Oscilloscope-Triggered Sweep using 555 IC

We can use the 555 timer to hold the cost down of adding a triggered sweep to an economy oscilloscope. The timer is triggered by the circuit’s input op amp, adjusting its flip-flop and cutting off its discharge transistor so that capacitor C can charge. The flip-flop resets and the transistor conducts when capacitor voltage reaches the timer’s control voltage (0.33Vcc), discharging the capacitor. By substituting a constant-current source for the frequency adjust resistor (R), we can achieve greater linearity. Here’s the circuit diagram:

[Source: Philips Semiconductors Application Note]