BJT monostable switch.

“Unexpected results”.
Hmmmm
Hmmmmmm

Can you describe what you expected and what actually happened ?

I expect 1.5 - 2 volts 10mA on the gate of the triac and that the triac turns on then off due to C1 and R4 time constant.

The Triac can ONLY be turned ON by the gate and then it will latch up.

Turn off ONLY happens when the main terminal current drops below the holding current. Read the data sheet to obtain this value.

But in your circuit as you are essentially shorting a battery, this will NEVER happen. The Triac will remain on until the battery is depleted. Which in a SPICE software, only happens when you close the simulation.

You could use a PUJT such as a 2N6027 as a pulse generator, but not a triac. The circuit arrangement is different.

I wonder what the purpose of your circuit might be.

Since you are switching a DC source, a TRIAC is almost certainly the wrong (or less optimal) part for the job.

What are you building? Tell us in detail what it is you are trying to achieve. Voltages, currents, timing relationships, external conditions - all of it.

ak