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.

  • 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.

Help with a project. (Laser Phototrigger)

Status
Not open for further replies.

Tijaiha

New Member
I am having some trouble with a circuit for a laser phototrigger (schematic courtesy of saskview.com). Here is the original schematic, and this is my current circuit. Photograph of my actual breadboard, with an overlay of the circuit to hopefully make it easier to read.

Basically the idea is to have a laser pointed at a photodiode, when the beam is broken, it triggers a brief timer which then fires the camera. When the laser is hitting the diode, there is 5V going in to pin 6, when the beam is broken, it drops to roughly 1.6ish (to fall into trigger range). I am having trouble getting the second timer to fire and briefly light the trigger LED (which would mean the voltage from the red wire would drop to zero, firing the camera).

My terminology and knowledge is pretty limited, so for that I apologize. If anyone can help troubleshoot this I'd greatly appreciate it (feel free to respond simply and as though I were a complete idiot on the subject, because.. well I am!).

Thanks in advance,
David
 
Last edited:

mneary

New Member
7805 needs 7 to 7.5 volts (6V is not nearly enough) to be a good source of 5V.
6V appears to be shorted to "5V".
If the 6V (corrected to >7V) is a few inches from the supply, it needs a bypass cap >0.22uF.
DO NOT ground unused pins of 7404 (74LS04).
There should be a "bypass" cap from 5V to 0V at every chip.
 

Tijaiha

New Member
Thanks for the reply.

The only pin on the 7404 that is grounded is the ground for the chip (pin7). I'll try upping the voltage and see how that goes.
 

ProFPGA

Banned
i agree the 6V input to 7805 is too less for it to regulate 5V o/p.
Also after you have a stable 5V dc output , disconnect trigger led and see what voltage variation it shows with or without laser on pin 9 . I would appreciate if you could post a just the schematic here for us to better understand the ckt .
 

mneary

New Member
Please consider at least editing the photo's notations so it doesn't look like you connected all the chips pins together.
 

Tijaiha

New Member
I apologize for the poor image notations. It was a fairly quick hack job just to get a fairly easy to read image up so you wouldn't have to sit and follow stuff on the breadboard. Reason for the overlay was a possible overlooking of a major mistake on my part on the actual board.

I was able to access the PDF today, so here is the schematic.
 

mneary

New Member
Here is a 'local' copy of the schematic so it won't disappear again. I've added pin names to the 556 so I can follow it.
 

Attachments

  • phototrigger.jpg
    phototrigger.jpg
    107.8 KB · Views: 348
Last edited:

mneary

New Member
OK, the circuit should work, but the LED only fires for 10 milliseconds. You could miss it easily. Try increasing R2 to 10k or higher to get a visible pulse.
 

Tijaiha

New Member
I replaced R2 temporarily with a 22k, the green light does fire briefly when I first power up the circuit. If I power down, discharge C6 and C7, then power back up, it fires again upon powering up. Still can not get it to fire when the beam is broken.

Pin 6 on the 556 is the trigger for the first timer, and from the datasheet I believe it said between 1.25 and 2.0 volts will fire the trigger, with ideal being around 1.67. Using VR1 I adjusted the voltage to around 1.67 volts (with the laser not hitting the photodiode). With the laser hitting the diode it is at 6 volts.

Could someone explain what is happening between the output of timer one, (pin5) and trigger of timer two (pin 8) and a quick explanation of R1, C6 and C7.

I've been trying to understand the circuit, but I only really grasp the concept behind it at the moment. I'm struggling to understand what the caps and resistors do. No worries if it's too much to explain it, I've been doing my best to read up and find what I can on the internet.

Appreciate the assistance from all of you! Very kind to take time aside to help an extreme novice.
 
Last edited:

mneary

New Member
Only one of C6 and C7 is necessary; in series they are 0.01/2 or 0.005 but this value is not critical. Following assumes we don' have C7:

Operation: A high to low on Tr1 (pin 6) starts timer 1 and O1 (pin 5) goes high. D1 has been holding Th1 low. At the trigger, it releases Th1 so it can charge C1,2, and 3. At the end of the time (VR2 *(C1+C2+C3)), O1 goes low again.

Here it is: However if the trigger remains low, the rising voltage on Th has no effect.

When O1 (pin 5) goes low, C6 should pull Tr2 low for about (R1*C6) = 10 microseconds. R1 pulls Tr2 back up, as the second element of the time constant. When Tr2 goes low, it should cause O2 to go high, illuminating the LED and triggering the camera.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top