I'm working on a personal project that will count the number of people entering a room. The circuitry for this is fairly simple, but the problem I am facing involves people being counted when they leave the room. The plan is to use infrared sensors to detect a person passing through the door. What I need is for the counter to increment once every two times the sensor triggers the circuit. I have considered latches, flip-flops, counters,and a mux but cannot find any way to accomplish what I need. Have I missed something or is there something I have not considered?

 

What will your detector do when somebody enters and another person leaves at the same time?

What about people who are side-by-side?

What about skinny or very fat people?

What about very short and very tall people?

 

The doorway is not wide enough for two people to fit thru at the same time. A person's body will break the beam as they walk thru the doorway so their general body type won't matter. The accuracy of the count doesn't need to be exact anyway. I am trying to make it as close as possible. I could just set it up and divide the total by 2 at the end of the day but I want something more sophisticated.

 

Well, you really need two sensors; one more towards the inside of the room, one more towards the outside.

If the sensor on the outside gets activated first, the counter needs to increment by 1 when the inside sensor gets activated.

If the sensor on the inside gets activated first, the counter needs to decrement by 1 when the sensor on the outside gets activated.

 

What SgtWookie is describing is referred to as a quadrature sensor.

Research "up-down counter" and "quadrature decoder" for something like this:
http://forum.allaboutcircuits.com/showthread.php?t=18154
and this: http://www.electro-tech-online.com/attachments/electronic-projects-design-ideas-reviews/16815d1200843588-up-down-counter-circuit-counter.png

Ken

 

Thanx a ton. I had considered using two sensors but I had no idea they made ICs for this application.