The circuit shown in the figure can be used to generate an oscilloscope scan or sweep signal triggered by an external signal. The trigger point is set in potentiometer P1 while P2 is a sensitivity control that must be adjusted according to the strength of the external signal. The circuit works with signals up to 1 MHz given both the characteristics of the 555 and the operational amplifier. Note that the output capacitors for the ramp generation are selected according to the frequency range. P3 precisely adjusts the ramp up time depending on the frequency of the input signal. The diodes are commonly used as 1N4148 or equivalent and the power supply for the operational amplifier must be symmetrical.
