low voltage alarm circuit

Thread Starter


Joined Feb 1, 2019
Tryin to implement a very simple low voltage alarm into my current project. I came across this one and all seems fine. Just having a little bit of trouble finding the correct resistor value... and making sure I have my buzzer connected correctly

I've included the original schematic and mine. I added a buzzer to the circuit and set it to go off at 3v. the original schematic chose to change the zener diode to the value you want the alarm to go off and depending on which zener diode you use to also change the resistor value. here is what I have. in the original schematic it shows using a zener diode of 3.6v the resistor should be changed to 680ohm. I've been playing around the slightly smaller resistors was wondering if anybody could tell me the exact resistance or close to what it should be. and I don't have a buzzer on hand to test with just the LED. is my buzzer connected in the right place?

My schematic :
low voltage alarm 2.JPG

Original Schematic:
low voltage alarm.JPG


Joined Jul 10, 2017
The buzzer should be connected between the collector of Q2 and VCC if you want it to act as a low voltage alarm.
The voltage across the zener diode is what sets the alarm threshold. The resistor R2 should be selected to limit the zener current at maximum VCC to the data sheet recommended value.


Joined Jun 22, 2012
You need the buzzer in Parallel with the led supply.
For better results use a TL431 zener, with resistors 2K and 10K for 3V trigger.
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Joined Jun 4, 2014
Also ypu should be aware that there two kinds of piezo 'buzzer'.
One kind you simply supply it with a DC voltage and it buzzes. This kind will work in your circuit providing the buzzer is intended for 3V.
The other kind needs to be supplied with AC, or pulsed DC at the frequency you want it to buzz, typically they work best at 3-4kHz. This kind will not work in your circuit.
It can be difficult looking at the description of the buzzer which kind it is.

Audioguru again

Joined Oct 21, 2019
Your zener diode has no part number for you to determine how much current it needs. There are some high current sener diodes and some low current zener diodes available.


Joined Jul 10, 2017
To way the circuit works:
The switching point is determined by the difference between the supply voltage and the zener voltage. If it is greater than approximately 0.7 volts (the base to emitter voltage of Q1), current will flow between Q1 collector and emitter, which will turn off Q2. If the difference voltage is less than 0.7 volts, Q1 will be off and Q2 will be on.
To change the switching voltage in this circuit, you will have to change the zener diode.


Joined Mar 14, 2008
Better to use a TLV431 (low current version of the TL431) adjustable reference instead of a zener.
It's more stable and can be adjusted to the precise trip-point you want.

LTspice simulation of example circuit below:
It trips when the Ref voltage crosses 1.24V,


Thread Starter


Joined Feb 1, 2019
Hey guys thx for your help.

The buzzer I am using is an active piezo buzzer. They're rated for 5v but will buzz at 3v also.

The zener diode I am using is a BZX55C3V0 (3v 500mw)

I also included a new schematic with the buzzer connected properly. If you would verify please.

I have the circuit connected exactly as the original schematic I found which I posted above. The LED lights very close to my intended 3 volts. But if anyone has some suggestions I'm using a different value resistor or anything else in the circuit please let me know.

I am implementing this circuit into one of my most elaborate PCB designs. Which I'm hoping to send off to JLBPCB later today. But since the circuit works and it may just need slightly different valued resistors and or zener, as long as the active buzzer is connected to the proper spots I should be good to go to have a PCB made and can always use different guy components when I build them.

Thank you for that. I was trying to use components I already have this is so constantly buying more and more stuff LOL I sure you I know the feeling. This low voltage alarm is for a large 4kwh DIY battery. The alarm is just an extra warning on top of all the normal necessary battery management for Stuff. I've designed a PCB that uses lm3914's and 10 seg led bar graphs to make a state of charge meter. Each PCB includes 4 meters for a 4S battery and I've made it so you multiple can be connected together to work with up to a 7s battery. And I wanted to put an alarm so if any Cells Go below 3 Volts and the BMS is not working properly for some reason it will set off an alarm. So with that said it doesn't have to be super accurate it's just there as a backup. If you say that the zener diode approach to the alarm we'll be close enough to my 3-volt alarm or if it will not be, will tell me which direction to go

Thx again. Below is my new schematic with the buzzer connected properly I believe and also includes the part number of my Zener diode. As well as the zener diode datasheet.

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Thread Starter


Joined Feb 1, 2019
I also have a ton of lm358 ic's... I just came across this on Amazon which is a lm358 low voltage alarm... and thankfully it has a picture of the back of the PCB what's the traces... It even labeled the transistor. It's using a s9014 tranny... could it be replaced with a ordinary everyday transistor? ex. 2n222, 2n2907,BC327etc...

  • Type - NPN
  • Collector-Emitter Voltage: 45 V
  • Collector-Base Voltage: 50 V
  • Emitter-Base Voltage: 5 V
  • Collector Current: 0.1 A
  • Collector Dissipation - 0.4 W
  • DC Current Gain (hfe) - 60 to 1000
  • Transition Frequency - 150 MHz

If it was between my zener diode low voltage alarm for this lm358 circuit, which one would you recommend?


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sparky 1

Joined Nov 3, 2018
The ON semi conductor datasheet for TLV431 figure 29 without introducing temperature drift to the chart and good layout on copper will improve the circuit. Avoid transistor thermal error the chart from 25 to 50 degree C should be nearly flat. Otherwise the TLV431 accuracy would be compromised.