warning circuit.........plz help ?

[G-wiz]

Hello all

Hopefully this will be a very easy one for you all !

What i need is a led to light up or go out when a bulb blows !

for example if your brake light bulb blows, the led would indicate this ?

Also for a motor too. Would like a warning if a fuel pump failed !!!

Please keep circuit as simple as possible, as i'm not to clever with "chips".

Thanks in advance for any help you can offer guys !!!!!!!!

 

[ljcox]

A very simple solution would be to connect a LED and resistor in series across the switch. It will glow when the globe is intact. It will go out when you apply the brakes and go out if the globe fails.

The same would work for the fuel pump, but it will only work if the motor goes open. There may be other types of fuel pump failures that will not be detected.

 

[mstechca]

A very simple solution would be to connect a LED and resistor in series across the switch.

I assume that this is connected to power. Could you imagine connecting the switch in parallel with the battery supply? :?:

As soon as you flick it on, BOOM! there explodes your battery for forcing unlimited current from +ve to -ve. :shock:

If there was a resistor in series with the switch, the battery might not explode, but the resistor could heat up.

Think of the "globe" as a resistor. Measure the resistance with an ohmmeter or DMM, and connect it between base of an NPN transistor that can handle the current, and ground (-ve). Connect a resistor of a larger value between base and +ve. Connect collector to +ve.
Connect the emitter to the output (which will be an LED and a resistor). Don't forget to ground the other end of the output.

Now when the globe blows up, the resistance between base and ground goes to infinity, and the voltage at the base is significantly higher. This turns on the transistor, and turns on the LED.

If the globe didn't blow up, it's resistance and the resistance between base and +ve make a voltage-divider network, and the LED does not light very bright.

You need to experiment with the resistor connected to the base to get the best results.

I think the motor is a different story.

 

[tkvenki]

I was just thinking weather we could do this.

In th eschematic i have attached, The 1M resistor will not supply enough current to drive the transistor. THis will be the case when the bulb is working.

If the bulb fails, then then more current is through the 1M resistor. THis will be enough to drive the transistor. which will turn on the LED.

Can we do this??

 

[stevez]

Just some thoughts:

Generally auto systems run at about 14 volts when the engine is running. At times the voltage will drop to 11 or 12 volts - but this doesn't happen often.

A tail light bulb draws about 2 amps. With 2 amps flowing thru it a 0.75 ohm resistor has a voltage drop of 1.5 volts - enough to power an LED. If you inserted this in series the bulb would see 1.5 volts less. LED would be on, not off, when the filament is intact and powered. That may not be what you want but it is simple. It's also less than ideal in that voltage to the bulb is reduced though that may be sufficient for your purposes.

A 0.05 ohm resistor would have a voltage drop of 0.1 volts under the same conditions. It would seem that a simple transistor circuit could be devised to indicate the condition. You'd also need some simple logic - AND would be a start - because the voltage drop only means something when the light is supposed to be on. In other words - power is going to the bulbs and the 0.1 volt is not present.

Sorry this isn't complete but it's time to get some work done.

 

[Roff]

I was just thinking weather we could do this.

In th eschematic i have attached, The 1M resistor will not supply enough current to drive the transistor. THis will be the case when the bulb is working.

If the bulb fails, then then more current is through the 1M resistor. THis will be enough to drive the transistor. which will turn on the LED.

Can we do this??

No, that won't work. The battery voltage is (nearly) independent of whether the bulb filament is intact or not.

 

[Roff]

A very simple solution would be to connect a LED and resistor in series across the switch.

I assume that this is connected to power. Could you imagine connecting the switch in parallel with the battery supply? :?:

As soon as you flick it on, BOOM! there explodes your battery for forcing unlimited current from +ve to -ve. :shock:

The bulb is in series with the switch, which limits the current.
Len's idea is the best one so far, IMHO.

 

[Roff]

Hello all

Hopefully this will be a very easy one for you all !

What i need is a led to light up or go out when a bulb blows !

for example if your brake light bulb blows, the led would indicate this ?

Also for a motor too. Would like a warning if a fuel pump failed !!!

Please keep circuit as simple as possible, as i'm not to clever with "chips".

Thanks in advance for any help you can offer guys !!!!!!!!

The only use for a fuel pump failure light is as a diagnostic tool, to tell you why your engine has died. Maybe that's what you want it for?

 

[DirtyLude]

Easiest and most reliable fuel pump failure warning is a pressure switch on the fuel send line.

 

[mramos1]

ljcox has the best idea for cheap and will work.. I agree.

 

[G-wiz]

The only use for a fuel pump failure light is as a diagnostic tool, to tell you why your engine has died. Maybe that's what you want it for?

I'm making a new wiring loom for a friends modified car. It is turbocharged and is running very high boost pressure's, therefore needs a lot of fuel. It will be running 2 fuel pumps to save any melted pistons, what I want is the driver to know if one of the pumps fail.

Many thanks to all who have answered will try some experiments !

 

[G-wiz]

A very simple solution would be to connect a LED and resistor in series across the switch. It will glow when the globe is intact. It will go out when you apply the brakes and go out if the globe fails.

The same would work for the fuel pump, but it will only work if the motor goes open. There may be other types of fuel pump failures that will not be detected.

This is a real easy one for the brake lights, thanks!

The problem i can see with the pump, is when the pump is turned on (switch closed) the led goes out, now if the windings in the pump burn out and it goes open circuit the led is still out, therefore you wouldn't know it had failed !

 

[ljcox]

The problem i can see with the pump, is when the pump is turned on (switch closed) the led goes out, now if the windings in the pump burn out and it goes open circuit the led is still out, therefore you wouldn't know it had failed !

Correwct, as I said initially it is not an ideal solution for the fuel pump.

However, you wanted something simple. A circuit to detect a motor failure would be more complex.

Someone's suggestion of a pressure sensor seems to be the best solution.

 

[zevon8]

Not sure about how the 2 pumps will be connected, but if they feed a common rail or similar, you will need to be able to tell if the pressure is from one pump or 2. A simple diphram sensor may not be of much use.

What about a basic pressure guage?

 

[DirtyLude]

Just get a single 1000hp pump like an SX. Adding a second fuel pump is like buying a second lottery ticket where the grand prize is a blown engine.

Pump/regulator failures are so uncommon it's not worth it to setup and warning system for them. If you really have a need for monitoring, just get a gauge.