I am using the current circuit to time a basic traffic light that changes every twenty seconds: enter image description here

However, I've been trying to figure out a way to add a way to detect a car stopping at the light, so that it changes to green faster, I've been thinking about adding a photo diode right where the potentiometer is in the diagram, with an emitter diode right in front of it, however this did no good.
What do you recommend?

  • \$\begingroup\$ Is this homework? \$\endgroup\$
    – tcrosley
    Jun 29, 2015 at 2:16
  • 1
    \$\begingroup\$ What's an emitter diode? \$\endgroup\$ Jun 29, 2015 at 9:00
  • \$\begingroup\$ @NickJohnson: I guess OP meant LED(?) by emitter diode, as he plans to place it right in front of a Photodiode. \$\endgroup\$
    – WedaPashi
    Jun 29, 2015 at 11:57

2 Answers 2


I believe traffic lights commonly use an inductive loop embedded in the road as a metal detector or an infra-red sensor to be triggered by the heat of the engines.

Perhaps come up with something that emulates either of those.


Put a cadmium sulfide photocell in parallel with potentiometer P. You may need to add a resistor in series with the photocell if the system runs too fast when the light is on.

Arrange it so that the photocell is pretty much in the dark and only gets directly lit when your "vehicle" is at the "traffic light." Adjust the potentiometer for the period you want then there's no vehicle. When you shine a light on the photocell, the timer will run faster.

This will, however, speed up all the lights. So, it could potentially make the green light turn red faster. That is, however, common to any scheme that changes the oscillator frequency.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.