LED Fuel gauge project - Electronics expert required !

[affinite]

My Moody has a bar LED display in place of the usual analogue fuel gauge but the display driver gave up the ghost several years ago.
This winters project was to try and remember enough electronics knowledge to coble together a replacement.
Well I dont know how but I did manage to come up with a circuit which seems to work, at least on the test bench.
(See attached circuit diagram)

It uses an LM3914 LED display driver chip and a green LED bar display.
I'd now like to add a flashing Red LED to come on when the last LED goes out (on pin 1) ie Fuel Tank is almost Empty.
I'm sure I just need a couple of resistors and a suitable transistor but I just dont know how to do it.
Any electronic expert care to throw me a bone here ?
Thanks in advance
Steve

 

[Rigger Mortice]

The easiest way is to wire up the LM3914 in dot mode, rather than bar mode. Then wire up a red led instead of the green one at position number 1. That will also warn you that you have one tenth of a tank full rather then you are just about to run out of fuel, which is arguably more useful .
Rather than, or addition to, a red led you could also get it to sound an audible alarm.

 

[nigelmercier]

Assuming pin 1 will float to the supply voltage when the LED is off, you just need to connect this to the base of an NPN transistor via a resistor, like this:

Connect pin 1 where it says 5V, and your power rail where it says 12V. I would use a 10k base resistor, and any general purpose NPN transistor with Hfe > 150.

I've got a power cut until 16.00, so can't turn on my PC to do a proper diagram.

 

[lw395]

If you want the last LED to flash, you'll need something like the circuit above switching on a 555 timer IC to do the flashing.
Or you can get LEDs with a flashing circuit built in.

 

[nigelmercier]

...Or you can get LEDs with a flashing circuit built in.

Quite.

 

[affinite]

Thanks guys. I do want bar mode not dot and I do have a flashing LED ready to go.
I'll Nigels circuit a go at the weekend

 

[GHA]

Thanks guys. I do want bar mode not dot and I do have a flashing LED ready to go.
I'll Nigels circuit a go at the weekend

I've had a similar circuit working on breadboard but to finalise, getting the display to read in a vaguely linear fashion needed a bit of work, have a look here....
http://www.ybw.com/forums/showthread.php?383859-One-for-the-electronics-wizards

 

[nigelmercier]

Assuming pin 1 will float to the supply voltage when the LED is off ...

I've now had a glance at the data sheet, it appears that the outputs are LED CC drivers. In this case, measure the voltage on the pin with the LED on and off, you should be able to use a Zener diode in series with the base to offset the switching point.

 

[affinite]

Thanks again chaps
GHA my tank is a nice simple square box with no difficult sloping sides to consider.
My sender is a Wema and the driver circuit/display is pretty linear so all in all I'm happy with my driver - its just the icing, ie the flashing LED, that Im trying to implement.
BTW looking back at your original thread, I used a LM317 voltage regulator to provide a regulated 10v supply (not shown on my circuit diag) to the driver circuit across which I have a simple voltage divider for the LM3914 to measure.

Nigel should the Zener be slected based on the LED on or off voltage?

 

[nigelmercier]

Zener selected so it drops enough voltage to keep the transistor off when you want it off. Note that I'm not 100% sure this will work, it depends how the LM3914 regulates the output. I've not played with one of these before.