GETTING TO KNOW IC 555 Through Experiments

The first six circuits to learn the theory and digital logic behind twelve simple experiments that can be performed using IC 555 were published in December 2022 issue. Here are the remaining six circuits.

- RAJEEV NILKANTH DESHPANDE

Although the circuits are non-conventional, they help understand the IC 555 comprising voltage divider, trigger comparator, threshold comparator, SR flip-flop, discharge transistor, reset transistor, and output stage. They also help understand the IC's three modes of operation: monostable, astable, and bistable. The visible operation of the IC in all the twelve experiments is shown by LEDs.

All the experiments are simple and can be performed on a breadboard. A power supply of 5V to 9V DC can be used for the circuits (although according to data sheet of IC 555, 4.5V to 15V may be used). In the following circuits two 555 ICs are used to create interesting situations and make the learning an enjoyable experience.

Experiment 7

As shown in Fig. 7, in this circuit both the ICS, IC12 and IC13, are wired as bistable multivibrator. However, the output pin 3 of each IC is connected to the discharge pin 7 of the other IC through an LED and a current limiting resistor. In normal bistable operation, the discharge pin 7 is not used as there is no capacitor in the circuit.

Initially, when the circuit is powered, both LED12 and LED13 remain off. When switch S9 is pressed, LED12 turns on, and when S10 is pressed, LED12 turns off. Similarly, when S11 is pressed, LED13 turns on, and when S12 is pressed, LED13 turns off. This is a normal bistable operation.

But if switch S9 is pressed to switch on LED12 and then S11 is pressed, instead of LED13 turning on LED12 turns off. Thus, both LED12 and LED13 remain off though the output at pin 3 of both the ICs is high! This unique situation arises because the internal discharge transistors at pin 7 of both the ICs turn off and the transistors cannot conduct.