Voltage level indicator

[markm6164]

Hi everyone.

I am looking for a circuit to help diagnose a vehicle problem. I have a vehicle module that's supply voltage is dropping to 6.25 volts. This supply voltage is supplied from another module. I want to monitor the supply voltage to both modules and see if only my first module loses power or the supplying module loses its power too. The only problem is it is very intermittent and usually goes a month inbetween faulting. I want to attach two LEDs to either light up or go out when the voltage drops below 8v? So I can see if both modules lose voltage or just the one. I have searched the Internet but found there are so many ways to do it. Can anyone suggest a nice easy circuit to achieve what I want?

Many thanks.

[Seekonk]

A TL431 would be a precise way to do it.  A two resistor voltage divider to the reference pin.  LED & resistor to 12V for indicator. Schematic with every data sheet.

[therew]

Was looking for ideas on the simplest way to indicate that a 12V Lead Acid battery has reached a low level, somewhere between 12.2V -12.4V. In other words, a low voltage dectect circuit. Any help?

[uncle_bob]

Hi everyone.

I am looking for a circuit to help diagnose a vehicle problem. I have a vehicle module that's supply voltage is dropping to 6.25 volts. This supply voltage is supplied from another module. I want to monitor the supply voltage to both modules and see if only my first module loses power or the supplying module loses its power too. The only problem is it is very intermittent and usually goes a month inbetween faulting. I want to attach two LEDs to either light up or go out when the voltage drops below 8v? So I can see if both modules lose voltage or just the one. I have searched the Internet but found there are so many ways to do it. Can anyone suggest a nice easy circuit to achieve what I want?

Many thanks.

Hi

Do you want the circuit to latch or simply indicate? A circuit that indicates might blink for 1 second each month and you would miss it. A circuit that latches would tell you "since you last looked, the voltage dropped".  An even more fancy approach would be a data logger that records "at 8:43:13AM on Feb 21 the voltage dropped for 0.73 seconds". A lot depends on what you expect to cause the outage and how much data you have from other sources.

Bob

[digsys]

To be serious, there's nothing better than a ~15-20V analogue meter ! Just 2 wires, no electronics, no nothing AND you can see EXACTLY
what's happening. Digital, or even bar graph meters don't come close in these type of situations.

[Ian.M]

OTOH, for a vehicle application, a data logger makes a lot more sense as if the voltage drop anomaly happens on the road, there is a very high chance the driver wont be able to take any readings desired safely, and if in heavy traffic may miss the event totally.   One can also log multiple other signals to help localise the fault in fewer events.

Depending on one's embedded programming and general electronics experience, either an off-the-shelf logger or an Arduino based one are possibilities.  The Arduino option is easier to customise but much harder to get up and running with basic functionality in the first place.

[markm6164]

I just want two led's wired to each power feed so I can let the driver have the vehicle back and when it faults he can tell me which led went out or even better which led lit up. They aren't loosing power it's  dropping to 6.25v so I need something to signal the low voltage. The TL431 seems a good solution. Can I use it to turn the led's on when the voltage drops instead of turning them off when the voltage drops?

[uncle_bob]

I just want two led's wired to each power feed so I can let the driver have the vehicle back and when it faults he can tell me which led went out or even better which led lit up. They aren't loosing power it's  dropping to 6.25v so I need something to signal the low voltage. The TL431 seems a good solution. Can I use it to turn the led's on when the voltage drops instead of turning them off when the voltage drops?

Hi

Have the TL431 drive a two transistor inverter if you need the LED to go out. Hook the LED to the collector of the second transistor.

Bob

[Seekonk]

OR just have a LM339 do the inverting and driving.   Add two more LED for latching indicators. 431 can still be the reference.

That is what I like about the $2, 328 minis.  Add two LED and you are done, including breadboard.  Tell you what happened first and for how long.  I've been ruined for life.

[Zero999]

Only one transistor is needed.

[Ian.M]

Von for a TL431 in comparator mode is about 2V and red LED Vf is typically 1.8V or greater so that should work nicely. 

I've been trying to think of a way to make it latch easily, and I believe a similar circuit with a 2N5061G thyristor in place of the transistor could be made to work.  It would need a RC network between the TL431 cathode and the thyristor gate to delay latching slightly so it could get past the initial powerup without latching.   Obviously it would need to be fed from the ignition switch accessory terminal so that the thyristor would be reset after use.  R4 would  need to be decreased to 470R to provide a minimum 10mA holding current.  It would be preferable to use a LED with a Vf>2V to minimise the chance of false triggering due to the TL431 not holding the gate low enough.   Worst case, it may need a diode in series with the gate to drop a little more voltage,