Hi,
recently i faced a problem with rectifier/regulator on my motorbike and long story short I'd like to be informed on what is happening with voltage to prevent any failures on the road.
I definitely don't want to put large digital voltmeter - there a tons of them on aliexpress.
I want to make 1 bi-color led which will tell me is voltage in proper range or not. And use as little components as possible since i want to mount this inside a dashboard (and drill the only hole for the led).

I came up with a schematics attached. Left part is TL431 voltage monitor circuit from datasheet, right part is something that came into my mind.
I have few questions regarding this
1) when i built this on breadboard I realized it works differently than in isis simulation.
At first voltages on U2 cathode are much lower, why could it be so? i had to play with resistor values to make it work (actual values are already on a scheme)
2) U1/U2 voltage dividers are calculated by datasheet and shall give me 13...15 volts range, however in real life this came to smaller range of 13.4-14.7v. any ideas?
3) i realized i'm out of two-pin bi-color LEDs, any ideas how to replace it with common-cathode LED?
4) do i need to put any kind of filter on input from battery?
5) any ideas how to minimize number of components? for example i tried to replace R6+Q1 with mosfet, however this didn't work well due to low range of U2 cathode voltages (it changes from 1 to 2 volts approx depending on battery voltage). Actually the whole R5+R6+Q1 sink looks a bit unstable to me. but this is just a thought.

I'm not a guru of electronics, so any advice will be fantastic.
Thanks a lot.

 

If you really want to know what the state of the charging system/battery on your motorbike is, you should fit a centre-zero ammeter. If you just monitor the voltage, by the time you realise there is a problem, it will probably be too late.

 

If you just monitor the voltage, by the time you realise there is a problem, it will probably be too late.

Good point, however I believe that parts are durable enough to withstand some extra load, but not for too long. And voltage monitor is very simple tool

 

As for circuit itself - any ideas how to switch to common-cathode bi-color led with minimum components?

 

You could try something like this :-

 

As for circuit itself - any ideas how to switch to common-cathode bi-color led with minimum components?

Here, one dual op amp (8-pins chip) can do the job. I assume your LED has only two leads and you need to reverse voltage through the device.

If you have three leads and common cathode, use the circuit above (not this one).

 

Thanks for ideas guys!

 

The single bi/tri colour led voltmeter is certainly compact and I like compactness on my motorbike projects
(as well as rugged.)
I have reservations about a single led though,
not about the circuit, but the info the I the rider gets.
Mostly, all I need to know is if the charging voltage is in the 14V range
as my bike regulates around 14-14.2V.

Mostly, if I have headlamps, heated grips and the cooling fan is working this will obviously drop a
bit which is normal and no cause for concern.
this why I like a bit of 'sweep' rather than thresholds
I make expanded scale voltmeters using the LM3914 bar graph IC
and 7 leds
2 yellow for low, 3 green for acceptable range with the middle one set for the sweet spot
and 2 red for over voltage.
It's simple, I can see trends with this and not panic unless there's a significant move out of range.
It isn't as compact as a single led of course but I potted the first one in a 35mm film canister
and later made PCBs for the IC, components and 3mm leds which were about 30mm square
so it isnt bulky and when potted will work under water
range and configuration can be set to suit the user and 3914 circuits abound on the net