Playground Sentinel

What it is

Playground Sentinel is a playful perimeter alarm for a garden or a kid’s room. A tiny ATtiny202 monitors a PIR sensor and, when triggered, wakes a 433 MHz OOK transmitter to send a short burst. A matching receiver counts signal edges in a 10 ms window and lights LEDs for the detected alarm bins.

Hardware at a glance

Transmitter

ATtiny202 @ 20 MHz (internal oscillator, prescaler disabled).
SEN-HC-SR501 PIR on PA4 (output pulled low internally).
FS1000A DATA on PA1, supply from the battery rail.
PA2 LED mirrors each RF burst.

Receiver

ATtiny202 + XY-MK-5V ASK module feeding PA1.
PA2 and PA3 drive LEDs for the active bands.
Other bands are defined for future use.

Power

Runs directly from a 3×AA/AAA rail.
Idle current in µA range.
linear regulators for components included.

Signaling

Every 500 ms the transmitter emits a 10 ms burst of a square wave. The carrier frequency encodes the alarm ID. The receiver starts a 10 ms window on the first edge, counts transitions, and compares the total to per-alarm ranges.

Carrier	Edges in 10 ms (ideal)	Notes
1 kHz	20	Reserved (future)
2 kHz	40	Alarm 2 (PA2)
3 kHz	60	Alarm 3 (PA3)
4 kHz	80	Reserved (future)
5–10 kHz	100–200	Reserved; practical ceiling near 10 kHz

Firmware architecture

Transmitter

TCA0 in single-slope mode with /16 prescaler.
CARRIER_FREQ_HZ (default 3000) defines burst frequency.
send_burst() aligns to TCA overflow and emits an exact-cycle 10 ms burst.
PA2 LED mirrors burst activity.

Receiver

PORTA interrupt on PA1 (both edges) counts transitions.
First edge arms a 10 ms window; counts accumulate until timeout.
Counts are compared to per-band ranges; matching bands latch LEDs for 200 ms.
Higher bands (beyond PA2/PA3) are defined but currently unused.

Build & flash

sudo apt install gcc-avr avr-libc avrdude   # once
./build.sh                                  # builds receiver + transmitter
./flash.sh transmitter                      # flash build/transmitter/transmitter.hex
./flash.sh receiver                         # flash build/receiver/receiver.hex

flash.sh requires you to name the image explicitly to avoid accidentally flashing the wrong device. Default port is /dev/ttyUSB0; override via AVR_PORT=/dev/ttyUSB1 ./flash.sh transmitter.

Repository layout

firmware/receiver, firmware/transmitter — C firmware for each ATtiny202.
build.sh, Makefile, flash.sh — helper scripts to build and flash.
pcb/PlaygroundSentinel_sender, pcb/PlaygroundSentinel_receiver — KiCad designs.
datasheet/ — PDFs and notes for ATtiny202, PIR, XY-MK, etc.
case/ — mechanical and enclosure assets.

Extending the system

Add more alarm IDs by defining additional frequency bins and matching thresholds.
Replace the fixed 500 ms cadence with PIR-triggered bursts once enclosure wiring is set.
Consider envelope modulation (e.g., double bursts) if more coding gain is needed without raising carrier frequency.