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I want to use a 555 PWM signal to control a motor.

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I also want represented the speed of the motor with an LED graph using a LM3914 led driver.

enter image description here

Can I use a digital signal to control the LM3914 circuit? And if so how do connect the two?

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    \$\begingroup\$ It's difficult to use a 555 to generate 0% to 100% PWM duty cycles. It's less difficult to generate 50% to 100% (or 0% to 50%) PWM with the 555, though. What exactly do you need to achieve? What circuit are you considering with the 555, exactly? \$\endgroup\$
    – jonk
    Commented May 24, 2019 at 5:31
  • \$\begingroup\$ I'm using the exact circuit found in the link. \$\endgroup\$ Commented May 24, 2019 at 5:32
  • \$\begingroup\$ I had looked at your link but didn't readily see any discussion of how to achieve the 0% to 100% PWM with the 555 timer. I think I'll leave this to other experts. \$\endgroup\$
    – jonk
    Commented May 24, 2019 at 5:38
  • \$\begingroup\$ Thanks JRE. That helps, somewhat. \$\endgroup\$
    – jonk
    Commented May 24, 2019 at 5:53
  • \$\begingroup\$ Thanks for your help. Can you recommend other motor driver circuits that would be compatible with the analogue input of the LM3914? \$\endgroup\$ Commented May 24, 2019 at 5:54

2 Answers 2

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Personally, I'd use neither any 555 nor the LM3914 to drive an LED indicator row:

The NE555 can't reliably do the full control range (i.e. go from 0% to 100% PWM duty cycle). Also, its properties depend on analog external components which leads to high tolerances. It also needs a lot of power and a relatively high supply voltage, but that won't be of much of a concern in a motor control system.

To use the LM3914 with a PWM signal, you'd need at least an RC low pass filter, designed just right to hit the full range of the LM3914 when the PWM is at maximum duty cycle.

You could safe yourself the effort of designing that filter and tuning the NE555's circuitry by simply replacing both ICs with a single microcontroller.

Many cheap microcontrollers (e.g. Arduinos and Arduino-compatible boards) can directly sink e.g. 5 mA per output pin, sufficient to make an LED shine bright enough.

All of these can do PWM with arbitrary, 0-100% duty cycles. And, since the microcontroller would be the one defining the duty cycle, it could also trivially show the percentage on LEDs (or on a seven-segment number display, or on an LCD character display, or print it on a thermal printer, or whatever you program it to do).

Bonus: you get a microcontroller that can take control functions, e.g. "press a button, increase speed", which sounds like something you'd want.

Also: with a microcontroller, you could not only generate a PWM, you could also drive a H-Bridge, meaning that you can control the speed and the direction of a DC motor.

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LM3914 needs an analog signal. Convert the PWM to analog with a low pass filter for example.

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  • \$\begingroup\$ Hi Justme I don't see anything under example. \$\endgroup\$ Commented May 24, 2019 at 5:22

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