Low voltage alarm circuit

AlbertHall

Joined Jun 4, 2014
12,345
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.
Does that work with a red LED? Does the '431 pull the voltage down low enough?
 

AlbertHall

Joined Jun 4, 2014
12,345
No it doesn't come on if P2 is connected to pin8.
OK. Then supply the voltage at which you want the alarm. Turn P1 until the LED comes on. Remove the supply voltage. Connect 14V (don't turn it up to 14V, just connect it.
If the LED comes on then you will need a capacitor between pin 2 and pin 8, value is a bit of a guess but try 100nF for a start.
 

crutschow

Joined Mar 14, 2008
34,285
The 555 has a TRIG (Set) threshold (pin 2) of 1/3 Vcc and a THRS (Reset) threshold (pin 6) of 2/3 Vcc, so the only way I see to get it to reset when it goes some above 12V is to have a separate pot for the TRIG and THRS inputs.
 

Thread Starter

Rich2

Joined Mar 3, 2014
254
OK. Then supply the voltage at which you want the alarm. Turn P1 until the LED comes on. Remove the supply voltage. Connect 14V (don't turn it up to 14V, just connect it.
If the LED comes on then you will need a capacitor between pin 2 and pin 8, value is a bit of a guess but try 100nF for a start.
Tried a 100n as suggested, now the LED comes on very dimly before coming on properly, apart from that, it's working the same as before.
 

Thread Starter

Rich2

Joined Mar 3, 2014
254
The 555 has a TRIG (Set) threshold (pin 2) of 1/3 Vcc and a THRS (Reset) threshold (pin 6) of 2/3 Vcc, so the only way I see to get it to reset when it goes some above 12V is to have a separate pot for the TRIG and THRS inputs.
Are you suggesting another 100k preset on pin 6 (so it would be above p1 on the diagram)?
20200326_164931.jpg
 

AlbertHall

Joined Jun 4, 2014
12,345
I think it would be better to start again with one of the two circuits above. We could be messing about with this for a long time.
 

crutschow

Joined Mar 14, 2008
34,285
Here's an LTspice simulation of the 555 circuit, modified with two pots.
U3 determines the LED turn-on threshold, and U3 determines the LED turn-off threshold.
For the pot wiper settings shown, the turn-on is nominally 12V and the turn-off is nominally 12.5V.

Note that I had to greatly lower R1 to get sufficient current for the circuit to operate properly.
Also note that the pots go to the battery, not the 555 supply voltage, otherwise they can't sense the change in battery voltage.

1585245434914.png
 
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Thread Starter

Rich2

Joined Mar 3, 2014
254
Here's an LTspice simulation of the 555 circuit, modified with two pots.
U3 determines the LED turn-on threshold, and U3 determines the LED turn-off threshold.
For the pot settings shown, the turn-on is nominally 12V and the turn-off is nominally 12.5V.
Note that I had to greatly lower R1 to get sufficient current for the circuit to operate properly.
Also note that the pots go to the battery, not the 555 supply voltage, otherwise they can't sense the change in battery voltage.

View attachment 202461
Is that Zenor diode the same one as I'm using? 9.1v?
 

Thread Starter

Rich2

Joined Mar 3, 2014
254
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.

View attachment 202458
I've ordered some Tl431a that 555 circuit was getting too complicated with 2 batteries involved... The nearest resistors I've got are 12k and 3.3k. Are they near enough?
Thanks
 

Thread Starter

Rich2

Joined Mar 3, 2014
254
It does in simulation (which is no guarantee of course).
But, depending on the LED forward drop, you may have to add a diode or two in series with the LED.
It works! Had to put one diode in series with the LED to make it go off fully.
Thanks all.
 
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