How can I use a transistor as a sensor switch?

Question

I need a schematic or diagram on how I can use 1 or 2 transistors as a sensor and switch to detect when a machine's indicator light is burned out. The system is 5V, and there is limited space for this sensor inside. I can use an LED remotely to indicate the burned out bulb (control room).

Answer 1

you need only one transistor connected in series to both the source and to another LED added to this we need one photo-diod .

when the led is lightened up-the bulb is burned out-the photo diod gives a logic signal that activates the transistor and thereafter lightens the indicator.  

Answer 2

i geuss if you insist on using transistors , a simple diffrential pare can help you good. try to bios it in saturation part as a limmit

Answer 3

OK, good answers, so far.

This can be tricky.

Transistors, probably not the best option.

Do you need to sense the bulb blown when the bulb is not supposed to be lit? ie, know its bad before it is needed?

If so, more difficult.

Guidelines along the lines of the other answers.

Bulb ok means low resistance.

So when good bulb is on, 5volts across bulb & some current.

When bad bulb should be on, 5 volts but no current.

Small resistor in series, is a reasonable way to gauge current.

Some sort of threshold in the system will make it more reliable, look for schmidt trigger (get the right spelling, I can't remember)

eg bias the sensor device appropriately.

Also watch ground levels. If you plan to remotely indicate something, then if there is any significant distance between the locations, sending analogue levels can be a bad idea.  If the levels are small, the ground difference between the locations can change a little & the indicator goes off.  Similarly, if you send small unbalanced levels over distances, you'll pick up all sorts of interference, radio stations, switching transients, &c.

If you application is critical, you may need something smarter.

Consider a relay? or two?

If you can get one that goes series with the bulb, without dropping too much voltage (=> dim bulb) then the contacts can run your remote indicator.  If you use two, one parallel & one series you can sense on, but not drawing current.

Not what you wanted, but sorry, not as easy as you might like, unless you have a really easy situation. If so lets us know, you may get something simple with more details.

Answer 4

This can be complicated. The most comprehensive way is to insert a small resistor in series with the bulb, enough for a 0.1 to 0.5 volt drop. Use a differential amplifier to amplify that difference, and wire it to your remote location. There use a comparator to light a LED when the current drops below a set point, indicating the light is open.

There are simpler solutions, but they depend on how the light is wired.

Cheaper to replace the indicator with a LED.

.

Answer 5

You dont need a transistor. A simple method would be to use an led and a resistor (in series)  wired in parallel with the indicator light. The bulb blows out, the led lights up! Not knowing the voltage, there is a quick and dirty way of calculating the resistor. Use a variable pot and adjust it until your circuit works...ohm the pot out for the value and viola! Replace the pot with the proper resistance. As an industrial machine tech, we used this method lots!