Is there a digital input that registers light? / Can only ADC pins register signals from an LDR?

Question

I have been building a traffic light over the past year as a GCSE project. Initially, what was unique about it was that, the pedestrian interface would light up when it was dark outside. Subsequently, I added a qualitative counting system for drivers so they would know how long was left but couldn't jump the lights at 1 or 2 seconds.

Now to my current dilemma. Apparently, all of the above doesn't differentiate my project enough from the current model. So I thought, what if you no longer needed to press the light button. What if you could just walk up, and an input would register the movement and start the process. I thought about having a laser pointing at an LDR and when the laser point was obstructed by a person for example, the ldr would register this and start the cycle. Unfortunately, my chip only has 1 ADC pin, so i can't use the LDR. I was wondering if there was an input that could register this obstruction of light and then transmit to the chip.

Side Questions: Can a non ADC pin register an ldr Are there such things as infra red lasers e.g. a laser point which can't be seen to the human eye Can an optoisolator replace a normal PTM switch

EDIT: I have done some research and found something called a photo transistor which by my understanding could replace a switch. Is this a viable option?

Answer 1

The LDR connected to a comparator (or opamp in comparator mode) can provide a simple make or break signal. On/Off. The below circuit is a IR Led and Photodiode Proximity sensor, and the LM358 compares the analog voltage at R2 with a Potentiometer RV1.

You could go with a better method, a digital ultrasonic sensor. It will provide a pulsed digital signal, and be much less susceptible to light reflections or anything, while having a greater distance. Bonus, it will give you a definitive distance.