Low voltage alarm circuit

Thread Starter

Rich2

Joined Mar 3, 2014
172
Hi all, I got this circuit off the Internet. It is a low voltage alarm for a 12v leisure battery.
I've had it working partially, but once it lights the LED it won't turn off again if I turn the preset down again, or even if I turn the voltage up.

also if I turn the voltage up to more than 12v, suddenly lots of current flows and the chip fails. I have put a 10k resistor between pin 2 and the preset to try and protect the chip but it makes no difference. Any ideas?
I've broken about 4 chips now :rolleyes:
20200326_132104.jpg
 

AlbertHall

Joined Jun 4, 2014
10,554
Pins 2 and 6 can be at higher voltage than the chip supply, pin 8. This is a no-no. Connect the top of the pot and pin 6 to pin 8.
 

Thread Starter

Rich2

Joined Mar 3, 2014
172
Thanks, done that but now the chip only has a 2.2v power supply. The input supply is 12v but 80% is being dropped across the resistor R1.

Seems like the zenor diode is in the wrong place?

At least it's trying to work a bit now I can make the LED come on very dimly by turning the preset.
 

Thread Starter

Rich2

Joined Mar 3, 2014
172
Thanks. Done that and now the LED is bright. But once it turns on it won't turn off by upping the supply voltage.

The voltage across the zenor goes up and down with the power supply voltage, is this right? I thought they are supposed to regulate. There is roughly half the supply voltage across it.
 

AlbertHall

Joined Jun 4, 2014
10,554
Just realised you're expecting the LED to go off when the voltage goes back up. That circuit won't do that. Once the LED is on it will stay on until power is removed.
You need a different circuit.
 

Thread Starter

Rich2

Joined Mar 3, 2014
172
No it doesn't matter, as long as it comes on with falling voltage it will be fine.
I've just set the voltage at 14, then adjusted the preset until the LED goes off... But when I lower the supply voltage to 10v it doesn't come on.
 

AlbertHall

Joined Jun 4, 2014
10,554
Turn the pot so pin 2 is connected to pin 8. Set the voltage at whatever you want to turn on the LED, then slowly turn back the pot until the LED turns on. That should set it correctly.
 

Thread Starter

Rich2

Joined Mar 3, 2014
172
No it won't set. The supply voltage doesn't make any difference to the LED, if it comes on when setting up it will always be on. I set it, disconnect the supply and turn it up a few volts and reconnect, the LED still comes on.
Latest diagram...
20200326_164931.jpg
 

AlbertHall

Joined Jun 4, 2014
10,554
If P1 is set so that pin 2 is connected to pin 8, the LED should not come on regardless of the supply voltage (up to 15V anyway).
If it does then something is wrong in the circuit.
 

Thread Starter

Rich2

Joined Mar 3, 2014
172
No it doesn't come on if P2 is connected to pin8.

No it's a new chip, I'll try my last one.. ie my last new one.
 

crutschow

Joined Mar 14, 2008
25,653
Here's the LTspice simulation of a simple circuit using a TL431 programmable reference as a comparator to light an LED when the voltage is below the setpoint (2.5V as the Ref input) and turn it off above that point.

1585242599863.png
 
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