Optical Pulse Generator


This little aid was
originally designed to test the Shutter Time Meter. This meter was
specifically designed for ‘analogue’ SLR
cameras. In order to measure the exposure time of a camera accurately,
it will first have to be checked with a well-defined signal first. This
circuit was designed for that purpose. But the circuit can also be used
if you need a well-defined pulse for some other purpose. The circuit is
build around a trio of standard logic ICs. Firstly a 74HC4060 (IC1) is
used to provide a quartz crystal accurate reference for the duration of
the pulses. For the crystal frequency we choose the common 4.096 MHz
value.

Picture of the Project

To test all the ranges of the shutter time meter, we choose three
different pulse lengths in three different decades, namely: 1 / 2 / 4 /
10 / 20 / 40 / 100 / 200 / 400 ms. With jumper J1 you select a frequency
of 1000, 500 or 250 Hz (see table). The frequency is then passed on to
J2 and the dual decade counter IC2 (a 4518). This does not need to be a
fast HC-type, since the frequency is at most 1 kHz. With J2 the
frequency can be reduced by 1, 10 or 100 times. This frequency is then
applied to IC3 (a 5-stage Johnson-counter). This has been set up in such
a way that in the end there appears only one single pulse at the
output.

Circuit diagram:

Optical Pulse Generator Circuit

Optical Pulse Generator Circuit Diagram

The advantage of the Johnson-counter is that each output is free
from glitches and has a duration that is exactly equal to the period of
the clock input. We choose Q2 as the output. Q4 is used to stop the
counter. Q0 is only active if we push the reset-button S1. IC3 will then
start to count. To ensure that the reset does not affect the duration
of the pulse, a differentiating RC-network R4/C3 generates a short reset
pulse. R3 ensures that C4 is discharged after releasing S1. Also, just
to be sure, we don’t use the second counter output but use the third one
instead. For the same reason, to stop the counter we use the fifth
output.

Parts and PCB layout:

Especially with longer times you will notice that the pulse will
arrive at the output a short time after pressing the switch. R5 drives a
current of nearly 20 mA through D1. D1 provides sufficient light for
this application to trigger the receiver diode in the shutter time
meter. An unusually fast type was selected for the LED,
which, with a switching time of 40 ns, has practically no influence on
the length of the pulse. If you would like to use another LED then you will have to look closely at the switching time.

{image5}

This needs to be small compared to the duration of the pulse. If you
want to use the circuit with a logic level output then you can just
omit D1. If necessary, the pulse lengths can be changed be selecting
another crystal frequency. The current consumption in the idle state is
less than 2 mA. In our prototype, while the circuit is delivering a
pulse, the current consumption increases briefly to about 18 mA. Do not
forget the wire link under IC2 when assembling the circuit.

Resistors
R1 = 1k
R2,R3 = 1M
R4 = 10k
R5 = 180
Capacitors
C1,C2 = 33pF
C3 = 10nF
C4,C5,C6 = 100nF ceramic, lead pitch 5mm
Semiconductors
D1 = HSDL-4230
IC1 = 74HC4060
IC2 = 4518
IC3 = 74HC4017
Miscellaneous
S1 = pushbutton, make contact, 6mm
J1,J2 = 3-way pinheader with jumper
X1 = 4.096MHz quartz crystal 1 wire link.

Source: Elektor Electronics


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