Modifying to change relay to a Triac

Hi

I found a BT138 (I'm sure there are better) to play around with. I don't know what I am looking at. It would appear as if the required gate voltage is between 0.7 and 1.5V max whilst the latching current is around 40mA.

If the gate current is to be limited to 40mA then I was hoping that the 330R with the LED would do this. The LED will hopefully illuminate when the Triac is conducting. Currently the 2N3906 is allowing a 12V signal to switch the relay. If the switching voltage for the BT138 is a max of 1.5V how will I reduce the voltage accordingly.

I have attached the original and working schematic with the modified schematic below, am I on the right track or am I going to kill myself?

Cheers
Andrew

I see a problem in the modified circuit: the GND of the analog circuit should be connected to the A1 pin of the triac while now the two circuits are electrically insulated. But by doing this connection, the analog circuit will be directly connected to power line so be very careful.

A better solution, keeping the analog circuit insulated from power lines, should be to use a MOC optoinsulator designed to trigger triacs: look at the datasheets and examples on Fairchild Semiconductor site.

Yes, the change makes the entire circuit live - you MUST use an opto-coupler (preferably a zero-crossing one) to drive the TRIAC.

The bottom circuit won't work because the 0V of the DC and AC side are isolated.

Nigel's right, you need an opto-coupler.

Another option is a pulse transformer but for that, you'll need an oscillator or a zero crossing circuit.