sawtooth synchronous to mains

7-26 elektor july/august 1980 This circuit is really meant to control a triac but it can also be put to other uses. The section around Al forms an inverting Schmitt-trigger which "squares up" the (low voltage) AC mains frequency input. This square- wave is fed to the differentiating network consisting of R5 and Cl. The non-inverting input of A2 therefore receives two pulses for each period, namely, a positive and a negative pulse. A2 is really an integrator which converts the signal into a "sawtooth" waveform. The mixer reacts to the positive as well as the negative slope of the input signal. This is made possible by the unusual internal architecture of the LM3900. The opamp reacts normally to positive pulses. As soon as the non- inverting input becomes "high", the inverting input will also have to become "high" to maintain the balance. This can only happen if the 5V 3/ sawtooth synchronous to mains output voltage is made to rise. The rise in voltage is then passed on through C2. ...