# Two-LED Voltage Indicator Schematic Circuit Diagram

#### Versatile Indicator Beyond Basic On/Off Signals

In various applications, a simple low/high indicator isn’t always sufficient, but not necessarily requiring the precision of a digital or analog display. A prime example is a car’s battery charge level indicator. This uncomplicated circuit, needing just two LEDs (preferably a single package containing green and red LEDs), a budget-friendly CMOS IC like the 4093, and a few resistors, can serve in numerous similar scenarios. When coupled with a suitable sensor, this indicator can vividly represent the relevant quantity through colors, transitioning from red to orange, yellow, and eventually green. IC1.A acts as an oscillator, typically running at around 10 kHz with the specified component values, although the exact frequency isn’t crucial. Assuming R1 remains unspecified for now, the output of IC1.A produces a square wave with an almost 50% duty cycle.

#### Adaptable Display for Various Quantities

This circuit caters to scenarios where a simple on/off indication falls short, offering a solution between basic indicators and complex digital or analog displays. A classic instance is a car’s battery charge indicator, where precision isn’t necessary, but a more nuanced representation than a mere high or low indication is valuable. Comprising minimal components – just two LEDs (ideally housed together, featuring green and red LEDs), a cost-effective CMOS IC like the 4093, and a handful of resistors – this circuit proves versatile. When paired with a suitable sensor, it translates the measured quantity into a visual spectrum, transitioning seamlessly from red to orange, yellow, and finally green. IC1.A, functioning as an oscillator at approximately 10 kHz (though exact frequency isn’t critical), generates a square wave output with an almost 50% duty cycle, assuming the value of R1 isn’t specified yet.

#### Generating a Triangular Wave for Color Control

At the junction of R2 and C1, a nearly triangular wave is generated. Its level dictated by the difference between the two threshold voltages of the NAND Schmitt trigger gate IC1.A. IC1.B, IC1.C, and IC1.D function as inverting and noninverting buffers, ensuring complementary switching of the outputs from IC1.C and IC1.D. With a 50% duty cycle, the red and green LEDs are illuminated alternately, creating an orange-yellow hue as both LEDs glow with similar brightness. When R1 is incorporated, the input voltage to IC1.A becomes a combination of the triangular waveform and the dc input Vin. As the input voltage varies, the oscillator’s duty cycle changes, extending the duration that either the red or green LED stays lit, altering the visible color of the combined LED. The range of this effect is determined by the relative values of R1 and R2, allowing customization for various supply voltages.

#### Flexible Voltage Monitoring with LED Response

With the specified component values and an 8-volt supply. The LED transitions from entirely red to entirely green in response to input voltages ranging from 2.5 V to 5.6 V. To monitor a car battery’s voltage, the battery itself can power the circuit, provided a zener diode and a dropper resistor are added to stabilize the IC supply voltage, outlined with dashed lines in the circuit diagram. Using an 8.2 V zener, the dropper resistor should be around 220 Ω, and R1 must be reduced to 4.7 kΩ. The brightness of the LED is regulated by R4, with the formula:

R4 = (Vsupply – 2) / 3 [kΩ]

It is important to note that the 4093 can only supply a limited output current. This versatile circuit finds application in non-critical areas like simple battery testers, basic temperature indicators, water tank level indicators, and similar contexts.

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