LM393 with NE555

Thread Starter

slevesque

Joined Jan 31, 2023
43
Hello guys,

I am designing a small circuit which will compare a Voltage from a 24V battery with a reference Voltage. If the battery voltage is low (<24V) the comparator (LM393) will output a current which will activate a NE555 to make a LED blink (Duty cycle of 25%).

So I was able to make the circuit for the LM393 which seems to work fine on LTSpice. I was also able to make the circuit for the NE555 with a duty cycle of 25% which also seems to work fine.

My concern here is I am trying to use the "Reset" pin of the NE555 with the comparator and that just doesn't work on LtSpice.

From what I understand if the "Reset" pin is High, the NE555 will be activated. I have connected the "Reset" pin of the NE555 with the output of the LM393 with a pull-up resistor. The thing here is no matter what I do with the "Reset" pin, the NE555 seems to just work constantly it never stop. Even if I don't connect the "Reset" pin to nothing.

I have attached my schematic. The zener diode is a 12V.

1675352748835.png
 

Alec_t

Joined Sep 17, 2013
14,390
Please post your asc file.
The schematic doesn't show the comparator output being used to control the 555, nor does it show any input to the RST pin of the 555..
 

dl324

Joined Mar 30, 2015
17,027
I am designing a small circuit which will compare a Voltage from a 24V battery with a reference Voltage. If the battery voltage is low (<24V) the comparator (LM393) will output a current which will activate a NE555 to make a LED blink (Duty cycle of 25%).
Have you read the datasheet for NE555? It won't tolerate 24V.

Do you know how to design this circuit yourself? Using LTspice to simulate a timer and comparator seems a bit much to me. The comparator symbol is horrible for conveying circuit intent. The timer isn't much better. The output should be on the right side and the inputs on the left.
 

crutschow

Joined Mar 14, 2008
34,722
The thing here is no matter what I do with the "Reset" pin, the NE555 seems to just work constantly it never stop. Even if I don't connect the "Reset" pin to nothing.
Leaving the "Reset" pin open is that same as connecting it to the high voltage.
It needs to be brought below about 0.5V to reset.

You need to look at the part's data sheets, since you are apparently designing blindly. :rolleyes:

The output of an LM393 can only sink a max of 4mA, not the 23mA from R2 (did you look at its output voltage?).
Increase R2 to ≥20kΩ.

Also the max 555 operating voltage is 18V, which you are exceeding by at least 6V from the battery.
You can use a regulator (e.g. LM317) to reduce the voltage to the circuits.
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
Have you read the datasheet for NE555? It won't tolerate 24V.

Do you know how to design this circuit yourself? Using LTspice to simulate a timer and comparator seems a bit much to me. The comparator symbol is horrible for conveying circuit intent. The timer isn't much better. The output should be on the right side and the inputs on the left.
Sir I use the library that came from Ti and LtSpice. I haven't modify the assembly.
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
Leaving the "Reset" pin open is that same as connecting it to the high voltage.
It needs to be brought below about 0.5V to reset.

You need to look at the part's data sheets, since you are apparently designing blindly. :rolleyes:

The output of an LM393 can only sink a max of 4mA, not the 23mA from R2 (did you look at its output voltage?).
Increase R2 to ≥20kΩ.

Also the max 555 operating voltage is 18V, which you are exceeding by at least 6V from the battery.
You can use a regulator (e.g. LM317) to reduce the voltage to the circuits.
Thanks, I will take a look at the LM317. I am not sure how modifiying the value of R2 will impact the sinking current of the LM393. Did you mean R7?
 

dl324

Joined Mar 30, 2015
17,027
Sir I use the library that came from Ti and LtSpice. I haven't modify the assembly.
How can the comparator portion be working correctly? You have the inputs wired backwards.

You should adjust the voltage divider values to account for the actual zener voltage.
 

dl324

Joined Mar 30, 2015
17,027
I am not sure to understand why would it be too much to simulate this circuit.
The circuit is so simple. You'll become more competent if you learn how to design it yourself; instead of slinging components around in a simulator until you find something that works.

If you use larger timing resistors, you can use a smaller capacitor.
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
How can the comparator portion be working correctly? You have the inputs wired backwards.

You should adjust the voltage divider values to account for the actual zener voltage.
What I was thinking is dividing the Voltage of the battery in half and compare it with 12V. So if the battery is <24V, the comparator will output current. Did I did something wrong? As form what I understand if Vin- is higher than Vin+ it will output current?
 

dl324

Joined Mar 30, 2015
17,027
What I was thinking is dividing the Voltage of the battery in half and compare it with 12V. So if the battery is <24V, the comparator will output current. Did I did something wrong? As form what I understand if Vin- is higher than Vin+ it will output current?
The zener voltage in an actual circuit won't be 12.0V. The tightest tolerance is 5%, so voltage would be 11.4-12.6V.

We think of the comparator output in terms of voltage, not current. If the non-inverting input is below the inverting input, the output will be low. That would disable the timer.

What is the battery chemistry for the 24V battery? What is the tolerance for detecting low battery voltage?
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
The zener voltage in an actual circuit won't be 12.0V. The tightest tolerance is 5%, so voltage would be 11.4-12.6V.

We think of the comparator output in terms of voltage, not current. If the non-inverting input is below the inverting input, the output will be low. That would disable the timer.

What is the battery chemistry for the 24V battery? What is the tolerance for detecting low battery voltage?
The battery is a lead-acid 24V battery.

Tolerance of detection should be 24V ± 0.2V

I should take the tolerance of the zener diode to design correctly my voltage divider. Sorry if I appear to be a noob, this is my first circuit conception.
 

WBahn

Joined Mar 31, 2012
30,256
The battery is a lead-acid 24V battery.

Tolerance of detection should be 24V ± 0.2V

I should take the tolerance of the zener diode to design correctly my voltage divider. Sorry if I appear to be a noob, this is my first circuit conception.
You'll also want to consider the tolerances of your resistors. That part should be pretty easy, but only if you don't ignore it.

Instead of using a zener, look at getting a precision voltage reference. They don't cost much more. If you do go with a zener, you'll want to pick something closer to 5 V as those are more stable as the temperature varies.
 

LowQCab

Joined Nov 6, 2012
4,228
A 24-Volt Lead-Acid-Battery will be permanently damaged
if it stays at 24-Volts for more than a few days.

A 24-Volt Lead-Acid-Battery needs to be "Maintained" continuously
above ~27.2-Volts to insure a long Service-Life.
Do You have a Maintenance-Charger ?
.
.
.
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
That's kind of low. What is the purpose for detecting "low" voltage? What is the load on the batteries and amp hour capacity?
To put you in context, the battery is installed in an aircraft. The main purpose of the battery is to run a system that drain 20A peak for 20min before the aircraft engine is turn on. Maybe 24V is way too low indeed. The purpose of the circuit is to tell the pilot if they need to charge the battery before starting the system or if they can start the system and there will be no problem for 20min only using battery power. When the engine aircraft is turn on, the system battery will be recharge by the alternator aircraft.

I think I should reconsider the reference, it should be higher at maybe 25.5V. The battery is a 24Ah.
 
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