This circuit is a simple visible light or IR detector for testing any equipment which outputs light. The circuit can be also used for measuring ambient light levels. The circuit is based on one photodiode which is used in photocell mode so that the photodiode generates a voltage and current signal when it get light.The circuit is best used when you connect it directly to the input of your oscilloscope. Then set the input to the DC inout setting.source

The circuit displays an understanding of the mechanics of displaying an analogue waveform. The timebase is simply a 555 generating a horizontal sweep, while the vertical amplifier is 3914 with a trimpot on the front. It’s extremely simple, but it works. source