You can build the alarm sequence in Charles Platt’s Make: project with either three 555 timer ICs or a Raspberry Pi Pico running MicroPython. The 555 version demonstrates how separate circuits create an exit delay, an entry delay and a continuing alarm; the Pico version expresses much of that logic in code. Both are educational breadboard projects—not certified or professionally tested home-security systems.
What the alarm sequence does
An alarm needs to do more than sound when a door opens. In Platt’s design, it first gives the user time to leave after arming. If a door or window opens later, a second delay provides time to disarm the system before the alarm stage begins. A third 555 timer handles that continuing audible alarm stage.
The Go button starts the exit delay. In the illustrated circuit, a diode prevents the sensor-trigger path from firing during that delay. Once the exit interval ends, opening an entry triggers the last-chance timer; when its interval expires, the third timer produces the alarm stage. The demonstration uses a short exit interval for testing, so the timing is an adjustable design choice, not a validated setting for a security installation.
How magnetic door and window contacts work
In the 555 version, magnetic reed contacts are wired in series. The contacts are described as normally open: with the magnet beside the contact while the opening is shut, the contact is held closed. Opening any protected door or window moves the magnet away and breaks continuity in the sensor loop. A cut wire also breaks continuity.
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That sustained break is different from pressing a push-button, which briefly connects the trigger to ground. The circuit therefore needs to turn the sensor change into a trigger pulse rather than leave the timer input in an undefined state.
How to build the 555 timer version
Understand the monostable trigger
In the monostable demonstration, pin 2 is the trigger and pin 3 is the output. Pulling pin 2 below one-third of the supply voltage makes pin 3 go high for a timed interval. Platt’s example uses a 47 kΩ resistor and a 10 µF capacitor for a pulse of about three seconds; that is an example circuit value, not a universal 555 setting.
The trigger must have a defined resting voltage. The test circuit uses a 10 kΩ pull-up to keep pin 2 near the supply voltage until the push-button pulls it to ground. As Platt puts it, “You must not allow Pin 2 to float at an indeterminate voltage, so a 10K pullup resistor keeps the pin near the voltage of the power supply until the pushbutton bypasses it with a direct connection to negative ground.”
Rank #2
- The Raspberry Pi Pico is a beginner-friendly microcontroller board that uses MicroPython to give you a taste of the Internet of Things and microcontrollers. The RP2040 is a well-designed microprocessor that can be utilized in almost any Internet of Things project. It has enough power to complete the task quickly.
- 【Raspberry Pi RP2040 Microcontroller】Raspberry Pi Pico features Dual-core ARM Cortex M0+ processor, flexible clock running up to 133 MHz. With 264KB of SRAM, and 2MB of on-board Flash memory.Supports up to 16 MB of off chip flash memory via a dedicated QSPI bus
- 【Multiple Software Support】Pico has rich and complete software support, it comes with a complete Rasberry Pi official C/C++ SDK, Micropython SDK.The programming and burning of Pico need to be carried out on the computer. Supported operating systems and computers include:Raspberry Pie with Raspberry Pi OS,Other platforms equipped with Debian based Linux system Computer with MacOS, Computers with Windows, etc.
- 【Rich Hardware Interface】Raspberry Pi Pico has 30 GPIO pins, 4 pins for analog signal input and 26 × multi-function GPIO pins, 2 × SPI, 2 × I2C, 2 × UART, 3 × 12-bit ADC, 16 × controllable PWM channels.USB 1.1 supported by host and device, The installation mode can be flexibly selected by users to facilitate welding with other development boards.
- 【Build Project in Tiny Size】Only 2.1cm*5.1cm ( as small as your thumb). Pico has been designed to use either soldered 0.1" pin-headers or can be used as a surface-mountable 'module'.
Adapt the trigger for the sensor loop
Because an opened sensor loop removes a connection instead of making a momentary button connection, the full alarm uses a pulldown and coupling capacitor to create a short trigger pulse from the sustained sensor change. Platt reports using a 47 kΩ pull-up, a 10 kΩ pulldown and a 0.47 µF coupling capacitor after checking the trigger voltage with an oscilloscope. These are the author’s values for this circuit, not general requirements for every 555 design. He describes the resulting action this way: “When a switch is opened, the negative pulse breaks through and triggers IC2.”
Wire the three timing stages
The three timers divide the sequence into exit, last-chance entry and alarm stages. Consult the project’s full schematic for component connections and reset wiring; the description alone is not a substitute for the diagram. For a breadboard build, use three 555 timer ICs and the resistors, capacitors, diodes and switches specified by that schematic. Check component package and electrical ratings against it before wiring.
Can you use a Raspberry Pi Pico instead?
Yes. The exact project’s “Raspberry Pi” option is a Raspberry Pi Pico microcontroller running MicroPython, not a Raspberry Pi computer used as a desktop. Code replaces much of the separate timer logic, while GPIO pins read the button and sensor and control indicator LEDs.
