From Don't Ride Route 13
A pressure strip hidden under a seat. Someone sits down — and the device knows. Three versions: dead simple, timed, and the real setup.
No soldering. No wiring diagrams. If you can tape two things together, you can make this.
Parts — where to buy each one →
The pressure mat has two wires. The LED has two legs — one long (positive), one short (negative). Wire the mat in series with the LED and the battery. Long LED leg to battery positive (+). Short LED leg to one mat wire. Other mat wire to battery negative (−). Wrap the connections with electrical tape.
Slide the mat under a chair cushion, a doormat, a rug — anywhere someone will step or sit. Run the LED wire to wherever you want the glow to appear. When weight hits the mat, the circuit closes and the LED lights up orange. Lift the weight, it goes dark.
If you use the RP-S40-ST thin-film sensor instead of a mat, it works the same way — wire it in series. The difference: the harder someone presses, the brighter the LED glows. Light step, dim glow. Heavy step, bright glow.
A thin-film pressure sensor, same LED — but now a tiny chip reads the pressure and controls the fade. Someone sits, it glows. They stand up, it fades over five seconds. Like the warmth leaving a seat.
Parts — where to buy each one →
Hold the chip so the small notch or dot faces left.
Bottom-left is Pin 1. Top-left is Pin 8. Pins count counter-clockwise.
Use a Tiny AVR Programmer board (~$10) — plug the ATtiny85 into the socket, plug the board into USB, and upload a short program from the Arduino IDE. Six lines. One time. After that, the chip remembers.
void setup() { pinMode(0, OUTPUT); }
void loop() {
if (analogRead(1) > 100) { // pressure detected
for (int i = 255; i >= 0; i--) {
analogWrite(0, i); // fade from bright to dark
delay(20); // ~5 seconds total
}
}
}
Five connections. No soldering. Bend the resistor legs and LED legs directly onto the chip pins — they're long enough.
Tape it together with electrical tape. The battery holder has a built-in on/off switch.
Tuck the circuit behind furniture, under a shelf, or inside a gap. Run the sensor wire to wherever you're hiding it — under a cushion, doormat, or rug. The sensor is thin enough to disappear under fabric.
Doesn't light? Check LED polarity (long leg to resistor side). Check 150Ω resistor is between pin 5 and the LED, not bypassed. Check sensor wires are on pin 7 and pin 8 (VCC). Check 10kΩ pull-down is between pin 7 and pin 4 (GND). Check battery is seated. Try flipping the LED. If it triggers too easily, raise the threshold in code (e.g. 200 instead of 100). If it won't trigger, lower it (e.g. 50).
The sensor: Under a doormat. Under a chair cushion. Under a rug at the top of the stairs. Under the seat of a porch swing. Anywhere someone will step or sit without looking down. The pressure mat (V1) or thin-film sensor (V2) is thin enough — they won't feel it through a cushion.
The device: Hidden nearby. Behind a table leg, under a shelf, inside a drawer with a gap. The LED needs to face outward so the glow is visible, but the device itself should be out of sight. Run the wire along the baseboard or under the rug.
The glow should feel wrong. Not a flashlight. Not a night light. A dim orange glow that appears when someone sits down — and stays just long enough to make them wonder. If it looks like a gadget, you did it wrong. If it looks like the room just noticed you — you got it right.
Want to know what the driver was really doing with these?
Read Book 2"We saw it . . ."