Schmitt Trigger circuit to monitor car battery voltage level

Thread Starter

Wai Khang Lim

Joined Oct 2, 2018
7
Hi, i am trying to design a circuit using STC(Schmitt Trigger Comparator) circuit to monitor the voltage level of a car battery which is 12 volts. Im trying to use two LEDs,green and red, to indicate the voltage level of the car battery. If the voltage is above 11.6v the green LED will light up, then if voltage is below 9.6v, the red LED will light up. This means that the upper threshold voltage is 11.6v and the lower threshold voltage is 9.6v . Im currently stuck with this circuit , but I was advised to use zener diode and to scale down the UTP and LTP voltage.How can i do that??
 

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ericgibbs

Joined Jan 29, 2010
10,177
hi Wai,
Welcome to AAC,
Your image looks like a LTSpice simulation, if yes, please post your LTS 'asc' file so that we can run the simulation.
E
 

dl324

Joined Mar 30, 2015
10,933
Im currently stuck with this circuit
How are you expecting to get a negative voltage to turn on D2?
but I was advised to use zener diode and to scale down the UTP and LTP voltage.How can i do that??
Without a stable voltage reference, you're going to get a voltage ratio indication, not absolute voltage.

You're using an opamp as a comparator. Why not just use comparators?
 
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dl324

Joined Mar 30, 2015
10,933
I think this circuit is easier to understand:
upload_2018-10-2_11-24-24.png
EDIT: corrected value of R4 (was 249)

The 9.6V trip is actually 9.5V. To make it 9.6, you could make R6 4.8k by putting a 49.9 ohm resistor between it and ground; and make R5 1.0k.
 
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crutschow

Joined Mar 14, 2008
25,108
The nominal open-circuit voltage of a rested, fully-charged car lead-acid battery is 12.6V.
A battery at your upper limit of 11.6V is already essentially dead (see below).
What state of battery charge are you trying to monitor?

upload_2018-10-2_13-35-10.png
 

Colin55

Joined Aug 27, 2015
519
Here are some clever, simple circuits, designed by me:
BATTERY MONITOR MkI
A very simple battery monitor can be made with a dual-colour LED and a few surrounding components. The LED produces orange when the red and green LEDs are illuminated.
The following circuit turns on the red LED below 10.5v
The orange LED illuminates between 10.5v and 11.6v.
The green LED illuminates above 11.6v


BATTERY MONITOR MkII This battery monitor circuit uses 3 separate LEDs.
The red LED turns on from 6v to below 11v.
It turns off above 11v and
The orange LED illuminates between 11v and 13v.
It turns off above 13v and
The green LED illuminates above 13v

 
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