Upcycled Lighting: Extend Your Motion-Activated LED's On-Time

A couple of years ago, I bought a few motion-activated lights for my yard, hoping they would stay on long enough while I took out the garbage or watered my plants. While they provided a convenient hands-free lighting solution, they had one major flaw—the lights turned off too quickly (around 40 seconds). This often left me in the dark, especially when I had my hands full with a garbage bag.

The constant need to wave at the sensor to keep the light on became frustrating. Instead of discarding these lights and upgrading to expensive smart LEDs, I decided to save some money and modify their trigger circuits to extend their on-time.

For most motion-activated lights, this can be done by changing one resistor! So instead of throwing out your old motion lights, bring them back to life with this easy fix!

Only basic tools/electronic equipment is needed for this fix:

1. Standard motion-activated LED light
2. Multimeter
3. Pliers
4. Wire cutters
5. Tweezers
6. Soldering iron and solder
7. Replacement resistor (value calculated in Steps 4-6)
8. Small hobby knife or tool to cut adhesive (e.g., a spudger)
9. Isopropyl Alcohol (optional, makes disassembly easier)

Before you start modifying the light, it’s important to measure the current on-time of the LED and, optionally, the trigger inhibit time. These measurements will help you fine-tune the adjustments later.

1. Power up the LED: Connect the LED light to its power source and turn it on. The light will remain on for a preset amount of time before automatically shutting off.
2. Measure the On-Time: Using a stopwatch, trigger the light by moving in front of it. Start the timer as soon as the light turns on, and stop it when the light goes off. Record this on-time for reference.
3. Measure the "Trigger Inhibit" Time: The trigger inhibit time is the period after the LED turns off when the sensor does not respond to motion.
4. To measure this, continuously move your hand in front of the sensor while the light is on and immediately after it turns off.
5. Note the time from when the LED turns off to when it responds to your motion again and turns back on. This step isn’t necessary if you only want to change the on-time, but it can help you understand the system better.

Before we get started, I want to stress that it is important to be safe and respectful of mains electricity! Do not work on live circuits, regardless of how low-voltage it may seem. Many of these motion-activated LEDs use non-isolated power supplies, so they are NOT SAFE to be tinkered with while powered.

Please take the appropriate precautions and only perform this modification while the bulb is disconnected from power.

The LED has two separate electronic modules: The LED assembly and the motion sensor. To adjust the timing, we need to get access to the motion sensor.

1. Remove the Bulb: Turn off power and remove the bulb from the socket.
2. Locate the Adhesive Seal: Most motion-activated LED lights are sealed with adhesive between the bulb and the body. You’ll need to carefully cut through this adhesive to access the internal components.
3. Tool Tip: Use a hobby knife or a small prying tool to cut or lift the adhesive gently. Be cautious not to crack the plastic cover or damage internal components.
4. Pro Tip: Applying a small amount of isopropyl alcohol to the gap between the bulb and the body can loosen the adhesive, making it easier to remove.
5. Remove the Bulb: Once the adhesive has softened, pull the bulb away from the body to reveal the motion sensor and LED circuitry.
6. Caution: Be careful not to pull too hard, as internal wires still connect the motion sensor to the bulb.
7. Disconnect the Motion Sensor: The motion sensor is typically connected to the bulb using a pin header or similar connector. Gently disconnect the sensor from the bulb, then set the bulb aside.

Once you have the sensor assembly in hand, you will need to remove its casing to expose the internal PCB (Printed Circuit Board).

1. Remove the Sensor Lens: The sensor lens is usually held in place with 2–3 clips. Use a small pry tool to gently release these clips and remove the lens.
2. Remove the PCB: Similarly, 2-3 clips hold the PCB to the lower casing. Taking care not to damage the PCB, pry these away and remove the casing.
3. Note: Be gentle while handling the PIR sensor and its circuit board, as these are fragile and small components.

Most PIR sensors use a BIS0001 or similar chip. If your sensor has this chip, you are in luck!

For a detailed summary of how PIR sensors, specifically the BIS0001 control chip, I highly recommend reading this document from Adafruit:

https://cdn-learn.adafruit.com/downloads/pdf/pir-passive-infrared-proximity-motion-sensor.pdf

Key Variables to Understand:

1. Tx: Duration the output remains high (LED stays on) after being triggered.
2. Ti: Time after a successful trigger during which motion is ignored (output remains off).

