3:24
  • \$\begingroup\$ Way too big.. Ic=C dv/dt is current limited to charge in dt duration. reduce current to stay within spec , using CMOS version? and define t pulse width and increase R to max and reduce C or change design to CD4060 counter. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 12 '17 at 4:01
  • \$\begingroup\$ TBH your problem is that you're trying to use a 555 as a re-triggerable one-shot - and it's not intended to do that. You'd be better off using a 74xx123 or something similar. You could try adding a P-MOSFET across the timing cap (with gate connected to trigger) to try to help to discharge it better than the 555's internal discharge transistor, but I wouldn't hold my breath... \$\endgroup\$ – brhans May 12 '17 at 4:12
  • \$\begingroup\$ Using a one-shot to re-trigger a one-shot is kind of silly. Any hardware you'd add to extend the reset pulse might as well just be used as the entire retriggerable one-shot from the get-go, getting rid of the 555 entirely. How narrow can your trigger pulse (I assume "active low") be? What's the rail voltage? How long to you want your re-triggerable one-shot to hold its output after the last trigger pulse? \$\endgroup\$ – jonk May 12 '17 at 5:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.