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I am trying to make a subsystem for my project that gives a low battery warning when the batteries are close to dying. How do I implement this?

Many thanks

Malted Wheaties
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    Have you even searched for "low battery detection circuit"? What have you found when you looked for common solutions to implement this function? – Bimpelrekkie Apr 21 '20 at 21:02
  • These would work for a circuit with a precise power supply as a reference voltage, however the tester needs to run off the battery it is testing. Is this even possible? – Malted Wheaties Apr 21 '20 at 21:04
  • How does the title of your question relate to the actual question? You can't measure the voltage of a battery without some form of voltmeter. However, you could test the voltage so see if it is above a minimum value with a oomparator type circuit. As suggested in the previous comment, you should search online for such circuits as they are readily avaliable. – Barry Apr 21 '20 at 21:06
  • however the tester needs to run off the battery it is testing. Is this even possible? What if there was a circuit or a component that can make a stable voltage from a varying input voltage? These exist, they are called voltage regulators. Don't ask "Is that even possible?" before you have searched for and studied existing solutions. Without knowledge about circuits and electronics there are hundreds of things that sound impossible. – Bimpelrekkie Apr 22 '20 at 07:47

3 Answers3

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Using an Op Amp Comparitor, a Zener Diode rated for the Low Battery Voltage, and a voltage divider, you could output s High or Low signal when the battery voltage gets too low. How you want the actual system to trigger would depend on you. You don't have a schematic posted, so idk how your circuit operates.

Jay
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If you want something simple and your battery voltage is maybe too small for using a zener diode, you can use some bipolar transistors for turning off the an LED in case the battery voltage is insuficient.

Consider the following circuit. If the battery voltage is high enough the LED will turn on with a current set by \$R_{LED}\$. The point at which the led is turned off can be regulated through the pot \$R_3\$. circuit

EDIT #1

Here is an updated version whose threshold can be more linearly controlled via a pot too:

The value of \$R_3\$ can be roughly calculated to turn off the LED for a given battery voltage threshold \$V_{BAT,TH}\$:

$$R_3=(V_{BAT,TH}-V_Z-V_{BE})\dfrac{R_2}{V_{BE}}$$

For example:

$$R_3=(3V-2.1V-650mV)\dfrac{100k\Omega}{650mV}\approx 38k\Omega$$

circuit_2

vtolentino
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  • This looks like it could work for my appliaction. Does it need a reference voltage? – Malted Wheaties Apr 21 '20 at 21:33
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    No, it uses the battery voltage itself. You just have to set the values of $R_3$, $R_2$ and $R_{LED}$ for your supply voltage application. If your supply is large enough, you can make it a bit more precise by adding some zener diode to create a reference voltage. – vtolentino Apr 21 '20 at 21:41
  • This is working great, however the thresholds need fine-tuning. Is there an equation for this? I need the led to go off when the voltage is ANYTHING under 3v. – Malted Wheaties Apr 21 '20 at 22:15
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    I will update my answer with a formula which might roughly suffice your needs. – vtolentino Apr 21 '20 at 22:38
  • Thanks so much. Just goes to show the lengths the people of this community will go to help others :) – Malted Wheaties Apr 21 '20 at 23:10
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The easy way: use a voltage supervisor part, which outputs a signal indicating the input is above a certain preset voltage, or not. Sometimes this is called "brownout detection", but the function is the same: monitor a voltage, and give go/no go status based on a preset threshold.

Maxim, TI, ST, On Semi and others make parts that do exactly that. More here: https://www.maximintegrated.com/en/design/technical-documents/tutorials/2/279.html

hacktastical
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