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Does a L293D return the same signal on the output as it gets on the input? Or does it change a pwm signal to a rather constant voltage?

FritzB
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  • Why do you want to use PWM? The LEDs need constant power. I can't think of a reason to drive the data line through a L293D. – joan Dec 07 '17 at 10:11

2 Answers2

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Is there a particular reason you need to drive a NeoPixel strip with L293D? It is a high voltage/current H-bridge for driving highly inductive loads like a DC motor. Your application instead needs high precision in the time domain, assuming you power the strip with a dedicated DC supply (it can damage your RPi otherwise).

If you are not married to the chip but absolutely have to use an RPi, I'd suggest you grab an arduino (a nano would do just fine if you are pressed for space) and hook it up to your RPi (either as I2C backpack or usb Firmata) and save yourself a whole lot of headache, given that you have access to all the NeoPixel libraries for Arduino and don't need to worry about chip level timing.

Here is a decent how-to guide.

cvb0rg
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  • I have a 12V led strip. The data line needs atleast 8.4V = 0.7*12V. I have a dedicated dc source and I want to step up the data line to the 12V from that source. I have these l293d around here and was wondering if I can use them for that. – FritzB Dec 10 '17 at 17:30
  • Are these BCM 2835 LEDs? – cvb0rg Dec 11 '17 at 15:15
  • It's a WS2811 LED strip with 3 5050 SMDs per pixel – FritzB Dec 11 '17 at 17:54
  • Does it say in the datasheet you need 8.4V on the data line? The WS2811 has a 74AHCT125 chip which runs on 5V CMOS logic. So to drive it you only need 0-5V on the gate (DIN or data input). Problem is the digital pin of the RPi are 3.3V logic. So you either need a level shifter or as I described, use and Arduino to drive it. Even with a level shifter the WS2811 require a unique timing which is difficult to generate off the SPI clock on the Pi and the light is just going to flicker or have some random color behavior. – cvb0rg Dec 11 '17 at 23:36
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The L293D is effectively a buffer as it passes 3V logic signals to the output which will have a larger voltage swing and can pass significantly more current (up to 1 amp I think) than the few milli-amps that are available on the RPi's GPIO pins directly. So in this case whilst it will provide the 3.3V to 5.0V logic shift I think you will need you will be paying for the high-power capability (which you do not need IIRC for WS2811 strips) with propagation delays that are likely to hurt the quite tight timing requirements to drive that type of strip with say the RPi_WS281x library.

SlySven
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