NE555DR

The NE555DR from Texas Instruments is the classic precision timer IC that has been a staple of electronic design since 1972. It can be configured as a monostable (one-shot) timer producing a single pulse of defined width, or as an astable (free-running) oscillator producing a continuous square wave. The internal architecture consists of two comparators, a flip-flop, a discharge transistor, and a resistive voltage divider that sets the trigger level at 1/3 VCC and the threshold level at 2/3 VCC. The 200mA output stage can directly drive relays, lamps, small motors, and speakers without external transistors. Timing is determined by external resistors and capacitors, providing flexibility from microseconds to hours. The DR suffix denotes the SOIC-8 package with tape-and-reel packaging.

The NE555DR operates based on three internal 5kΩ resistors forming a voltage divider that sets reference voltages at 1/3 VCC (trigger comparator) and 2/3 VCC (threshold comparator). In monostable mode, the trigger pin (pin 2) is normally held above 1/3 VCC. When a falling edge pulls the trigger below 1/3 VCC, the trigger comparator sets the internal flip-flop, driving the output HIGH and turning off the discharge transistor. The external timing capacitor (connected to pin 6/7) charges through the timing resistor. When the capacitor voltage reaches 2/3 VCC, the threshold comparator resets the flip-flop, driving the output LOW and turning on the discharge transistor to rapidly discharge the capacitor. The output pulse width is T = 1.1 × R × C. In astable mode, the capacitor charges through R1+R2 and discharges through R2 only. The charge time (output HIGH) is T_H = 0.693 × (R1+R2) × C, and the discharge time (output LOW) is T_L = 0.693 × R2 × C. The frequency is f = 1.44 / ((R1 + 2×R2) × C), and the duty cycle is (R1+R2)/(R1+2×R2). The control voltage pin (pin 5) provides access to the 2/3 VCC node; applying a voltage here modulates the threshold and trigger levels, enabling pulse-width modulation or frequency modulation.

NE555DR uses the SOIC-8 (D) surface-mount package with the standard 555 timer pinout: Pin 1 = GND, Pin 2 = TRIG, Pin 3 = OUT, Pin 4 = RESET, Pin 5 = CTRL, Pin 6 = THR, Pin 7 = DISCH, Pin 8 = VCC. The DR suffix denotes SOIC-8 tape-and-reel from TI. The pinout is identical to the PDIP-8 version (NE555P), enabling seamless PCB layout migration between through-hole and surface-mount. The CTRL pin (Pin 5) is at 2/3 VCC internally — adding an external voltage to this pin modulates the threshold and trigger levels for pulse-width modulation. For CMOS low-power version, TLC555DR provides 1.5-15V operation with only 170µA supply current.

• Monostable Timer: Push-button trigger creates a fixed-width pulse for relay hold, LED timeout, or debounced output
• Astable Oscillator: R-C network sets frequency for LED flasher, tone generator, or clock source
• PWM Controller: Control voltage pin modulates threshold for motor speed or LED brightness control
• DC-DC Oscillator: 200mA output directly drives small flyback or boost converter switching transistor
• Schmitt Trigger: With TRIG and THRESH tied together, functions as an inverting Schmitt trigger with 1/3 VCC hysteresis

NE555 is the industry-standard precision timer generating accurate time delays from microseconds to hours. It operates in both astable (oscillator) and monostable (one-shot) modes. CMOS versions (TLC555, LMC555) offer 2-15V supply and 1µA standby for battery applications. The NE556 provides dual timers in DIP-14. SA555 extends temperature range to 125C for automotive use.