Falling Edge Triggered 555 Monostable Timer
A falling edge triggered 555 monostable, also known as negative going, or negative edge triggered monostable, is simply a timer circuit that has an active LOW trigger. The terms negative edge and falling edge sometimes confuses students, because they start thinking of waveforms, however if we were to mention active-LOW, then straight away you should have a picture in your mind of a pull-up resistor, and switch to ground forming a potential divider type network.
In this circuit, there is a pull-up resistor establishing the HIGH level, and switch to ground providing a negative going pulse to activate the monostable timer. The value of the pull-up resistor is sufficient enough that it keeps pin 2 HIGH all the time and we usually use a value of 1 kΩ and no less. This is because we do not want the pull-up to be so strong that it will draws unnecessarily large amounts of current from the positive voltage rail when we close the switch.
As you can imagine, pin 2 remains HIGH all the time, however when we close the switch momentarily, the voltage there goes LOW and falls to zero volts, thereby triggering the timer. The duration of time for which the LED remains lit depends upon the values of R and C in the circuit.
Calculate Timing
The timing components are, the 47 µF capacitor, and 100 kΩ resistor. The values of R and C determine the duration of the output pulse, and given by the following standard formula.
T = 1.1 × R × C
T = 1.1 × 100 × 103 × 47 × 10-6
T = 5.17-seconds
Calculating LED Series Resistance
In this circuit, we are using an LED with a forward voltage drop of 2 V. This means that the voltage across the resistor R will be 4 V because the output from the 555 IC will be the same as the positive rail voltage of 6 V, hence 6 - 2 = 4 V.
We would like the LED to light brightly and therefore chose 24 mA (maximum) to pass through it. Since we know the voltage across resistor R and the current through it will be 24 mA, we simply use Ohm’s Law to calculate the value of R.
R1 = V / I
R1 = 4 / 0.024
R1 = 167 Ω
We therefore use a value of 180 Ω as the preferred value from the E24 Series standard resistor list.