We have already learned a number of LM3915 projects such as vibration meter, 10 step battery charger circuit etc, yet one more circuit using the same chip is discussed below wherein it's configured as a 10 step battery voltage monitor circuit.
The circuit allows you to visualize precisely what voltage your battery has at any particular instance while it's being charged.
The LM3915 is basically a 10 stage dot/bar mode LED driver circuit which provides a sequential 10 step LED display corresponding to the varying voltage levels set at its signal input pinout#5.
This input can be set with any voltage level right from 1 to 35V for acquiring a correspondingly sequencing readout of the voltages fed on that pin.
In the proposed circuit of a 10 step battery charging indicator we assume the battery to be a 12V which is to be monitored, the circuit functioning may be understood as follows for the aforesaid condition:
The transistor at the right end is configured as an emitter follower replicating a high current, constant voltage zener diode, fixed at 3V.
This is required so that the LEDs are restricted from drawing excessive current, unnecessarily making the IC warm.
The battery voltage is also fed to pin#5 via a voltage divider network made from a 10K resistor and a 10K preset.
The outputs of the IC are all connected with 10 individual LEds for producing the required 10 step indications. The color of the LEDs can be as per your preference.
How to Set up the above explained battery status indicator Circuit.
It's pretty simple.
Apply the full-charge voltage level across the point indicated "to battery positive" and ground.
Now adjust the preset such that the last LED just illuminates at that voltage level.
Done! Your circuit is all set now.
For calibrating, simply divide the above mentioned full charge level with 10.
For the present case, let's assume the full charge level to be 15V, then 15/10 = 1.5V, meaning each LED would stand for an increment of 1.5V. For example with the 8th LED just ON would indicate 1.5 x 7 = 10.5V, 8th LED = 12V, 9th LED = 13.5V and so on.
Similarly, the circuit can be used with any battery and just needs to be set as per the above guidelines for achieving the proposed 10 step battery level monitoring.