New user, info for tansisto

Hi everyone, I'm new and I have little experience with electronic components. I would like to activate a relay when the voltage of a led from positive +2.7 volts becomes negative. Can anyone help me with the components to use?

Not much to go on however.....

One possibility would be to get a TIP31 transistor. Most any will do but this one is heavy enough to take some beating and survive, and it was the first appropriate one I found on eBay.

  • Disconnect the LED from Ground,
  • Connect the LED to the transistor base,
  • Connect the transistor emitter to ground
  • Connect the transistor collector to the relay coil
  • Connect the other end of the relay coil to V+
  • Connect a 1N4001 or 1N4002 or 1N4003 or 1N4004 across the relay coil with the banded end (cathode) connected to V+

thomas_zagati: Hi everyone, I'm new and I have little experience with electronic components. I would like to activate a relay when the voltage of a led from positive +2.7 volts becomes negative. Can anyone help me with the components to use?

Perhaps a link to the hardware this LED is part of? Why would the voltage across an LED ever become negative?

Hi, sorry I explained myself wrongly.

The LED is connected to a hardware is a status LED of the burglar alarm unit. I can't disconnect it from the hardware to put the transistor. When the LED is off I would like to activate a relay that is triggered with gnd. I thought that: with the LED off the signal at 0 is active the relay, but even with the LED off there is always a small voltage of a few mw.

How can i do? many thanks.

Glue a light sensor on top of that LED, then you can easily detect it's on or off.

A proper burglar alarm will have lots of self checks to detect it's been tampered with, which of course should set off the alarm.