# Schmitt Trigger circuit to monitor car battery voltage level

#### Wai Khang Lim

Joined Oct 2, 2018
9
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??

#### Attachments

• 83 KB Views: 22

#### ericgibbs

Joined Jan 29, 2010
12,264
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

#### Wai Khang Lim

Joined Oct 2, 2018
9
Here is it

#### Attachments

• 1.7 KB Views: 7

#### dl324

Joined Mar 30, 2015
12,681
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?

Last edited:

#### dl324

Joined Mar 30, 2015
12,681
I think this circuit is easier to understand:

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.

Last edited:

#### crutschow

Joined Mar 14, 2008
26,775
The nominal open-circuit voltage of a rested, fully-charged car lead-acid battery is 12.6V.
What state of battery charge are you trying to monitor?

#### 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

#### Wai Khang Lim

Joined Oct 2, 2018
9
So this is the circuit now but im not sure why the Vout of the circuit is not a square wave. I tried scaling the voltage by the ratio of 2.1

#### Attachments

• 26.2 KB Views: 14
• 2.1 KB Views: 4

#### crutschow

Joined Mar 14, 2008
26,775
My simulation results:
Why did you simulate for only 5ms when the battery ramp is 2 seconds?

#### eetech00

Joined Jun 8, 2013
2,299
Hi

There are two LM358's in a package. You could do something like this: