In this instructable we will be making a simple voltage range indicator. i designed the circuit to show voltage levels from 3v to 30v in eight steps, you can increase the number of steps or the voltage or both depending on your need.

Step 1: Components Required

1. Zener diodes (of various values. i took zeners of values 1.2v, 3.3v, 5.1v, 7.2v, 10.2v, 13.2v, 22v and 27.3v)

2. LEDs of any colour

3. resistors ( 12Kohm )

Step 2: A Little Knowledge About Zener Diodes

A normal diode acts as a short circuit (very low resistance) in forward bias condition while in reverse bias condition, it acts as a open circuit ( very high resistance). a zener diode also acts as a short circuit in forward bias condition but in reverse bias condition when the voltage across the zener diode crosses its break down voltage then current starts flowing through the zener (i.e it acts as low resistance path in reverse bias).

Step 3: Single Led Voltage Indicator

consider the voltage level at which we need the led to light is v1 and the led on voltage is v2 then the zener diode to be selected for this voltage level is = v1-v2

example:

*voltage level to be indicated is 5v

*led forward voltages for various leds are :

red led- 1.7v

green,yellow led -1.8v to 2v

white or blue led -2.8v to 3v

lets take red led so forward voltage is 1.7v

*zener diode to be considered is = 5v-1.7v = 3.3v

now connect the zener diode to led and a series resistor of 12Kohm to limit the current going to the led. now this led will light up when the voltage across this setup crosses 5v.

Step 4: Multiple Steps

Now add many such modules in parallel using various zener values such that each module will indicate a different voltage value starting from lowest to the highest voltage. i added 8 steps of 3v(1.2v zener), 5v(3.3v zener), 7v(5.1v zener), 9v(7.2v zener), 12v(10.2v zener) ,15v (13.2v zener), 24v (22v zener), 30v (24v+3.3v zener). you can also add a series resistor of 330ohm to this whole setup to drop the current more.

Step 5: Solder, Casing and Caliberation

The setup can be soldered on a pcb and this can be covered with paper tape or cardboard to provide protection to the circuit and also to diffuse the led light. the voltage levels can be calliberated using a multimeter and these voltage levels can be marked on the voltage indicator.

Step 6: Applications

This voltage indicator can be permanently fixed to a variable power supply(like i did in this picture) to monitor the voltage or it can be used as a portable voltmeter.