6.3  Blink Circuit Analysis

The LED 555 Timer Circuit

We will be building the circuit diagram of figure 1. This circuit creates a blinker by alternately charging and discharging the capacitor \(C_1\) in a continuous cycle.

Circuit Operation

The operation of this circuit can be understood in two distinct phases:

Charging Phase (LED ON)

• The capacitor \(C_1\) charges through resistors \(R_1\) and \(R_2\) with time constant \(\tau_{charge} = (R_1 + R_2) C_1\)
• The capacitor voltage is applied to both pins 2 (TRIG) and 6 (THRES)
• When \(v_{C_1}\) reaches \(\frac{2}{3}V_{CC}\) (the upper threshold), the comparator connected to pin 6 triggers
• This sets the SR flip-flop, causing the output (pin 3) to go HIGH
• The HIGH output drives current through the LED and current-limiting resistor \(R_3\), turning the LED ON

Discharging Phase (LED OFF)

• When the flip-flop is set, the discharge transistor (connected to pin 7) turns ON
• This connects pin 7 directly to ground, providing a discharge path for \(C_1\)
• The capacitor now discharges through \(R_1\) only, with time constant \(\tau_{discharge} = R_1 C_1\)
• When \(v_{C_1}\) falls to \(\frac{1}{3}V_{CC}\) (the lower threshold), the comparator connected to pin 2 triggers
• This resets the SR flip-flop, causing the output to go LOW
• The LOW output turns the LED OFF, and the cycle repeats

Timing Analysis

The timing periods can be calculated as:

• HIGH time (LED on): \(t_{high} = 0.693(R_1 + R_2)C_1\)
• LOW time (LED off): \(t_{low} = 0.693 R_1 C_1\)
• Total period: \(T = t_{high} + t_{low} = 0.693(2R_1 + R_2)C_1\)
• Frequency: \(f = \frac{1}{T} = \frac{1.44}{(2R_1 + R_2)C_1}\)

The duty cycle (percentage of time LED is on) is: \[\text{Duty cycle} = \frac{t_{high}}{T} = \frac{R_1 + R_2}{2R_1 + R_2}\]

Note that with this configuration, the duty cycle is always greater than 50% because \(R_2\) contributes to the charging time but not to the discharging time.