Although addressable LED strips are all the rage, [Mike] of [mikeselectricstuff] has been committed to using more basic two-line strips for installation, which can be simply controlled by PWM dimming. He recently figured out a neat way to send sensor signals to these strips without adding any additional cables.

The construction uses 24 V LED tape, which consists of 6 LEDs connected in series with a forward voltage of 3V. Therefore, these strips do not start to light up until approximately 18V passes through these strips.

The LED is dimmed by adding a 15 V Zener diode and a resistor across the MOSFET. When the MOSFET PWM dimmer is in the off phase, a voltage of approximately 9 V can be applied across the LED without lighting they. With the help of a 3.3 V regulator in parallel with the LED, the PIC10F322 microcontroller and accelerometer can operate at this voltage. When the LED is on, the regulator must also be able to handle the full 24 V.

A transistor is also connected, and a 2.2 K resistor connected in parallel with the LED is switched. When turned on by the PIC, this transistor will cause approximately 10 mA of current to flow through the Zener diode and its series resistance. When the transistor is turned on and off, the voltage generated on the series resistance can be measured. In this case, the pulse width used to turn on the transistor is related to the motion detected by the accelerometer at the end of the LED strip.

Turning on the LED with a 100% duty cycle will prevent the system from working because the pulse width generated by the sensor circuit cannot be detected when the LED line remains high. However, in practice, this is not important-operating the LED with a maximum duty cycle of 98% can eliminate this problem.

This is a clever way to send the sensor signal to the two-wire LED strip, even if it does take a second to wrap your head around it. It also seems to have done a good job in adding motion response effects to related LED light bars. This is not the first LED project we have seen from [Mike]. Video after the break.

[Thanks Tim Gremalm for the tip! ]

It is no different from the addressable fire alarm device I used before. In order to be able to communicate with one or more devices (smoke detectors, alarms, sounders) on a pair of wires, the fire alarm panel sends a series of voltage pulses superimposed on the nominal 24vdc power supply. The responding device will then send a digital reply by absorbing or not absorbing current, similar to OOK.

Any example circuits that allow this 2-wire date to pass through a DC power supply?

Use a standard h-bridge motor controller, such as DRV8838. Pull the enable pin high to provide DC power to the receiver on the line or pull it low to stop providing voltage. By controlling the phase pin to modulate your signal, switch which line has power + and which line has power -.

At the receiving end, connect two wires to the AC pin of the bridge rectifier to restore the DC power supply voltage. Connect the two wires to the input end of the operational amplifier and obtain the data signal at the output end. Since this is a balanced circuit, if you use twisted-pair wiring, the op amp will also eliminate most of the induced noise on the line.

Thank you, but if I understand your description well, this is not superimposing voltage pulses on the nominal DC power supply, such as the device described by ColinM, but modulating the DC power supply, which is quite different. It seems more interesting to me to be able to modulate the signal with a constant power supply.

Common adjustable buck and boost regulators have a feedback pin that allows the output voltage to be adjusted. Usually this means setting a voltage divider at the output. When the output voltage is equal to the output you want, this will cause the feedback pin to get a specific target voltage. The adjustable power supply uses a potentiometer with two ends across the output, and the wiper is connected to the feedback pin. You can use a digital potentiometer to change the output voltage under the control of a microcontroller, but I'm not sure how fast the voltage can be changed.

It reminds me of the LED rope lights that Costco sold three years ago. The LEDs are connected in series to the rectified DC from the power supply, with only two wires. But-people can call multiple colors and patterns-but there is no control signal.

It turns out that the control box sends short pulses of no current during part of the (rectified) AC cycle, which each LED will explain. Each LED in each series string receives the signal and enters the desired mode (color and selectable pattern).

Alas, the same signal goes to all (enhanced chip) LEDs-they cannot be controlled individually, which is exactly what I want. In the multi-color mode, each LED randomly selects the color.

Reminder to yourself-When looking for transient signals (isolating transformers and careful detection), be very careful when using the power-powered O'scope on the power-derived high-voltage line. The scope survived, but a rope light control box did not.

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