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- with pre-soldered header Raspberry Pi Pico. RP2040 microcontroller chip designed by Raspberry Pi in the United Kingdom
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- Castellated module allows soldering direct to carrier boards. USB 1.1 with device and host support. Low-power sleep and dormant modes. Drag-and-drop programming using mass storage over USB. 26 × multi-function GPIO pins.
- 2 × SPI, 2 × I2C, 2 × UART, 3 × 12-bit ADC, 16 × controllable PWM channels.Accurate clock and timer on-chip.Temperature sensor.
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The Pico example uses GPIO 8 for the Go button and GPIO 20 for the sensor input. LEDs on GPIOs 17, 16, 14 and 15 indicate sensor state, exit time, alarm triggered and alarm, respectively. Pressing Go starts a ten-second exit delay; after the sensor circuit opens, the code waits another ten seconds before turning on the alarm output. Those are literal code settings and can be changed; they are not measured performance results.
Unlike the 555 section’s normally open reed contacts held closed by magnets, the pictured Pico setup describes its sensor switches as normally closed. Do not assume the two diagrams use identical contact behavior. The code leaves the alarm behavior active until reset or power-off, so stopping it requires a reset or power cycle in the example as described.
The Pico reduces the number of timing ICs, but it adds software setup: type or download the program, upload it to the board, and troubleshoot syntax errors if they arise. Platt notes, “The components look wonderfully simple, but of course you do have to write the program, upload it, test it, track down your syntax errors, and upload it again.”
Rank #4
- New Flexible Microcontroller Board --- Raspberry Pi Pico is a tiny, fast, and versatile board. It's based on RP2040 chip, which features a dual-core Arm Cortex-M0+ processor with 264KB internal RAM and support for up to 16MB of off-chip Flash, flexible clock running up to 133 MHz.
- Multi-Function GPIO Pins---It has 26 multifunction GPIO pins, including 3 analogue inputs, 2 × UART, 2 × SPI controllers, 2 × I2C controllers, 16 × PWM channels.
- Rich Peripheral Set---A wide range of flexible I/O options includes I2C, SPI, and — uniquely —8 × Programmable I/O (PIO) state machines for custom peripheral support.
- Multiple Software Support---Raspberry Pi Pico has rich and complete software support and community resources. Programmable in C and MicroPython. Drag-and-drop programming using mass storage over USB.
- Low-power sleep and dormant modes; Accurate on-chip clock; Temperature sensor; Accelerated integer and floating-point libraries on-chip
555 timers or Pico: which should you choose?
| Choice | What it teaches | How timing and state are set | Sensor setup described | Main trade-off |
|---|---|---|---|---|
| Three 555 timers | Pull-ups, pulldowns, diode isolation, coupling capacitors and monostable behavior | By timer connections and circuit components | Normally open reed contacts held closed by magnets while openings are shut | More discrete parts and circuit debugging |
| Raspberry Pi Pico | GPIO inputs and outputs, program flow and code deployment | By explicit delay settings and program logic | The pictured/code setup is described as using normally closed switches | Fewer timing ICs, but software setup and debugging; example alarm behavior persists until reset or power-off |
Choose the 555 path if you want to see the timing and trigger circuitry directly. Choose the Pico path if you want to work with GPIO and MicroPython. The project does not establish a scientifically fair comparison of cost or reliability between the two approaches.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What this project does—and does not—establish
Make: lists the project as moderate difficulty, estimates one to three hours, and displays a $20–$30 price estimate. Those are the project page’s figures from its 2022 publication, not an independently timed build or a current price check.
The project is an educational breadboard build. Its published description does not establish certification, tamper-resistant enclosure performance, backup-power behavior, alarm-audibility compliance or reliable protection for an occupied property. Do not treat it as a substitute for an appropriately designed and installed security system.
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- Easy to Use: Just connect the board to your computer (installed IDE) with the USB cable to program it
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A separate Raspberry Pi laser-and-sound example
Raspberry Pi Official Magazine describes a different alarm project; its sensors are not part of the exact-title Pico circuit above. That build uses a reflected 650 nm laser sensor and a digital sound sensor, powered from 3.3 V. It reads the laser on GPIO 21 and sound on GPIO 14, then uses an LED on GPIO 16 and an active piezo buzzer on GPIO 25. Its logic triggers when the beam is broken or sound exceeds the module’s threshold.
That tutorial suggests placing the beam near the floor, with a nearby-wall arrangement described at up to 1.5 m, and warns not to point the laser emitter at anyone’s head. It also cautions that sensor and device voltage and current matter: a larger siren needs a separately powered switching arrangement rather than direct drive from a Pi GPIO. Suggested extensions such as PIR or camera sensing, larger lights, recorded audio, email or push alerts are proposals, not tested features of the project discussed here.
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