Need help to modify circuit to work with scanning laser

[[email protected]]

Hi,

This circuit was designed for me a couple of years ago to detect an
interruption in a static laser beam by a photocell.

+5>---+-------------+
| |
[68k] [LDR] +---[1M]--+
| | | |
| +----+---|+\ |
| | | >--+---->OUT
[100K]<--[10K]---|--------|-/ |
| | LM324 [1.5k]
| [3.9k] |A
| | [HLMP4700]
| | |
GND>--+-------------+--------------+---->GND

[You might need to copy this to notepad to see it properly]

It worked really well, but I've since built a scanning laser that
creates 10 beams from one single source, switching the beam on and
off, and moving a galvanometer mirror very quickly.

So now the beam passing through the sensor is "pulsed" - the laser
software is set to 10,000 points per second - so you can imagine how
fast this occurs.

I suspect that I need to add a capacitor to the circuit, either at the
input, output, or the feedback section to smooth out the pulses to a
more acceptable TTL signal. I'm not sure which is best, or the value
of the cap I should use.

Any help is greatly appreciated.

Dave

 

[ehsjr]

Hi,

This circuit was designed for me a couple of years ago to detect an
interruption in a static laser beam by a photocell.

+5>---+-------------+
| |
[68k] [LDR] +---[1M]--+
| | | |
| +----+---|+\ |
| | | >--+---->OUT
[100K]<--[10K]---|--------|-/ |
| | LM324 [1.5k]
| [3.9k] |A
| | [HLMP4700]
| | |
GND>--+-------------+--------------+---->GND

[You might need to copy this to notepad to see it properly]

It worked really well, but I've since built a scanning laser that
creates 10 beams from one single source, switching the beam on and
off, and moving a galvanometer mirror very quickly.

So now the beam passing through the sensor is "pulsed" - the laser
software is set to 10,000 points per second - so you can imagine how
fast this occurs.

I suspect that I need to add a capacitor to the circuit, either at the
input, output, or the feedback section to smooth out the pulses to a
more acceptable TTL signal. I'm not sure which is best, or the value
of the cap I should use.

Any help is greatly appreciated.

Dave

Try a small electrolytic cap in parallel with the 3.9K
resistor. You didn't give the spec on how long the beam
must be interrupted before the circuit notifies you, so
the value is a guess, but try 2.2 uF. A larger value will
give a longer time before the circuit notifies you.

Ed

Ed

 

[Lord Garth]

Hi,

This circuit was designed for me a couple of years ago to detect an
interruption in a static laser beam by a photocell.

+5>---+-------------+
| |
[68k] [LDR] +---[1M]--+
| | | |
| +----+---|+\ |
| | | >--+---->OUT
[100K]<--[10K]---|--------|-/ |
| | LM324 [1.5k]
| [3.9k] |A
| | [HLMP4700]
| | |
GND>--+-------------+--------------+---->GND

[You might need to copy this to notepad to see it properly]

It worked really well, but I've since built a scanning laser that
creates 10 beams from one single source, switching the beam on and
off, and moving a galvanometer mirror very quickly.

So now the beam passing through the sensor is "pulsed" - the laser
software is set to 10,000 points per second - so you can imagine how
fast this occurs.

I suspect that I need to add a capacitor to the circuit, either at the
input, output, or the feedback section to smooth out the pulses to a
more acceptable TTL signal. I'm not sure which is best, or the value
of the cap I should use.

Any help is greatly appreciated.

Dave

A missing pulse detector can provide an output if the beam is gone
for a time longer than the scanning interruption. The basic idea is
a monostable that is reset when the beam is present but will output
a signal if it isn't reset in the correct time frame.