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.
Drive or Fire Thyristors SCR with Pulses
Learn about Thyristors here Power Electronics
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.
This is a very simple -5V supply using one 555, useful for analog blocks using FET Opamps using low power. This circuit came up when i had to design limited by inventory. It worked well for its need. It converts Positive Five Volts to Negative Five Volts to create a dual supply.
This +5 to -5 using a 555 Astable Multivibrator.is not a high efficiency design, in fact it cannot take a heavy load. Circuits having some CMOS Opamps and a A/D convertor is ok. Even very small battery designs must avoid this circuit.
Home Made Circuits
This suits well when you want to power an analog amp which has to measure voltages which swing on either sides of zero. It can be used in a LCD based portable measuring instrument running on a rechargeable 9V battery.
Powering a strain gauge amp may be one use, another may be like a RTD temperature meter for -50 to +150 deg C.