I need a little help with a logic circuit that I'm hoping can be fairly simple but I'm an analog guy trapped in a digital world.

I have been asked by a friend in Australia for a modification to a camera trigger circuit that uses a photo beam sensor to make it directional. He wants to install a second sensor and only trigger the camera when something passes both sensors in one direction.

This is the sensor he is using, output can either be a NO or NC relay contact.
http://www.takex.com/images/content/security/downloads/Man_PB-10NS.pdf

The one triggering the camera is using a NO contact. When triggered it sends 12 volts DC to a simple transistor switch that triggers the camera. He wants to add a second sensor and only have the Camera sensor trigger if the added sensor is triggered first.

May be a slight time delay between when the two sensors are triggered so whatever we come up with will need to latch until the second sensor is hit. We have 12 volts DC power to work with.

Open for ideas and suggestions..

 

You could enable (arm) the camera 'trigger' with the output of a one-shot triggered by the 'sensor' input. With this arrangement a subject must first trigger the sensor THEN the camera within the TO interval of the one-shot

Best regards
HP

 

Doing what your friend wants to do isn't all that difficult adding another sensor circuit and some logic. However, here is what concerns me. The maximum distance between the Sensor Transmitter and Sensor Receiver is about 10 meters or 33 feet. My guess is the transmitter beam starts out narrow and grows like a cone with distance. Now should that be true, depending on sensor spacing, will the sensors interfere with each other and how will they interact? That could be a potential problem with the scheme of things as to determining direction of travel as the beams are broken.

Ron

 

You mean like using a 555 in a one-shot configuration. I can build the one shot with a 1 second delay. Actually I didn't say this because I didn't feel it was necessary info to solve the problem but we are already using a 555 one shot to trigger the camera for debounce. The 555 is simply turning on a 3904 to trigger the camera.

How about something like this.. Using another 555 as a one shot. No power to the 3905 except for when the first 555 is triggered.

 

Doing what your friend wants to do isn't all that difficult adding another sensor circuit and some logic. However, here is what concerns me. The maximum distance between the Sensor Transmitter and Sensor Receiver is about 10 meters or 33 feet. My guess is the transmitter beam starts out narrow and grows like a cone with distance. Now should that be true, depending on sensor spacing, will the sensors interfere with each other and how will they interact? That could be a potential problem with the scheme of things as to determining direction of travel as the beams are broken.

Ron

Thanks for the thought but he only has about 2 feet between the Trans and Rec sensors. Shouldn't be an issue..

 

I think a simple screen (e.g. blackened tube) over/around the IR emitter to collimate the beam would overcome Ron's concern.

 

I think a simple screen (e.g. blackened tube) over/around the IR emitter to collimate the beam would overcome Ron's concern.

Yes, with the close spacing between transmitter and receiver and using a small shield that shouldn't be a problem. So it's a matter of taking the relay outputs and putting together a logic circuit to determine direction of travel. That is my thinking anyway. Sound like that should work? Anyone?

<EDIT> Just saw your post #4. Yes, that should work, along those lines. </EDIT>

Ron

 

Post #4
The only caution I see is the 10k on the base of the transistor and the 570k load on the opto. You need to allow for bleed through current from the transistor by making the load require more current. With a 1 to 1 opto, the bleed through could be more than 1 ma.

As for the rest of it, I didn't say anything because it looks good.

 

Not exactly sure I understand what you are saying, when the opto is triggered it charges the cap through the 100 K. Are you saying the 100K should be smaller to increase the current? It's working great as-is with the 100K, maybe we just got lucky...

 

Depending upon the nature of the intended subject's motion, you may wish to consider a shorter interval -- thus obviating any necessity of forced (asynchronous) reset upon exposure...

Best regards
HP

 

It depends on the opto. (12V-.6V)/10k = 1.14 ma through the base of the transistor and the LED in the opto with no voltage supplied from the first timer. If the opto has a 1 to 1 current transfer ratio, that would allow up to 1.14 ma going through the 100k and 470k resistors. 12 volts won't push 1.14 ma through that resistance, so the collector of the opto would be close to zero volts while waiting. The opto output can't pulse low if it's already close to zero.

Still, the proof is in the pudding. If it works, it works.

 

Noted, in the future if this ever comes up again, I'll eliminate the 470K across the cap and tie another 100K between the Collector and 12 volts, this should still discharge the cap while increasing the off voltage on the transistor.

 

Look at this drawing for how to do a monostable trigger.

http://www.allaboutcircuits.com/vol_6/chpt_8/4.html

 

Noted, 10K resistors and a .1 uf cap.
Thanks...