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.
Electronic – Reset and Trigger connected in 555 timer circuit
555
Related Solutions
The threshold triggering could be done using a rail-to-rail comparator, such as the Linear Technologies LT1711, using a 0.5 Volt reference voltage.
Depending on the precision required, the voltage reference can be obtained by using a sub-bandgap voltage reference IC such as the Analog Devices AD130 which has a 0.5 Volt output, or through a resistor voltage divider between the supply voltage and ground.
For a hybrid compromise, a Zener diode shunt reference of say 1.8 Volts can be used with a resistor voltage divider connected across it:
simulate this circuit – Schematic created using CircuitLab
The triggering slope (rising or falling) can be switched by interchanging the inputs to the comparator.
Connect the switch to the reset input of the 555, and connect the trigger through a small RC delay:
simulate this circuit – Schematic created using CircuitLab
The 555 is held in reset as long as the switch isn't pushed. When the switch is held in, the 555 goes through its normal timing cycle once, but it aborts if the switch is released early.
Best Answer
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.