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What is the effect of connecting Reset and Trigger together in a 555 monostable circuit?

I am modifying someone else's circuit and don't see why they say a need to connect Reset to Trigger instead of just tying Reset to Supply Voltage.

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  • \$\begingroup\$ I also don't quite see how would you want to connect an analog Trigger signal to the digital Reset. The Reset pin connects directly to an internal SR flip-flop. Connecting a signal that might not be TTL high or low would have unpredictable results. Do you have an example schematic, maybe? And a clue about what is the circuit supposed to do? \$\endgroup\$ Oct 6 '12 at 12:10
  • \$\begingroup\$ The reset pin must be below 0.7V to turn the chip off. Maybe by that logic you can work how your particular circuit operates. \$\endgroup\$ Oct 6 '12 at 12:19
  • \$\begingroup\$ Can you post the schematic? \$\endgroup\$ Oct 8 '12 at 18:14
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The 555 won't work 100% in the monostable mode anymore,that's for sure.In your suggested configuration,when trigger is pulled low,reset is pulled low,too.That means it sets and resets itself at the same time.

The result is rather unpredictable,as someone states in a comment.If reset turns on first,chances are the IC will work in monostable mode.If trigger switches on first,then the output won't turn on at all.It's like playing the lottery.You don't know what's going to happen.

I don't see why they would need such a setup,either.It can only lead to an erratic functioning of the IC.Setting and resetting at the same doesn't sound like a good idea.It is supposed to be in one of the states at a given time,not in both at once.

Perhaps the person you speak of thought of using the 555 in astable mode.That requires the trigger to be tied to the threshold pin,not to the reset pin.That way,after the voltage across the capacitor reaches 2/3 of \$V_{cc}\$ ,the discharge pin activates,until the voltage changes to 1/3 of \$V_{cc}\$,trigger activates,discharge is deactivated and the capacitor starts charging again.

Neither the bistable mode involves your suggested configuration.In this case,trigger and reset are pulled low independently.

To conclude,nothing useful would happen if you tie reset to trigger.The suggestion is a mistake in my opinion.

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  • \$\begingroup\$ If one ties the trigger input to the reset input, the trigger input will operate at voltages between 0.7 volts and control/2 (which is usually Vcc/3). If there were a resistor between the reset pin and the other pins, and there were some other means of forcing the reset pin high or low, one could add an extra form of latching to the 555: initially the oscillator would be stopped and the output would be jammed low, but pulling reset high long enough to charge the cap above 0.7 volts would start the oscillator and allow it to run until the next time reset is pulled hard low. \$\endgroup\$
    – supercat
    Dec 27 '16 at 22:51
  • \$\begingroup\$ @supercat Why not post this as an answer? \$\endgroup\$ Dec 28 '16 at 7:06
  • \$\begingroup\$ I was initially just going to mention that reset and trigger have different thresholds, but then got carried away with the implications. >:*3 \$\endgroup\$
    – supercat
    Dec 28 '16 at 15:15
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If one ties the trigger input to the reset input, the trigger input will operate at voltages between 0.7 volts and control/2 (which is usually Vcc/3). If there were a resistor between the reset pin and the other pins, and there were some other means of forcing the reset pin high or low, one could add an extra form of latching to the 555: initially the oscillator would be stopped and the output would be jammed low, but pulling reset high long enough to charge the cap above 0.7 volts would start the oscillator and allow it to run until the next time reset is pulled hard low.

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