Electronics > Beginners
Making an analog bar graph voltmeter 1-10V using 10 LEDs
Beamin:
Is it possible to make a bar graph style volt meter with LEDs by attaching a transistor to each LED with the Base resistor having high and higher values to get the transistors to turn on the LEDs as the voltage rises? So the first LED would have the smallest resistor allowing the transistor to conduct at 1 volt then higher value for 2 v etc etc. Would this waste power when not in use or only use power proportional to the LEDs being lit?
Audioguru:
A single resistor biasing the base of a transistor will turn on some transistors but not other transistors, even if they have the same part number because transistors have a wide range of current gain.
Also, temperature affects transistors, heat turns them on and cold turns them off.
A transistor turns on gradually, not suddenly unless it is in a circuit that switches it on and off quickly at a certain voltage. The circuit is called a comparator.
An LM3914 IC has a voltage divider feeding 10 comparators and it is made to do what you want, making an LED voltmeter graph. Sorry, they are obsolete and are not made anymore.
sokoloff:
TI still shows them as an active and in-stock part:
http://www.ti.com/product/LM3914
Siwastaja:
LM3914, an absolute classic, works fine if you don't mind their design bug which causes one of the LEDs to be on at very low current all the time, annoyingly visible with modern high-efficiency LEDs.
Audioguru:
The LM3914 is available today in a tiny surface-mount package that can be soldered in an oven by a robot.
The datasheet says that a 10k resistor can turn off the LED that is dimly on all the time.
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