This is a phase angle control of a Triac which is a evolution from an SCR. The 50 Hz or 60 Hz Sine wave of mains can be turned on at any point after the Zero Crossing
The Voltage ramps up in the sine wave which gives a near “Linear” slope which can be used to trigger the ON at a time delay after zero crossing when the voltage is zero. This is like a PWM but works on low frequency only. Some of the early SMPS(HV for TV and Mains Inverter) by Siemens were built around SCRs. Turning off a SCR is a difficult job for a designer, MOSFETS and IGBT are now used for PWM drive stages.
You can use it with a bulb to vary brightness of bulb, this is a live circuit it can give a shock, enclose in plastic box, the pot should have a plastic knob, use a fuse, you can also use it for temperature control of soldering irons.
The Source file in CadSoft EAGLE format is here del00004.zip
This will monitor telephone status without loading the telephone line, this way if you have two phones in parallel you will know if one of them is busy. Connect the two ends of circuit in parallel to phone lines.
D1 to D4 make a bridge so that LED’s are powered in correct polarity. LED1 indicates line ok no broken line, LED3 can light only when a 12V Zener breaksdown, this shows if line is busy or free, so that you dont go online when someone is talking on the phone in another room. LED2 lights on an incoming call ringing signal. This is a higher voltage at least in older exchanges, so the cap protects the LED, and LED Lights when a AC ringing signal occurs. D6 protects LED2 from reverse polarity.
The Source file in CadSoft EAGLE format is here del00003.zip
This circuit gives a burst of pulses to fire 2 SCRs, when pin 4 is taken to 12V the SCR is turned on, to use this circuit you need 12V short pulses phase shifted with respect to AC sine wave on bridge, like this you can control the bridge from near 0% to near 100% ON,
That way battery banks can be charged, electroplating can be done, current and voltage can be controlled with opamps, thyristors are very rugged compared to transistors and MOSFETS in that order.
Edit the circuit eagle cad file del00015.zip