Two Wire Sensors on LED Strips

While addressable LED strips are all the rage, [Mike] from [mikeselectricstuff] has been working on an installation using the more basic two-wire strips that are simply controlled via PWM dimming. He’s recently figured out a tidy way to send sensor signals down these strips without adding any additional cabling.

The circuit in question.

The build uses 24 V LED tape, which consists of gangs of 6 LEDs in series with a forward voltage of 3V. Thus, these strips don’t even begin to light until approximately 18V is across them.

By adding a 15 V Zener diode and a resistor across the MOSFET which dims the LEDs, a voltage of around 9 V can be put across the LEDs without lighting them up when the MOSFET PWM dimmer is in its off phase. A PIC10F322 microcontroller and an accelerometer can then be run from this voltage, with the aid of a 3.3 V regulator wired in parallel with the LEDs. The regulator must also be able to handle the full 24 V when the LEDs are switched on.

A transistor is also wired up, switching a 2.2 K resistor in parallel with the LEDs. When turned on by the PIC, this transistor causes roughly a 10 mA current to flow through the Zener diode and its series resistor. The voltage developed across that series resistor can be measured as the transistor is turned on and off. In this case, the pulse width used to turn that transistor on is relative to motion detected by the accelerometer on the end of the LED strip.

Turning the LEDs on at 100% duty cycle prevents the system working, as the pulse widths generated by the sensor circuit can’t be detected when the LED line is held high all the time. However, in practice, it matters not — running the LEDs at a maximum 98% duty cycle eliminates the issue.

It’s an ingenious way to send sensor signals down a two-wire LED strip, even if it does take a second to wrap one’s head around it. It also seems to do a great job of adding motion-reactive effects to the LED strips in question. It’s not the first LED project we’ve seen from [Mike], either. Video after the break.

[Thanks to Tim Gremalm for the tip!]