Practical Circuit 2: Monostable Multivibrator
A monostable multivibrator, also known as a one-shot timer, is mainly used when there is need to produce a pulse with a fixed duration following another pulse signal often known as a ‘trigger’. The trigger pulse duration must be shorter that the output pulse duration for the monostable multivibrator to work appropriately. A use case for a monostable multivibrator can be a controller for an electric bell. Let’s say you want a bell to ring for ten seconds after the bell push has been pressed; In this case, you will need a monostable multivibrator designed to produce a ten- second output pulse following a press of the bell push (a trigger). The 555 timer is well suited for this job. Below is a circuit of a 555 timer monostable multivibrator that can produce an output pulse of around eleven seconds when a button is pressed.
Bill Of Materials
Bill of materials required to build the Monostable multivibrator circuit shown below.
|Breadboard||–||1||₦1600 View Add to Cart|
|100KΩ Resistor||R1||1||₦10 View Add to Cart|
|10KΩ Resistor||R2, R4||2||₦10 View Add to Cart|
|470Ω Resistor||R3||1||₦10 View Add to Cart|
|100uF Capacitor||C1||1||₦10 View Add to Cart|
|0.1uF Capacitor||C2, C3||2||₦20 View Add to Cart|
|Red LED||D1||1||₦10 View Add to Cart|
|9V Battery with connector||BAT 1||1|
|Tactile Pushbutton Switch||Pushbutton||1||₦30 View Add to Cart|
|555 timer IC||U1||1|
The circuit can easily be tested on a breadboard. Follow the illustration above to assemble and test out the Monostable Multivibrator circuit on your breadboard.
The duration of the output pulse of the monostable multivibrator is set by the component values of resistor R1 and capacitor C1. The duration of the output pulse can be derived using the formula below:
The push button as shown in the circuit, is used to trigger the monostable multivibrator. Since the trigger pulse duration must be shorter than the output pulse duration for the circuit to work, the circuit arrangement of R4, R2 and C3 ensures that no matter how long the push button is pressed, the trigger pulse at pin 2 of the 555 timer remains far shorter than the output pulse. The duration of this trigger pulse is roughly the time constant of R2 and C2 (R2 * C2); around 1 millisecond. Once the push button is pressed, the circuit gets triggered, pin 2 of the 555 timer goes LOW momentarily setting pin 3 HIGH and disabling the internal discharge transistor connected to pin7. As the discharge transistor has been disabled, capacitor C1 starts charging through R1. As C1 charges, voltage across it rises. When the voltage across C1 has risen above 2/3 of the supply voltage (9V), the 555 timer output goes LOW and the discharge transistor is activated which discharges the capacitor C1, getting it ready for the next trigger.