Re: Can someone explain this (simple) schematic
- From: "Anthony Fremont" <nobody@xxxxxxxxxxx>
- Date: Fri, 16 Nov 2007 14:39:28 -0600
Kasterborus wrote:
Hi,
This is a schematic I've been given for a laser detector circuit. The
laser is being pulsed at about 600Hz and the 'PD' (photodiode) is
supposed to detect reflected light from this laser:
http://www.flickr.com/photos/20929480@N03/2037052153
The last stage is a comparator - but not well labeled. I think that
the inverting input is taking a copy of the TTL 600Hz clock pulse that
is driving the laser, and using it to flip the transistor.
The diode is probably an LED (?) and this represents the output. I
think that the LED should be connected to the -5v rail.
Does this make sense? Or have I missed something.
The use of -5v is confusing, why not just +5v?
It's an op-amp thing. You can do the same thing with a single rail supply,
but you have to bias the + inputs of the op-amps to half the supply voltage.
This is done so that the output idles at a "virtual" ground and can carry an
A/C signal. By my estimation, the first two op-amp stages act as high gain
amplifiers with a high frequency roll-off characteristic.
The final stage is likely a comparator used to switch the LED on and off at
whatever rate the the PD is being modulated. The LED is connected to the
+5V rail since most common comparators have an open collector output and
can't source any current. The comparator has one input biased at some
positive voltage (with respect to circuit ground) that is adjustable. The
rectangular blocks are resistors.
Of course IANEE and I could have this all wrong, so it's worth everything
you paid for it. :-)
Or is this so that the signal being compared is inverted to the clock
pulse...?
This circuit came from a french designer, and I don't get to talk to
him often.
.
- References:
- Can someone explain this (simple) schematic
- From: Kasterborus
- Can someone explain this (simple) schematic
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