The schmitt trigger circuit is built around a single LM741 op-amp, its output buffered by a transistor, which in turn energizes a relay. The relay may be replaced by a red LED if desired. The LM741 may be replaced by a LF411 or other JFET input op-amp with better slewing characteristics.
An ideal op-amp would have infinite open loop gain, be able to swing its output to the full supply rails and have infinite slewing rate, bandwidth etc. In practise op-amps get swing their outputs to within 0.5V of the supply rail but have finite open loop