6
SIGNAL PROCESSING CIRCUIT
Since the UVTRON output pulse waveforms are the same for incident ultraviolet light and for background noise, the
waveforms cannot be distinguished. Therefore, the pulse generation frequency (interval between pulses) is used to can-
cel out the BG. Figure 7 is a block diagram for the signal processing circuit and Figure 8 is a timing chart for its opera-
tion. The operation of this circuit is explained below.
Point (a):
The output pulses from the UVTRON are input to the gate timer and the counter at the same time. The
counter counts the pulses sequentially.
Point (b):
The gate timer maintains the open state as long as the pulses enter in succession at time intervals shorter
than the setting time T
1
. When the pulse interval is greater than T
1
, the gate timer closes the gate and resets
the counter.
Point (c):
When a series of pulses are input, the counter adds them up. When the number of pulses reaches the setting
value, a pulse is generated to the output circuit and the counter is reset.
Point (d):
At the output circuit, the output pulses from the counter are lengthened to the necessary time interval (T
2
)
and are output.
*1 (Figure 8): The setting time T
1
must be shorter than the interval at which background noise pulses are generated. It is
usually safe to set T
1
to 5 seconds or less. If set too short, weak ultraviolet light cannot be detected.
*2 (Figure 8): The sensitivity of the device can be adjusted by the counter setting. To trigger the device with weak ultravio-
let light, set the counter to 10 or less. To have the device only detect higher intensity of ultraviolet light, or
to have the device operate for weak ultraviolet light if it is received for a long period of time, set the counter
value to more than 10. If the counter value is set to 3 or less, the BG may not be cancelled out, so use caution.
Q
5 V
5 V
GND
GND
–
Q
Q
–
Q
10 µs
10 V
0 V
T
2
(10 ms)
(d)
(a)
(c)
(b)
1000 pF
10 k
TPT C0014EA
TPT C0014EA
UVTRON
CATHODE
UVTRON OUTPUT PULSE
GATE TIMER
RESET
OPERATION SENSITIVITY
SETTING SWITCH
COUNTER
OUTPUT SIGNAL
WAVEFORM
OUTPUT
CIRCUIT
Signal Processing Circuit
Ω
Figure 7: Signal Processing Circuit
