Source: https://mokxi.com/learn/555-timer-projects Updated: 2026-09-27 Learn # 555 timer science projects (no coding needed) Written by the Mokxi team, updated September 27, 2026 Sign up free or see every lesson - 192 parts on the bench - 25 boards running now - 1.00x real time, on every board Hold the button The 555 timer chip has been around since the early 1970s and is still one of the best first chips to learn, because two resistors and a capacitor set its timing and you can check the math with a stopwatch. None of the five projects here need a microcontroller or any code, which makes them a good fit for middle school science fairs and for anyone who wants to understand the circuit rather than the sketch. Each project has a working circuit, the formula that sets its timing, and a question you could test for a fair. The circuit above is the one-shot, which is the simplest to measure: press the button and the LED stays on for 1.1 seconds, however briefly you press. ## 1. The blinker (astable mode) In astable mode the 555 charges its capacitor through two resistors and discharges it through one, forever, and the output flips each time. The frequency is f = 1.44 / ((R1 + 2 x R2) x C). With 10 k, 47 k and 10 uF that is about 1.4 Hz, and the slide switch in this circuit swaps in 4.7 k for about 7.4 Hz. Fair question: does the measured frequency follow the formula as C changes? Change the capacitor, count blinks for 30 seconds, and graph measured against predicted. ## 2. The doorbell (a tone you can switch) The doorbell holds the 555 in reset until the button is pressed, which is how you switch any 555 circuit on and off without cutting its power. While the button is down, the astable runs at about 4 Hz and beeps the buzzer. Fair question: which beep rate do people notice fastest? It measures people, so human participant rules apply, even if you only test yourself. ## 3. The one-shot pulse stretcher (monostable mode) In monostable mode, a short dip on the trigger pin starts one timed pulse. The output goes high, the capacitor charges through R, and when it reaches two thirds of the supply the output drops. The time is t = 1.1 x R x C. With 10 k and 100 uF that is 1.1 seconds. In the simulator, a 30 ms press gave an output pulse of 1.10 seconds from the moment of the press. Fair question: how close is t to 1.1 x R x C across a range of R values? Measure with the scope or a stopwatch. ## 4. The shadow alarm Replace the button with a light sensor and the one-shot becomes an alarm. The LDR runs from 5 V to the trigger pin and a 10 k runs to ground. In good light the trigger sits high. A shadow raises the LDR’s resistance, the trigger falls below a third of the supply, and the buzzer sounds for 1.1 x 470 k x 10 uF, about 5.2 seconds, even if the light comes straight back. The simulation measured 5.16 seconds from the moment of the shadow. Fair question: at what light level does the alarm trigger, and how could you move that level? Swapping the 10 k resistor is the answer to test. ## 5. The PWM dimmer Two diodes split the capacitor’s charge path from its discharge path, and a 100 k potentiometer divides its resistance between the two. Turning the knob changes how long the output stays high compared with how long it stays low, while the total, and so the frequency, stays about the same. That is pulse width modulation, the way many LED dimmers work. The meter on this circuit is on its duty range. With the knob in the middle it read 49.9 percent. Near one end it read 11.0 percent, and near the other 87.2 percent. The LED looks dimmer or brighter because it is on for a smaller or larger share of each cycle, around 110 times a second, which is too fast to see as flicker. Fair question: does the LED’s perceived brightness match its duty cycle? People usually say no, because eyes do not see brightness linearly, which makes this a good experiment with a surprising answer. It asks people what they see, so human participant rules apply. ## Build it for real All five run on 5 V from a USB power bank or four AA cells with a regulator, and use common parts: an NE555, resistors, capacitors, an LED, a pushbutton, an LDR and an active buzzer. Electrolytic capacitors have a polarity: the stripe marks the negative lead, which goes toward ground in these circuits. Put one in backward on a real board and it can bulge or burst, so check before you power up. Real parts vary. Capacitors are often only within 10 to 20 percent of their marked value, so your measured timing may miss the formula by that much. That is not a failure; it is a result worth explaining in your write-up. ## Which 555 to buy The original NE555 runs from about 4.5 V to 16 V, and its output can drive an LED or a small buzzer directly. CMOS versions such as the TLC555 and LMC555 draw far less current from the supply and run from lower voltages, which suits a battery project, but their outputs cannot drive as much current. The timing formulas are the same for all of them, so any of these five projects works with either kind. Check the datasheet for your exact chip before you power it up. ## Questions What is the difference between astable and monostable? Astable runs by itself forever, making a square wave. Monostable waits for a trigger and makes one pulse of a set length, then waits again. Why does my 555 not start? Check that the reset pin is tied to the supply. Held low, the chip keeps its output off no matter what the timing parts do. Related ## Keep going The 555 Astable: Timed by a Capacitor, Not a Crystal Blink an LED with a 555 Timer, No Microcontroller Engineering Science Fair Project: How to Write It Up Arduino Science Fair Projects With a Real Hypothesis The NE555 part and its pins ## Build this for real Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed. Start building Open the editor