A simple sawtooth wave generator consists of an inverting Schmitt trigger that rapidly charges a capacitor through a diode and slowly discharges it through a resistor. If a light dependent resistor is used then the frequency will depend on how much light is hitting it. Cheap LDRs will very likely contain toxic heavy metals such as Cadmium. What if you were to replace both the resistor and capacitor with a LED? This component has a junction capacitance that will be charged when reversed biased, and when light hits it there will be a photocurrent that slowly discharges it. Using an LM358 as a Schmitt trigger, the circuit oscillates at audio frequencies if you shine a flashlight at the LED. Cadmium based light dependent resistors react very slowly to changes in light. Photodiodes react rapidly enough to hear the PWM in the flashlight's dimming circuit.