Source: https://mokxi.com/parts/ne555 Updated: 2026-09-27 Part # NE555 timer The real 555, astable or monostable, with the timing the capacitor gives it. or see every part - 8 pins Drawn live by the editor's own code, at the size you see it. Reference ## Every one of its 8 pins Pin Role What it does GND Input or output 0 V reference. TRIG Input Sets the flip-flop when it falls below a third of the supply. OUT Output High or low, a bipolar totem-pole output. RESET Input Below about 0.7 V, forces the output low regardless of the comparators. CTRL Input or output The upper threshold tap: normally two-thirds of the supply, movable. THR Input Resets the flip-flop when it rises above the upper threshold. DIS Input or output Open-collector, saturated to GND while the output is low. VCC Input or output Supply, 4.5 V or more. This part ## What it does The NE555 is the timer chip in every beginner’s kit, and it has no clock of its own at all. Inside it are three equal resistors from the supply to ground, setting two comparator trip points at one third and two thirds of the supply, an internal flip-flop, an open-collector discharge transistor and an output stage; every timing interval it produces comes entirely from the resistor and capacitor a circuit wires around it. Below about 4.5 volts on the supply the chip is unpowered and its output pins sit high, not driving anything, exactly like a chip that has not been plugged in. Comparator crossings reach the output 100 nanoseconds later, the datasheet’s own typical figure, and the control pin genuinely moves both trip points if something is wired to it, rather than being decorative. The astable and monostable circuits people build around this chip are entirely resistor-and-capacitor math; the chip itself only compares and switches. How it is modeled ## What is true about the NE555 timer, here ### What is modeled CTRL really is the control pin: the chip drives it with its own internal divider, so a decoupling capacitor there behaves as it does on the bench, and forcing a voltage onto CTRL genuinely moves both trip points. Below 4.5 V the chip is not running and OUT/DIS go high impedance, the way an unplugged chip would. OUT's high is a diode drop and a transistor drop below the rail (about 3.3 V on a 5 V supply) because it is a real bipolar totem pole, not an idealized rail-to-rail driver. The timing accuracy is the capacitor's, not the chip's, and the capacitor is solved by the matrix rather than stepped: a textbook astable measures 0.05% fast against 1.44 / ((RA + 2 RB) C) at 4.7 k, 10 k and 47 k of RB alike, held by a test in crates/parts/src/parts/ne555.rs. ### Not modeled The monostable used to run about 1.7% short and does not now. A textbook one-shot on 10 k and 10 uF should run for RC ln 3 = 109.8 ms; until September 17, 2026 it ran for 108.0, and it now runs for 109.8, within a twentieth of a percent, the same figure the astable holds. The fault was never the 555: it was in the analog solver, which ran one integration step ahead of the rest of the simulation, so anything that happened inside that step (a button contact bouncing, for instance, which is exactly how a one-shot is started) moved the clock back to the event without moving the capacitor's charge back with it. The capacitor kept a step's worth of charge it had not been given the time to collect, once per bounce. It is fixed, and both the monostable and the astable are now held to 1% by tests that run the real circuits. The discharge transistor's 10 ohms is not in the textbook formula, and it is in this model. Where RA is small (a 1 k RA against a 10 k RB), that 10 ohms moves the frequency about 1% below what 1.44 / ((RA + 2 RB) C) says. The model is right and the formula is the approximation, but a lesson that checks the formula to better than a percent should know which of the two it is testing. The output levels are fixed numbers rather than load-dependent ones: the high is always VCC − 1.7 V and the low always 0.1 V, where a real bipolar 555's drops depend on how much current the pin is passing. The comparator delay is a flat 100 ns from a crossing to the pin, inertial, which is the datasheet's output rise and fall time standing in for the whole path. Below 4.5 V of supply the chip simply stops rather than degrading, and there is no supply current, no internal noise, no temperature drift of the trip points, and no CMOS-variant (7555) behavior. From NE555 timer, in full. Projects ## See the NE555 timer in a project Learn ## Where it turns up in a lesson ### In the twelve week course - Week 4: Timing without a processor: the 555 ### In a learn article - The 555 Astable: Timed by a Capacitor, Not a Crystal - Blink an LED with a 555 Timer, No Microcontroller - Measure It: Read a Circuit With a Scope and Plotter - The 555 Monostable: One Press, One Timed Pulse - Multisim Live Is Gone: A Chromebook Alternative - 555 Timer Projects for a Science Fair (No Coding) More parts ## The rest of the bench Every one of these is drawn and simulated the same way. Full-size breadboard A real 0.1 inch grid with the rails and the center channel, 63 columns wide. Jumper wires Drag from any pin or hole to any other. Corners snap, and you can drag them. Power A supply rail at the voltage you choose. Ground The other end of every circuit. LED Lights when current flows. Five colors, and the brightness is what your eye would see. Resistor Any value you like, with the color bands drawn to match. ESP32-C3-DevKitM-1 A RISC-V board that runs your firmware at 160 MHz on the real memory map. Pushbutton A 12 mm tactile switch. Hold it while the simulation runs. Raspberry Pi Pico The RP2040 board on our own Cortex-M0+ core. Pick a program and press Run. Raspberry Pi Pico W Raspberry Pi Pico W (RP2040). The same board and the same pinout as a Pico, with the CYW43439 on it. The WiFi is simulated (no radio, no real internet), and the on-board LED, which hangs off that chip rather than off GP25, is driven through the same path. Everything else is the Pico. STM32F411 Black Pill The Black Pill on our own Cortex-M4 core. Pick a program and press Run. BBC micro:bit V2 The nRF52833 board with its 5x5 LED matrix, buttons A and B and a speaker. Wire the rings to a breadboard. See every part ## Wire up the NE555 timer Open the editor and push it into the breadboard. It is free, and it runs on your own machine. Start building Open the editor