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Topic Title: Build a simple low battery indicator circuit with two NPN transistors
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Created On: 06 November 2017 09:22 AM
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 06 November 2017 09:22 AM
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Joined: 06 November 2017

Here is a design that describes a simple low battery indicator circuit by using just two inexpensive NPN transistors. The main feature of this circuit is its very low stand by current consumption.

We have so far seen how to make a low battery indicator circuits using a 741 IC and a 555 IC, which are no doubt outstanding with their abilities of detecting and indicating low battery voltage thresholds.

However the following post relates yet another similar circuit which is much cheaper and employs just a couple of NPN transistors for producing the required low battery indications.

The main advantage of the proposed two transistor low battery indicator circuit is its very low current consumption compared to the IC counterparts which consume relatively higher currents. A IC 555 would consume around 5mA, a IC741 around 3 mA, while the present circuit would just consume around 1.5mA current.

Thus the present circuit becomes more efficient especially in cases where stand by current consumption tend to become an issue, example suppose in units which depend on low current battery supplies such as a 9V PP3 battery.

Another advantage of this circuit is it's ability to work even at voltages around 1.5V which gives it a clear edge over the IC based circuits.

As shown in the following circuit diagram, the two transistors are configured as voltage sensor and inverter.

The first transistor on the left senses the threshold voltage level as per the setting of the 47K preset. As long as this transistor conducts, the second transistor on the right is held switched OFF, which also keeps the LED switched OFF.

As soon as the battery voltage falls below the set threshold level, the left transistor is no longer able to conduct.

This situation instantly triggers the right hand side transistor, switching ON the LED.

The LED switches ON and provides the required indications of a low battery warning.

The above circuit was successfully built and installed by Mr. Allan in his paranormal depletion detector unit. The following video presents the implementation results:

Upgrading the above Transistorized Low Battery circuit into a Low Battery Cut-off Circuit

Referring to the above diagram, the low battery indicator is formed by the two NPN transistors, while the additional Link RemovedBC557(datasheet) and the relay are used for cutting OFF the battery from the load when it reaches the lower threshold, in this state the relay connects the battery to the available charging input.

However when the battery is in its normal state the relay connects the battery with the load and allows the load to operate through battery power. Please tell me are there any error in this circuit?If is,please leave a comment below.Thanks for reading.
 06 November 2017 10:20 AM
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I have no idea who designed this circuit but it is pretty unsatisfactory. It will suffer with temperature instability and dependence on the transistor characteristics of particular devices. The voltage sensor should at least use a long tailed pair to make the design work properly.


 06 November 2017 03:31 PM
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Joined: 22 July 2004

That's why it needs the trimmer - occasionally really needed, but here just a sign of a poorly centred design, as the turn on voltage will vary by severla millivolts per degree, and the threshold will be perhaps 600-700mV. So drifts of >10% of threshold voltage over entirely believable temperature excursions.
I suggest better is the use of a TL 431 or equivalent, designed for the role and cheaper than a half decent transistor.
I presume Mr Allens paranormal detector is designed to the same standard and is in fact just detecting thermal drifts and possible RF pickup.
Power fail detector possibly, but please not battery levels on any thing critical

regards Mike

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