HP 13255 Technical Information page 35

132~5
Cartridge Tape Module
13255-91032/33
Rev
AUG-01-76
The pulsed diode output is detected, shaped and fed into the monostable
multivibrator (one-shotl. The one-shot is retriggerable and its time
out is slightly longer than the time period of 13 kHz. Therefore, the
output Of the one-shot will remain true as long as it receives an input
of at least a nominal
13
kHZ.
By
measuring the time that the one-
shot's output remains true, it can be decided wtlether the detection
circuit has seen a scratch, a normal hole, or the tape has run com-
pletely off the end. This technique eliminates ambient light problems.
3.2.4.2
The pUlse generator consists of
U2,
R6, R7 and
C2.
the resistors and
capacitor control the frequency and symmetry of the output. The fre-
quency is
13.125 +1-1.5
KHz. The output of
U2,
Pin 3 has the following
waveform.
I
+SV
-,-
!<-----76 microseconds----->!
nominal
!<--18 microseconds
nominal
I
I 1
1 _ _ 1
I
I 1
1 ___ 1
3.2.4.3
el, CRt, Pt, R2, and
1/2
of Ul make up the diode driver. During the
time that the output Of U1 is high (oft) Cl charges through R2. When
the output Of U1 goes low, C1 is discharged throuqh Rl and CPt. Pl
limits the current throUqh CR1 and R2 isolates this low impedence path
from the +5 volts. The peak current through infrared emittinq diode
(CRt) is approximately 200 rnA, and should provide a beam power output
(minimum, peak) of
2.5
milliwatts.
3.2.4.4
The detection circuit conSists of Ql,
Q2,
Q3, CR2,
U3,
R4, R5, R8, R9,
RIO, and C4. Irradiance striKing its base reqion causes current to
flow through Ql. Most of this current flows through Q3 and into the
base of Q2. 02 saturates, prOViding a negative transition Which ini-
tiates the time out of the one-shot multivibrator,
Ul.
B5 and CR2
provide a de level to the base of Q3 that will allow the collector of
Q3 to overcome the
V(BE)
of Q2. P5 also keeps the capacitance of CP2
and Q3 cnarqed allowing faster circuit response. CR2 and Q3 provide a
low impedence load to the photo-transistor Ql, which has a high C(be)
thus alSO increasing response time. R4 sets the threShold ot the de-
tector circuit. Ql must provide enough current to raise the voltage
drop across R4 above the
V(SE)
of Q2 before Q2 will turn on. R9 is the
collector load for Q2 and provides current for the inputs of U3 in
their high state. RIO provides a high state for the other inputs of U3.
R8 and C4 are the timing components tor the monostable multivibrator
U3.
The time out is 110
+1-20
microseconds. Rli provides approximate-
ly
5 to 10 ner cent positive feedback giving the detection circuit a
SChmitt-trigger characteristic. This provides a more detinite detec-
tion of the hole edges.