555 As A Monostable Mulivibrator

This configuration of the 555 is called a ONE-SHOT Mulivibrator. The name comes from the circuit configuration allowing one action after the trigger. Once triggered, the output will stay in the triggered state for a period of time based on the RC time interval. Then switch back to the wait state and wait until another trigger is sent. NOTE: As we move on in the study other oscillator circuits using different components will also be referred to as a ONE-SHOT Mulivibrator.

This schematic is very similar to the prior one. A Switch S1 has been added to trigger the circuit. Here is how it works.

At first powered on, the output (pin 3) is 1 or on, and the LED is lit. R2 is holding the trigger (pin 2) high, causing the flip-flop S to be state 0. C1 is discharged and starts to charge through R1. NOTE: C1 is considered discharges at the beginning.

When C1 is charged to 2/3 supply voltage the upper comparators non-inverting input will cause it to switch setting the flip-flop R to state 1. At that moment the flip-flop will switch output states, Q will be 0, and the LED goes dark. There is more going on though. The discharge circuit will discharge C1, the upper comparator will switch back to its output state 0 setting and the flop-flop R to state 0.
Now the circuit will wait for a trigger. Until the battery runs down if need be.

There are three ways to look at the trigger.
1) Press and never releasing the trigger, the led will stay on. No real value in the fancy circuitry here.
2) Press and hold the trigger longer then the RC interval the LED will be on while triggered and shut off when it is released. No real value in the fancy circuitry here either.
3) Press and immediately released the trigger and the LED will stay lit for the RC time interval.

What are the parts in the circuit?
555 is the IC
R1 is the circuit timing resistor
R2 is a Pin 2 pull-up resistor for a logical high state.
R3 limits LED current
C1 is the circuit timing capacitor
S1 is the trigger.
One shot has only one time cycle, T = 1.1 (R1 x C1)

Experiment - Monostable Mulivibrator and an LED



Go to the work bench.
Review the Resistor Color Code chart and wire up the kit as described. Perform the lab, and record your results.

Parts List:
1 9-volt battery
1 Battery power clip
1 555 IC - U1
1 10 mmf capacitor - C1
1 1 k ohm resistor - R3 (brown, black, red)
2 100 k ohm resistor - R1, R2 (brown, black, yellow)
1 red light emitting diode
1 experimenters board
Miscellaneous: hookup wire

NOTES: The flat side of the LED is toward the V- side of the circuit. The minus side of the C1 capacitor is toward the V- side of the circuit.


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