• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

optical 4 ch LDR object detection

Status
Not open for further replies.

earckens

Member
Hi,
a project requires detection by optical means of the presence of objects.
Design requirements:
1. any Vcc between 5Vdc and 18Vdc
2. use of LDR sensors
3. 4 channels
4. detection independent of ambient light
5. TTL HIGH level when detection occurs
6. relay activation when detection occurs, per channel
7. 4 separate channels but with possibility to gate any of channels 2, 3 or 4 in OR gate to the first detector output channel 1
8. rising edge detection time to be delayed by Time Constant 1 (LDR + C2, 4, 6, 8): when detection occurs LDR value increases
9. falling edge detection time to be delayed by Time Constant 2 (LDR + C1, 3, 5, 7): when detection stops LDR value decreases
10. LED indication of detection present, per channel
11. Time Constant 1 < Time Constant 2

I based my project on a little diagram attached here as well.

Please scrutinize my little own schematic, any feedback is greatly welcomed.
I use p-channel MOSFET because only the last few days did I learn it would be better to use n-channel and I already had a draft ready; but my inertia to follow the road of least resistance made me finish this diagram with a p-channel. Unless there are compeling reasons to change to n-channel..

Thanks for your feedback,
Erik

EDIT: some minor cosmetic changes in the Eagle schematic
EDIT2: requirement 5 changed to TTL HIGH level when detection occurs
EDIT3: improve my english in the text
 

Attachments

Last edited:

earckens

Member
I removed some serious mistakes: delay timing requires 2 comparators, so I had to reduce the schematic to 2 channels. The R6/C1 and R5/C2 give a delay of about 1 second when the 339 output goes from low to high.
optical detector 2 channel v1.png
My question: how do I add a relay output from each of the 2 channels without sacrificing the open collector output which has to be used as TTL logic level (when detect = low)?
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top