Battery Tester Circuit with 3 LEDS

Question

I'm designing a small system that uses an ATMEGA328P and a LoRa module that runs on 3.3V using a AAA 1.5V battery. The battery is connected to a Boost Converter which steps up the 1.5V to 3.3V to power the entire system. So obviously the whole system needs to have a low power consumption to save battery power. I want to attach a push button in the circuit that displays the battery level when pressed through LEDs.

The LED indicators are as follows.

• Green LED - V_Bat > 1.4V
• Yellow LED - 0.9V < V_Bat < 1.4V
• Red LED - V_Bat < 0.9V

Here is the design of my circuit using the following components

• Voltage Regulator - TPS61221DCKT
• SCHOTTKY Diode - STPS1L30A
• Analog Comparator - TSX393IYDT
• XOR Logic Gates - 74AC86SC
• Green LED - KP-2012LSGC
• Orange LED - KPT-2012LVSECK-J4-PRV
• Red LED - KPT-2012LVSECK-J3-PRV

^{simulate this circuit – Schematic created using CircuitLab}

Here are my questions.

1. Will the circuit work?
2. Will the components I picked out worked? (I picked components that are all low power)
3. Are there any necessary resistors or passive components needed in the circuit to limit current anywhere?
4. In my voltage divider, how high can I set the resistors to reduce power consumption before it starts giving problem?
5. Any suggestions to improve the circuit?

Thanks for the help in advance and sorry for the many questions

Answer 1

It looks good but there is a problem as spotted by @Bimpelrekkie above - the comparator inputs always have volts applied, which means they will draw current and may fail as you violate the max ratings at the input.

This modification should correct those problems:

^{simulate this circuit – Schematic created using CircuitLab}

Now the circuit is completely isolated when the (double pole) switch is not pressed. The diode protects the comparator inputs, given that the two poles of the switch will not close at exactly the same time.

Answer 2

You could use the LM3914 (or similar) that is designed for this sort of application -

There are lots of examples in the datasheet

You could obviously modify an example to just have 3 LEDs.

Answer 3

I'm not sure you will get enough current to illuminate your LEDs. The comparator datasheet specifies that the output voltage will be at least 1.8V when sourcing 4mA. That same 4mA passing through 150\$\Omega\$ will drop an additional 0.6V so you may not even get your green LED to light up. This circuit might work as a one-off hack if you omit the resistors and use low current LEDs but I wouldn't use it for a mass-produced product.

I don't think you need the XOR gates. The top comparator's output would go to the green LED's anode, as you have shown. Connect the yellow LED's anode to the output of the bottom comparator and its cathode to the output of the top comparator. Connect the red LED's anode to the 3.3V supply and its cathode to the output of the bottom comparator. Suitable current limiting resistors will be needed, of course.

^{simulate this circuit – Schematic created using CircuitLab}