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I have a PWM based speed control circuit using an NE555 for the speed control of the DC motor. A potentiometer is used to control the speed of the motor. I am using one 9V battery.

I want to add 5 LEDs to the circuit to indicate the speed of the motor, such that when the potentiometer is at 10%, LED 1 is lit. When the potentiometer is at 20 % LED 1 and 2 should be lit, when the potentiometer is at 30% LED 1+2+3 are lit and so on until 100% of the potentiometer.

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  • \$\begingroup\$ (PP3 batteries will not last long. If you have a very small budget, try to find a 12 V regulated power supply, with maybe a 1 A or 2 A current capability. 12 V because more things work on 12 V than 9 V, and regulated because it has a more reliable voltage than unregulated. You can then use cheap buck converters to get from the 12 V to other common voltages like 5 V or 3.3 V.) \$\endgroup\$
    – Andrew Morton
    Commented Sep 6 at 17:54

2 Answers 2

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As always with electronics, there are different possible solutions. Since you're doing the PWM with an NE555, I guess you don't want to use a microcontroller just for the LED bar. We won't talk about that possible solution.

What you're looking for is an LED bar (bargraph) solution. The easiest way is probably using an analog voltage. You use an R/C-filter to make an analog voltage and use that on a cicuit that controls the LEDs.

One possibility is to use comparators. One comparator for each voltage level. You can use a specialized IC like the LM3914 instead (which I think would fit pretty neatly with your project.)

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Here's an approach using jellybean parts- one or two quad op-amps and some resistors plus a filter capacitor. An LM3914 would simplify things (they're officially 'in the process of being discontinued' by TI who acquired the original maker National Semiconductor and the recommended alternatives require an MCU, but for hobby purposes parts will be around for many years, and in any case, at least one Chinese semiconductor company is a second source). You can still find small quantities of the 5-LED bar graph drivers made by Telefunken, albeit at a fairly high price.

schematic

simulate this circuit – Schematic created using CircuitLab

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Adjust R13 to get the top LED D1 turning on where you want it to - with 5 LEDs the switching points would be 1/6,1/3,1/2,2/3,5/6. The op-amps come 4 to a package with 14 pins so it would be simpler to reduce it to 4 LEDs (maybe plus one that is on all the time) then it would switch at 20/40/60/80%. To do that just dump the top op amp, D1,R1 and replace R8 with a short.

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