The timing diagrams on the datasheet explain how these variables work. When motion is detected, the output is on for Tx seconds. After this time, the output is disabled and remains off for Ti seconds regardless of motion. After Ti seconds, the output responds to motion again.

The datasheet includes timing diagrams that show how these variables behave. For this project, we’ll focus on adjusting Tx (on-time) to extend how long the light stays on after being triggered. The relevant formula from the datasheet is:

Tx = 24576 * R10 * C6

Where R10 is a resistor and C6 is a capacitor. Our goal is to adjust R10 and C6 to change the on-time.

Now, it's time to locate the specific resistor and capacitor that control the on-time.

1. Find the Resistor and Capacitor: On the example schematic in the BIS0001 datasheet, we can see that R10 and C6 are connected to pins 3 and 4 of the chip. By following the PCB traces starting at pin 3/4, locate the corresponding resistor and capacitor (will likely not be labeled R10 and C6).
2. Tool Tip: Use a multimeter with a continuity function to find which components connect to the desired pin.
3. Measure the Resistance of R10: On my PCB, the resistors had their value printed on them, which made it easier to get their value. If your sensor does not have the value printed, use a multimeter to find its resistance. For my case, R10 was 120K ohms.
4. Determine the Capacitance of C6: Since not all multimeters can measure capacitance, use the on-time you measured in Step 1 to estimate C6’s value. For example, if the on-time was 38 seconds, you can plug that into the formula:
5. 38 = 24576 * C6 * 120000
6. C6 = 38 / (24576 * 120000)
7. C6 = 1.2885e-8 farads
8. In my case, the value of C6 was approximately 1300 pF, which is a common capacitor value.

With the values of C6 and the current R10 known, you can calculate the necessary resistance to achieve your desired on-time.

1. Desired On-Time:
2. I wanted the light to remain on for ~100 seconds. Plugging that into the equation, we solve for the new R10:
3. 100 = 24576 × 1.2885e-8 × R10
4. Solving for R10:
5. Rearrange the equation to solve for R10:
6. R10 = 100 / (24576 × 1.2885e-8)
7. R10 = 315789.47 ohms
8. In my case, I needed a resistor value of approximately 316K ohms. I chose 330K ohms because it was available.

This can be tricky as the components are small and surrounding parts are sensitive. Double-check that you are removing the correct resistor using a multimeter in continuity mode.

1. Remove the Resistor: Use a soldering iron and a blob of solder to remove the existing resistor.
2. Solder the New Resistor: Cut the leads of the new resistor to fit the solder pads and solder it onto the PCB. Ensure that the leads are not touching any other components.

If you have an oscilloscope + power supply available, it is useful to make sure the sensor is working as intended before assembling the LED. This isn't required, but is a good sanity check.

1. Reassembly: Place the motion sensor in its casing, and reassemble the LED, making sure that the sensor is connected correctly to the LED module.
2. IMPORTANT: DO NOT connect the LED to power until it is completely reassembled. Attaching scope probes/messing with the sensor wiring while connected to mains power is incredibly dangerous and can cause electric shock. Please treat mains voltage with care and stay safe!
3. Bulb Testing: Power on the LED and time how long it remains on after moving in front of it. If it is working within 1-5 seconds of your desired time (due to resistor/capacitor tolerances), your sensor is working correctly!

Congratulations! You’ve successfully modified your motion-activated LED light to stay on longer. This simple modification can make a big difference—now you won’t have to constantly retrigger the light while taking out the garbage or watering your plants!

Plus, you’ve upcycled old lights instead of throwing them away, which aligns with the contest’s focus on sustainability and reuse.

If you’re also interested in adjusting the Ti (trigger inhibit) time, the process is very similar to what we did for Tx, with a different formula and components. Another option is to change the trigger mode (pull pin 1 high on the BIS0001 or low). If you’ve managed to adjust this too, feel free to share your results in the comments!

I hope you enjoyed the project, and don’t forget to vote for this entry in the On a Budget Contest!