Photo Sensitive Clock Signal

[omega3387]

Well I will try to give you as much information as possible. Am am trying to build a circuit that will produce a clock signal for a counter. However this clock signal should be triggered by a change in light for a photo transistor or photo resistor. I have VTT7122 photo transistor and VT906 photo conductive cells available.

Per say:

A cup is resting on top of the photo transistor. When I lift the cup or object it will create a clock signal that will increment a counter by one. The table will be have plenty of ambient light to ensure that the photo transistor work properly.

Sorry I did not specify earlier, but i would like to make it using only IC's if possible. IF this is not possible what micros are good to work with that you have had good experience with.

Any help is greatly appreciated.

 

[ecerfoglio]

Your bigger problem will be to ensure that there will be only one count each time the cup is lifted.

Use a schmidt (spelling?) trigger between the phototransistor and the counter. Perhaps one gate from a CD 4093.

Also some low pass filtering (a small capacitor between the phototransistor's output and ground) will help.

 

[Roff]

If you use a microcontroller for your counter, software filtering could eliminate the multiple transition problem.

 

[Shyamal]

ooooooo......

 

[Noggin]

eeeeeeeeeoooooooaaaaaahhhhhh!!!!!!!!!!

 

[blueroomelectronics]

Shyamal's posts seem to be mostly random gibberish or stuck on a 1Hz 555 clock. Not sure why yet.

 

[Reloadron]

There are a number of ways to go about it. You may want to start by reading this link to gain a working idea of comparator circuits. Then scroll down the linked page and look at the two very simple photocell circuits. A comparator is just one way to get your clock pulses. If there is concern about multiple triggers when the cup is lifted or replaced add some hysteresis. The LM3911 comparator would likely be a good start.

Next to count the triggers (pulses) think about using a BCD Up/Down counter like the 74192 chip or similar. The counter will increment up for each clock pulse. See the data sheet for the 74192 which can be found here.

Since you don't mention a full count number as in 0 through 9 or 0 through 99 or whatever trips your trigger this is about as good as it gets. You also don't mention displaying the total count?

Ron

 

[DerStrom8]

I agree with Ron. A comparator is probably the easiest way to go about it. The circuit is easy to construct and takes relatively few discrete components. Here is a simple circuit for a light sensor built around an LM393 comparator (I apologize for the poor quality):



As previously suggested, you probably should find a way to make sure it doesn't trigger more than once at a time. You might be able to do this by connecting the collector and emitter of the transistor (on the output of the 393) between pin 2 (trigger) of a 555 and ground. The 555 could be wired as a 1-shot, so the output could stay on for a set amount of time before turning off. This would eliminate any extra pulses from the 393, as the output would be from the 555, instead.
If something about this is incorrect, please (anybody) let me know and I will make necessary revisions to the circuit diagram.
Hope this helps!
Der Strom