Description
5 VOLTE POWER SUPPLY MODEL
Trigger
Source - http://www.electronguide.com/circuits_pages/InstantFlashTriggerForPhotography.html
The heart of my flash trigger is an LM555
timer and a DM7473 flip-flop. The 555 provides an adjustable delay
between an event being triggered and the flash being fired — for
example, we want to wait for a falling drop to fall a few more inches
before the flash goes off. The flip-flop triggers at the end of the
delay and fires our flash.
A circuit is required that would give a change in voltage when a
light beam is interrupted. A phototransistor allows current to flow only
when light is present. A large resistor between the phototransistor and
ground causes 5V to be output when light is present, but pulls the
output to ground when the beam is interrupted.
Unfortunately, there’s bound to be ambient light hitting the
phototransistor even when the LED beam is interrupted. The
phototransistor may only reduce the voltage at its output by 1V when the
beam is interrupted — we normally output 5V, and the 555 timer won’t
trigger until the voltage dips below 1.67V, so the change in voltage
won’t even be noticed. However, we can divert some of the current from
the output of the phototransistor to ground with a variable resistor —
this allows us to subtract a constant voltage from the circuit’s output.
Now, instead of GIVING 5V normally, we can divert some current to
ground until we’re only GIVING 2V. A 1V drop in voltage will reduce our
output to 1V, which is low enough to trigger the 555.
Circuit Diagram
Since our circuit will use 5V all over the place, our first task is
to turn a 9V battery into the voltage we need. Maxim’s MAX603 linear
regulator is a handy chip that will output 5V given a wide range of
input voltages — with a few supporting capacitors, the chip will do all
the work for us. I simply followed the suggested circuit in the
datasheet.
Next, I needed a circuit that would output a change in voltage
when a light beam is interrupted. for an infrared LED and matching
phototransistor. Fairchild Semiconductor has a good application note
about designing with phototransistors — essentially, a phototransistor
allows current to flow only when light is present. A large resistor
between the phototransistor and ground causes 5V to be output when light
is present, but pulls the output to ground when the beam is
interrupted.
Unfortunately, there’s bound to be ambient light hitting the
phototransistor even when the LED beam is interrupted. The
phototransistor may only reduce the voltage at its output by 1V when the
beam is interrupted — we normally output 5V, and the 555 timer won’t
trigger until the voltage dips below 1.67V, so the change in voltage
won’t even be noticed. However, we can divert some of the current from
the output of the phototransistor to ground with a variable resistor —
this allows us to subtract a constant voltage from the circuit’s output.
Now, instead of output 5V normally, we can divert some current
to ground until we’re only outputting 2V. A 1V drop in voltage will
reduce our output to 1V, which is low enough to trigger the 555.
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