Doorbell
A pushbutton lifts the 555 out of reset and a 4 Hz astable beeps the buzzer. There is no microcontroller in it: the simulator solves the 7 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
Teaching? Assign it to your class in one click.
Beginner About 25 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $6. On Mokxi they're free.
What you'll learn
- A 555 astable oscillator and its timing formula
- RESET as an on and off switch for a 555
- Pull down resistors keep an input at a known level
- Active and passive buzzers differ
- Why a series resistor can silence a buzzer
- Difficulty: Beginner
- Time: About 25 minutes
- Parts: 9
How it works
A 555 timer chip is wired as an astable oscillator, which means it flips its output high and low by itself. Here it does it about four times a second. That output goes straight to an active buzzer, which has its own tone generator inside, so the 555 only sets the beeping rhythm.
The 10k and 47k resistors and the 3.3 uF capacitor set the speed. The formula 1.44 divided by ((10k + 2 x 47k) x 3.3 uF) gives about 4.2 Hz.
The pushbutton is the doorbell. Pressing it lifts the 555's RESET pin to 5 V, so it runs. Letting go lets the second 10k resistor pull RESET to ground, which holds the output low.
Parts list
9 parts, plus the jumper wires. They retail for about $6; on Mokxi they're free.
- 1 × Full-size breadboard
- 1 × Power, 5 V
- 2 × Resistor, 10k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × NE555 timer
- 1 × Resistor, 47k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × Capacitor, 3.3 µFIn Power Bank Kit.
- 1 × PushbuttonIn Starter Kit, Christmas Lights Kit, Logic Lab Kit.
- 1 × BuzzerIn Starter Kit, Rocket Flight Computer Kit.
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| Resistor, 10k Ω (1) pin 1 | 5 V |
| NE555 timer pin VCC | 5 V |
| Pushbutton pin 2a | 5 V |
| NE555 timer pin GND | Ground |
| Capacitor, 3.3 µF pin 2 | Ground |
| Resistor, 10k Ω (2) pin 2 | Ground |
| Buzzer pin 2 | Ground |
| NE555 timer pin DIS | Resistor, 10k Ω (1) pin 2 |
| Resistor, 47k Ω pin 1 | Resistor, 10k Ω (1) pin 2 |
| NE555 timer pin THR | NE555 timer pin TRIG |
| Resistor, 47k Ω pin 2 | NE555 timer pin TRIG |
| Capacitor, 3.3 µF pin 1 | NE555 timer pin TRIG |
| Buzzer pin 1 | NE555 timer pin OUT |
| Resistor, 10k Ω (2) pin 1 | NE555 timer pin RESET |
| Pushbutton pin 1b | NE555 timer pin RESET |
The notes that come with it
There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.
// A doorbell: a button, a 555 and a buzzer.
//
// The buzzer here is an active one: it has its own oscillator inside, so DC
// across it makes its note and nothing across it is silence. The 555 is not
// making the tone, it is making the rhythm: about four pulses a second, so the
// bell beeps instead of droning.
//
// f = 1.44 / ((R1 + 2 R2) C) = 1.44 / ((10k + 94k) x 3.3 uF) = 4.2 Hz
//
// Hold the pushbutton while it runs. It pulls RESET (pin 4) up to 5 V; let go
// and the 10 k resistor pulls RESET back to ground, which clamps the output
// low whatever the capacitor is doing.
//
// The buzzer is wired straight to the output with no series resistor, on
// purpose: it is a 160 ohm load that needs most of the supply across it to
// sound at all. Put the usual 220 ohm "LED resistor" in front of it and the
// divider leaves 2.1 V across the element, and it goes quiet.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The bell is silent when you press the button This project
The buzzer needs most of the 5 V. Do not put a 220 ohm resistor in front of it. - It beeps all the time or never stops This project
Check that the 10k resistor from RESET to ground is in place, so the pin falls low when you let go. - The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure. - A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground. - The output does nothing or stays high. NE555 timer
Pin 8 goes to the supply, pin 1 to ground, and RESET (pin 4) must be tied high. Floating reset is the most common mistake. - The frequency is unstable or wrong. NE555 timer
Tie pin 5 to ground through a 10 nF capacitor. In astable mode f is about 1.44 divided by (R1 + 2 x R2) x C. Watch the polarity of electrolytic caps. - An electrolytic capacitor heats up, bulges or pops. Capacitor
Electrolytics have a polarity. The stripe marks the negative leg, which goes toward ground. Also keep the voltage under the rated value, such as 10 V or 16 V. - An RC circuit charges much slower or faster than expected. Capacitor
The time constant is R times C. A 10 k resistor with a 100 uF capacitor gives 1 second, and about 5 seconds to charge fully. Check that you read the value on the part right.
Change it and keep it
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