# Battery indicator connected with bicolor led

Battery indicator connected with bi-colored LED.

I need to make a system that indicates when a 3.7V battery is connected to a 5V source or not. When the battery is NOT connected a red LED stays on, when the battery is connected the red LED turns off and the green LED is on. I do not want to show a low battery or something like that, I need to show through the LEDs if the battery is ON or not, just that. Below what should be the circuit, but do not know how to make the connections to make the system work as I want.

simulate this circuit – Schematic created using CircuitLab

• You'll need to provide more information. What is the battery connected to? How will you power the red LED when no battery is connected? Dec 12, 2017 at 13:41
• I have a source that is always with 5v. My battery is 3.7 V. This power supply charges a battery. Dec 12, 2017 at 13:48
• I presume the voltage across the battery will never reach 5V? You could use a comparator to measure the +ve battery terminal connection and if the voltage goes above a threshold (say 4.7V), the red LED is turned on. Dec 12, 2017 at 13:50
• This is not a free design service. What circuits/chips/modules have you found which can do this? Why do you need us to draw a circuit for you? Dec 12, 2017 at 13:51
• I'm doing it as a hobby. Bimpelrekkie did not asked for you to draw for me, I asked who can help me. It is not your duty to help, simple Dec 12, 2017 at 15:19

I'd do it this way using two MOSFETS if it's individual or common anode bi-color LEDS.

This way the LED current comes from the 5V supply in both cases and does not drain the battery significantly other than the 3.7uA through R3.

When the battery is present M1 will be on lighting the green LED, and turning off M2. When absent, M1 will be turned off by R3, and the gate of M2 will charge up through the green LED, and will turn on illuminating the red LED.

simulate this circuit – Schematic created using CircuitLab

Common cathode would require P-Channels with a suitable turn on threshold like this.

simulate this circuit

• +1 for the current consideration! (and for noticing the bi-colour LED part!)
– MCG
Dec 12, 2017 at 15:27

You could use a MOSFET as a switch to do something like this:

simulate this circuit – Schematic created using CircuitLab

So essentially, when the circuit is powered up, Q1 is on (it is a P-Channel MOSFET, the default symbol in this editor is a bit dodgy!) and so LED1 is on. When you connect the battery, it pulls the gate high, turns off the FET, which turns off LED1 and turns LED2 on. You may need to add a bit of extra circuitry, but the general idea is there and this should be a good starting platform to something a bit more ideal.

NOTE:

This circuit is not perfect, depending on your voltage supplies etc this may not work 100%, however, the basic idea is there and this is intended to serve as a general idea for the OP to work from.

• +1 I like the simplicity of this one, other than the current drain on the battery. Dec 12, 2017 at 15:15
• Hmm.. also no worky for a bi-color common whatever LED Dec 12, 2017 at 15:22
• Thanks, I admit I did not see the bi-colour part of it, I only saw his (part) schematic showing 2 separate LEDs so went off that! I wanted to keep this simple, to allow the OP to use it as a starting point and expand/improve from it
– MCG
Dec 12, 2017 at 15:24
• ya neither did I.... I am trying to figure out a more typical common cathode version now. Dec 12, 2017 at 15:26