A sequential timer circuit device are used in many applications for initializing conditions during start-up or for activation of test signals in sequences such in test equipment device. The circuit diagram below shows a sequencer circuit with possible applications in many sistems.
As you can see, it uses the NE555 precision timer IC which capable of producing accurate time delays or oscillation. These timing circuits can be connected to provide such sequential control with various combinations of astable or monostable circuit connections. You can used it with or without modulation for extremely flexible waveform control.
You may see the NE555 Datasheet here (source: ti.com)