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I wired up this battery level indicator using a 9V battery (power supply is for ease of changing voltage.)

It works well with only D1 lit at 6 volts up to all five LEDs at 9 volts. My problem is that I need to be able to dim the LEDs with the potentiometer at the top of the diagram.

In actual practice the LEDs dim very little when I increase the resistance to the maximum. I need them to go from full brightness to fully off dependent upon the setting of the potentiometer. I am relatively new to electronics and I have no education in the field. Any thoughts?

Dim-able 9v Battery Level Indicator

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    \$\begingroup\$ Have you read the data sheet? \$\endgroup\$ – Andy aka Apr 23 '20 at 8:43
  • \$\begingroup\$ @Andyaka May be time to update that standard question :-) - even though it's often a good one. In this case it's more the subtleties addressed by the answers - ~= "It can't fly slowly enough for modern LEDS. Travesty: Maybe a resistor across each LED! \$\endgroup\$ – Russell McMahon Apr 24 '20 at 0:13
  • \$\begingroup\$ Tyler: Maybe a resistor across each LED to sink the residual current when set to low. Not a nice way to do it, but may work. \$\endgroup\$ – Russell McMahon Apr 24 '20 at 0:14
  • \$\begingroup\$ @RussellMcMahon - is there a standard question that can serve to point to these problems of not reading data sheet? How's the lock down in NZ BTW? Same as everywhere else? \$\endgroup\$ – Andy aka Apr 24 '20 at 8:22
  • \$\begingroup\$ "@Andyaka My point is that while reading the data sheet is always a good idea, in this case it is/was a matter of reading the data sheet of everything involved and then applying the deep magic of experience. The last requirement is hard to attain as a newcomer. Whereas "once upon a time" the low level current may not have done much for the LEDs of the day, modern LEDs don't know how to turn off, almost. (Like a U2 where the hardest task is to persuade them to stop flying :-) ). || NZ is, SO FAR, managing things about as well as anywhere on earth, with the very large part of the populace ... \$\endgroup\$ – Russell McMahon Apr 24 '20 at 10:11
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CURRENT PROGRAMMING

A feature not completely illustrated by the block diagram is the LED brightness control. The current drawn out of the reference voltage pin (pin 7) determines LED current. Approximately 10 times this current will be drawn through each lighted LED, and this current will be relatively constant despite supply voltage and temperature changes. Current drawn by the internal 10-resistor divider, as well as by the external current and voltage-setting divider should be included in calculating LED drive current. The ability to modulate LED brightness with time, or in proportion to input voltage and other signals can lead to a number of novel displays or ways of indicating input overvoltages, alarms, etc. Source: LM3914 datasheet page 9.

As Dave Tweed points out and the datasheet clarifies, the minimum current is determined by the internal reference network.

If you really need to dim it you could consider a PWM feed on the LED anodes (positive supply to all the LEDs).

enter image description here

Figure 1. PWM signal transitioning from high pulse width (75%) to low (25%) and back again. Note amplitude remains constant. If used, this will result in brightness control respectively giving 75% and 25% of full brightness. Image source: mine.

A web search for a 555 PWM control circuit with switching frequency set at > 100 Hz should appear flicker free.

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  • \$\begingroup\$ Good idea. Add it into your answer. The circuit is linear so it should work. \$\endgroup\$ – Transistor Apr 23 '20 at 12:46
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You can't get "fully off" with this chip. Even if you disconnected VR1 altogether, the 45 µA current through the reference network would still put about half a milliamp through the LEDs.

A second issue working against you is the fact that modern LEDs are a lot more efficient than those available back when this chip was designed — they're quite bright on half a milliamp.


You could try adding a voltage follower between the reference output and RH, which would reduce the reference load to negligible levels. You'll also need a much higher value for VR1 — 1 MΩ or more. But I don't know whether this would shut off the LEDs altogether even if you open-circuit VR1.

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