555 Timer Animation

This animation of the 555 timer in astable configuration was extremely enjoyable to make. I wanted to make the clearest animated diagram so that anyone could understand its operation.

It was only when I drew the animation that I noticed another dimension to its operation, that the discharging cycle occurs when the flip-flop is in RESET mode, and the charging cycle occurs when the flip-flop is in SET mode. Another way to look at it is that the threshold signal is responsible for initiating the discharge cycle, and the trigger initiates the charge cycle.

Notice that the capacitor discharge path is through resistor R2 and the open collector transistor, during which time, the output (Q) is logic 0. However, notice that charging occurs through R1 and R2 by the current taken from the Vcc rail, during which time the output (Q) is logic 1.

Interestingly, in an astable mode, which is a free running oscillator, we usually connect the threshold and trigger inputs together, and a single external timing capacitor controls their voltage levels. Usually, pin 7 is where we connect the timing capacitor through a resistor, which then connects to the open collector pin of the transistor. The inverted output (/Q) of the flip-flop drives the base junction, so that when the flip-flop is RESET, /Q goes HIGH, which makes the transistor conduct causing the capacitor to discharge through it to ground. As it discharges, the voltage across the capacitor reaches 1/3 Vcc, and the trigger input asserts the SET function causing /Q to go LOW, during which time the transistor stops conducting and the capacitor starts charging again through the external timing resistors R1 and R2. As the capacitor continues charging, the voltage across it reaches 2/3 Vcc and this time threshold asserts the RESET function. This then makes /Q go HIGH causing the transistor to conduct and discharge the capacitor again. Therefore, the capacitor is always charging up to 2/3 Vcc and discharging down to 1/3 Vcc. How fast the capacitor charges and discharges obviously depends upon the timing resistor value.

The 555 timer IC is a very simple IC to understand, however most textbooks, and teachers, never explain its operation as clearly as this, because they do not have a clear idea either! I therefore decided to draw this new animated block diagram, which is the most detailed, clearest, and easiest to understand. It has all the main details a GCSE student would need to understand how a 555 timer operates.

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