Bulb faulty sensor

[Joelin]

this is my circuit. i wanna test if one of the bulbs fail. the 12v source is too high for ic's and the 0.2v coming from the transistor is too low. any ideas??

 

[dknguyen]

Won't the bulb just turn off if it's faulty? It is its own indicator.

 

[The Mad Professor]

Won't the bulb just turn off if it's faulty? It is its own indicator.

Not if you cannot see the bulb, a good example of this problem in the real world are the brake lights of an automobile.

 

[crutschow]

Here are two ways to sense a bad bulb:

1) Put a small resistor in series with each transistor source to ground. The resistor value should be selected to give about 100mV for the bulb current when the transistor is on and the bulb is conducting. You can sense this voltage with a comparater such as an LM339.

2) Sense the transistor drain voltage when the transistor and bulb are off. It should be 12V if the bulb good. You can sense this with a large resistor (say 100kΩ) in series with the base of a bipolar transistor (emitter to ground). This will switch over a mA at the transistor collector (so a 10kΩ collector resistor to 12V will go from 12V to 0V if the bulb is good when off).

 

[Externet]

Alternative, you could make it simpler by showing the opposite ; to confirm that the lamp is on.
Some Cadillacs have it, a strand of optic fiber from the bulb chamber to the dash shows it's lit, an elegant way to do it.
Or more elaborated, a photodiode in the bulb chamber can turn a led on (or off) at the dash.
The fiber can be canibalized at a wreck yard for nearly $0.
Miguel

 

[meyrickt]

Try a LED with suitable resistor across the bulb.
If the bulb is working, there should not be enough voltage drop across it to light the LED, however if the bulb has blown, then the LED will light !!

Just my 2p worth...

 

[dknguyen]

Go with the optic fiber because that's the sexiest method so far.

 

[crutschow]

Go with the optic fiber because that's the sexiest method so far.

Depends somewhat on what you want to do with the signal for a bad bulb.

 

[on1aag]

Hi Joelin,

You could detect the drain to source voltage of the mosfet.
When the mosfet is not driven, the drain to source voltage
will be close to the supply voltage.
When the mosfet is driven, the drain to source voltage will
be the product of the drain current and the Rdson of the fet.
About 833 mA * 0,16 Ω = 133 mV.
As long as the drain to source voltage of the fet (divided
by two) is well above the "reference voltage" applied to the
inverting input of the comparator, the led will not light.
However, when the bulb fails the drain to source voltage of
the fet will drop to zero and the led will light.

One remark: I would select a different, more modern mosfet
for the job because the IRF530 might not fully conduct with
only five volt drive. The bulb will light but the fet might
run hot.

on1aag.

 

[rezer]

Hi Joelin,

You could detect the drain to source voltage of the mosfet.
When the mosfet is not driven, the drain to source voltage
will be close to the supply voltage.
When the mosfet is driven, the drain to source voltage will
be the product of the drain current and the Rdson of the fet.
About 833 mA * 0,16 Ω = 133 mV.
As long as the drain to source voltage of the fet (divided
by two) is well above the "reference voltage" applied to the
inverting input of the comparator, the led will not light.
However, when the bulb fails the drain to source voltage of
the fet will drop to zero and the led will light.

One remark: I would select a different, more modern mosfet
for the job because the IRF530 might not fully conduct with
only five volt drive. The bulb will light but the fet might
run hot.

on1aag.

Yeah, the fiber optic idea is far less complicated. Besides, it's cooler.

 

[Joelin]

thanks

Hi Joelin,

You could detect the drain to source voltage of the mosfet.
When the mosfet is not driven, the drain to source voltage
will be close to the supply voltage.
When the mosfet is driven, the drain to source voltage will
be the product of the drain current and the Rdson of the fet.
About 833 mA * 0,16 Ω = 133 mV.
As long as the drain to source voltage of the fet (divided
by two) is well above the "reference voltage" applied to the
inverting input of the comparator, the led will not light.
However, when the bulb fails the drain to source voltage of
the fet will drop to zero and the led will light.

One remark: I would select a different, more modern mosfet
for the job because the IRF530 might not fully conduct with
only five volt drive. The bulb will light but the fet might
run hot.

on1aag.

thanks for the help, i wanted to use the IRL540, but unfortunately i couldn't get hold of any. i gonna try out your ideas, and see which works best. thanks

 

[Joelin]

Here are two ways to sense a bad bulb:

1) Put a small resistor in series with each transistor source to ground. The resistor value should be selected to give about 100mV for the bulb current when the transistor is on and the bulb is conducting. You can sense this voltage with a comparater such as an LM339.

2) Sense the transistor drain voltage when the transistor and bulb are off. It should be 12V if the bulb good. You can sense this with a large resistor (say 100kΩ) in series with the base of a bipolar transistor (emitter to ground). This will switch over a mA at the transistor collector (so a 10kΩ collector resistor to 12V will go from 12V to 0V if the bulb is good when off).

thanks for the help. will try it out

 

[Joelin]

Alternative, you could make it simpler by showing the opposite ; to confirm that the lamp is on.
Some Cadillacs have it, a strand of optic fiber from the bulb chamber to the dash shows it's lit, an elegant way to do it.
Or more elaborated, a photodiode in the bulb chamber can turn a led on (or off) at the dash.
The fiber can be canibalized at a wreck yard for nearly $0.
Miguel

Thank you, didnt think of it this way. it actually seems like a better idea

 

[Hero999]

Why not replace the bulb with an LED?

A 3W LED should give as much light as a 10W bulb and won't burn out.

You can buy bulbs with built in switching regulators or you can build your own (very cheap and easy) which I recommend over using a series resistor.