Flashing LED Battery Low Indicator

The post explains a simple low/normal battery voltage status indicator circuit through a flashing and a constant LED, where the flashing LED indicates a normal status while the solid indicator warns against a low battery condition. The idea was requested by Mr.Alf.

Technical Specifications

Hi, I found and built your “Low Battery Indicator Circuit Using Two Transistors Only
On your website because I am growing vegetables in a raised bed and have erected 12v electric fence to prevent slugs and snails eating my plants.

for this I am using an old car battery which is holding a good charge.

The low battery indicator circuit is working fine but I have to put my meter across the terminals in order to check it is still working.

Is there any way to add a second LED that will flash to indicate power is there and working?

 The Design

The figure below shows the circuit configuration of the proposed battery voltage status indicator circuit through a flashing LED indicator.

The left side stage which uses two BJTs is a simple low battery indicator circuit, the red LED lights up as soon as the supply voltage falls below the predetermined threshold level, as set up by the 47K preset.

This stage has been elaborately explained HERE, you may want to go through it for further details.

Low Battery Indicator with Astable LED

The low battery stage produces only a low voltage indication and the relevant LED shines only until the level dips below the set threshold, this might well keep the user guessing if the actual or the normal condition of the battery was OK and if the battery was functioning correctly in the meantime.

For satisfying the above results, a flashing LED indicator could be seen attached with the low battery section, using our old compatriot IC 555.

The IC 555 is wired in its conventional astable mode so that the connected green LED produces a flashing effect as long as its reset pin#4 stays above a certain positive voltage level via the red LED and the series 10k resistor.

Until the set lower threshold is not reached, the above pin of the IC is allowed to receive the specified amount of positive potential sustaining the green LED in a blinking mode, which in turn indicates that everything is well with the battery.

The moment the red LED starts glowing and gets illuminated sharply the green LED completely shuts down, making the conditions clear to the user regarding the battery which may be assumed to be fully down and  gone below the dangerous low voltage mark.

The flashing rate of the 555 stage could be adjusted as per user preference by adjusting the 1M pot appropriately.

Solving the Low Supply Issue

For lower voltage applications, the IC 555 stage may not function correctly since its minimum operating voltage is 4.5V, under such conditions the following configuration could be tried.

Here we find a third BJT being added to the existing stage, which holds a multicolor RGB flashing LED for the required indications (the same may be replaced with a simple LED if the flashing effect is not required).

The results obtained are identical, the left LED illuminates when a low voltage is detected, until this happens the flasher LED keeps blinking indicating a normal operation of the battery.

Improving the Circuit Outcome

The above circuit has an issue and may not work as intended, because the left LED would be always ON due to the conduction via the right hand side BC547 emitter.

The following circuit corrects the above issue and may be used for implementing the proposed flashing LED battery status indication flawlessly.

Upgrading the Circuit with an Opamp

If a single LED is intended to be used for indicating the above effects, the following design may be tried. The idea is designed and explained by Mr. Abu-Hafss.

The circuit presented here features a single LED which:

Simulation and Working

a) when light up constantly would indicate the power has been switched ON and the battery charge level is good.
b) when flashes would indicate battery is low

The design is pretty straight forward, consisting of two parts. In the green part, op-amp 741 configured as comparator, along with its corresponding components compares the voltage with the reference voltage preset using a zener diode. If the voltage is higher than the threshold level, the output of 741 remains low which causes the PNP Q1 to conduct hence, the LED is powered on constantly.

As soon as the voltage level falls below the threshold level the output of 741 goes high.  This causes the Q1 to stop conducting and the LED goes off. At the same time, the high output of 741 also switches on the flasher circuit in blue part (built around a pair of an NPN and a PNP transistor) which causes the same LED to flash. The flashing rate can be adjusted by varying the values of R8 and/or C1.

Alternatively, to make the flashing circuit more compact, this circuit https://www.homemadecircuitsprojects.com/2011/12/single-transistor-led-flasher.html may be tried.

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I build the above mentioned circuit having 2 led and 3 bc547 and connected a 12v battery but the problem is that the left led is always glowing and the flasher led only blinks when i adjust thr 47k preset. What mistake am i doing??
Swagatam said…
use a variable power supply instead of battery and apply 11V to the circuit, now adjust the 47K preset until the left LED glows brightly.

Now slowly increase the voltage to some higher point may be to 13V and see the response of the LEDs, you'll find the left LED shutting off and the right LED beginning to flash.
I have built a 0-12v regulated voltage supply with lm317 and 12v lead acid battery. When i apply 11v to the indicator circuit the left led is glowing constantly and adjusting the 47k prest has no effect on the led. I have replaced the flashing led with a normal led but its not glowing but after i adjust the 47k preset it glows but the left led als glows with it.
Swagatam said…
Something's not correct in your circuit or in the adjustment procedure, first make the following design successfully, and then you can proceed with the flashing LED stage.

Swagatam said…
OK, I understood the fault, the left LED is finding its conduction path through the flashing LED transistor's emitter.

I'll try to find a remedy for this and update the new diagram soon...please keep it pending until then.
Swagatam said…
I have updated the corrected diagram, please do the shown modifications in your circuit accordingly.
Thanks sir....
I will do the modification.
If i add a 12v zener diode in place of 1n4148 fiode will it work??
Swagatam said…
No, you can use 1N4007 or any other similar diode.
brett said…
can I swap the (24003 and 2n4001) for either (BC547 or BC556) or (BD140 or BD139) or a 555 timer?
Swagatam said…
please compare the datasheet values of the devices for getting the correct info....;look for the V, I specifications in the datasheet....
Unknown said…
is there a way to make these circuits handle 18 volts input instead of 15 volts? thank you.

Keep up the good work
Swagatam said…
18V can be also used for the above circuits, except the IC 555 which will need a 7812 regulator at its pin8/4 for the a 12V regulated supply
Hi Swagatam,

Thank you for sharing all your circuits.

In my caravan I have added an extra 12V car battery to supply the caravan with power when not on a campsite.
Is there a way to show when the battery is being charged either by the car or a battery charger?

Best regards,
Swagatam said…
Thanks Henriks, you can add a small value resistor in series with the battery positive, and add two LEDs in parallel to this resistor. Connect the LEDs with their polarities in the opposite directions, so that one LeD illuminates whenever the battery is charging and the other when the battery is being used by the load. you can put a diode in series with each LEDs to improve their polarity response

the values of the resistor could be calculated with the following formula

R = 3 x 10/battery AH
Hi Swagatam,

Thank you for your fast answer. I will try this out soon.
I have a question regarding power supply.

I have built a touch sensitive circuit with a pot to adjust the sensitivity.
Connected to my lab power supply set to 12V it works very well. I can adjust the sensitivity so that I don’t need to touch the touch plate in order to make it react. Even at lower voltage (5V) it works well.
When I try the same circuit with 9V battery I have to touch the touch plate to make the circuit react.

My lab PSU shows that the circuit uses 10 mA

Can you explain me why?

Thank you in advance,
Swagatam said…
Thanks Henrik,

Any mains operated power supply will have some ripple content in the line, may be this ripple content is making your circuit a little unstable, and which in turn could be causing it to be more prone to stray fields....whereas a battery voltage may be extremely constant with absolutely no ripple, and this could be allowing the circuit to be more stable and calm, and unresponsive to weak triggers....it's my interpretation, not sure whether it's correct or not.

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