I have a couple of questions about this burglar alarm circuit

[nullpoint]

This is a burglar-alarm circuit. There should always be a lightsource pointed to the LDR, and if the line of light is broken, the buzzer goes off.
I have a couple of questions. How does the alarm stay on after the lightsource is back, and how does it reset after the switch is pressed?

[Andy Watson]

T1 and T2 form a bistable.  When T1 conducts, the voltage on its collector is low (and the buzzer sounds), T2 has no bias. When the switch is closed, T1 has no bias and stops conducting, it's collector voltage rises, cause in T2 to conduct. The voltage on T2's collector is now low - removing the bias for T1.

[Delta]

Is that an old symbol for NPN BJTs, or something weird and unique to a manufacturer?

[Andy Watson]

When you first point a lightsource towards the LDR, and turn the circuit on, why does T2 conduct first?
What stops T1 to conduct before T2?

I assume that you turn it on with SW1 closed. If SW1 is open, it could power-up in either state - it would be a matter of luck. It might be possible to force the state by bridging LED1 with a resistor - causing T2 to turn on first.

Where did the circuit come from? Those are very old style transistor symbols!

[Richard Head]

I remember seeing those transistor symbols in the old Salora TV sets from the 70's. I think it may be a Scandanavian convention.

[nullpoint]

Thanks!
The circuit is from a relatively new exercise book. I've never seen those symbols before either.

[nullpoint]

I remember seeing those transistor symbols in the old Salora TV sets from the 70's. I think it may be a Scandanavian convention.

They are actually Scandinavian!