Timers or Multi-vibrators 

Today we are going to discuss the digital electronics application called timers or multi-vibrators “Timers are those circuits which provide specific periodic signals to the digital circuits. These signals are capable of changing the condition of those systems” it can be defined as that “multi-vibrators are those circuits which provide those signals which are consists of high and low levels only called multi-vibrator. In general, multi-vibrator is a two-stage amplifier in which one amplifier’s output is connected to the input of the second amplifier as positive feedback. Both amplifiers are connected in such arrangement that if one transistor is in saturation mode or ON then the second one will be in cutoff mode. These circuits or devices are usually used as pulse generating, storing, counting circuits. 

Types of Multi-vibrators:

Multi-vibrator has the following three common types.

1. Astable multi-vibrator
2. Monostable multivibrator
3. Bistable multi-vibrator   

Astable multivibrator is that type of multi-vibrator who’s both output levels are unstable. It means both output levels are consists of semi-stable levels.

Monostable multi-vibrator is also called the single shot or single swing or one-shot multi-vibrator. Monostable multi-vibrator is a digital device whose output is consists of one stable and semi-stable state. When the multivibrator is triggered externally it switches its state from stable position to semi-stable/unstable state.

Similarly, bistable multi-vibrator is that type of multi-vibrator whose output signal is consists of two stable states. Its output never changes until an external trigger signal is applied to the device or circuit. This multi-vibrator is also known as binary or divided by two elements.

The above waveforms can be generated through different digital devices but the most common digital device which is more frequent use as a clock generator is 555 timer. It is a digital monolithic integrated circuit which is used as a clock generator. It can generate astable and monostable waveforms. It is a versatile and most using digital device. It provides a timing delay of few microseconds up to few hours. 555 timer IC having 8 pins is given below in the figure.

555 Timer IC is Consists of the Following pin which are Explained in Detail

1. Ground:  This terminal is called the common point or ground point. All the external ground terminals and the power supply ground terminal is also connected with this terminal.

2. Trigger: This is the terminal in 555 timer IC on which 1/3rdvoltage value of the applied voltage is given then the circuit output is changed from low to high state.

3. Output: This is the terminal from which output signal can be received. This terminal is also connected with the load which can be low or high at a time.

4. Reset: This terminal is used for clearing the output state ignoring the previous statement means changing the output to the low level.

5. Control voltage: About 2/3rd positive voltage of the applied voltage is available on this terminal so it becomes the part of a comparator. Usually, there is a capacitor is connected in b/w this terminal and ground terminal.

6. Threshold voltage: Threshold voltage and control voltage are the input of comparator circuit. The comparator circuit compares threshold voltage with the reference voltage. If the threshold voltage is more than or same as the reference voltage comparator circuit output becomes low. And if the threshold voltage is less than the reference voltage then comparator output becomes high.

7. Discharge: This terminal provides a low resistive path for the externally connected capacitor when circuit output is low but it behaves open circuit for the capacitor when the circuit output is high.

8. +Vcc supply voltage: Operating all the terminals of the timer it needs to be energized. For that purpose, 15v is supplied to the IC with this terminal.

In the above figure, a circuit diagram is given which clearly illustrates the connections b/w the components. The voltage divider circuit is consists of three 5kΩ resistors. This provides 10v to the comparator 1 and 5v to the second comparator. When the trigger input is increase than 1/3rd of the applied voltage then comparator B becomes low to high. In result, RS flip-flop becomes set and its output change to high. And the discharge transistor switches off. The output will be high until the threshold voltage value equals to2/3rd of Vcc. When the threshold voltage value increases, the comparator A output changes from low to high, in result the RS flip-flop resets and discharge transistor turns ON. By controlling the threshold and control input with external components it can be used as astable and monostable multi-vibrator.