He
epc134/epc135/epc136/epc137/epc138/epc139
Application Information
Light Barrier Application
The following circuit is recommended to operate the epc134/135/136/137/138/139 as a photo diode amplifier in a single beam light barrier:
+VLED
D2
Light path
D1
C1
R1
VDD
PD
OUT
C3
digital output
to logic, PLC
or μC
epc13x
Type
T1
R2
from digital
oscillator or
μC output
CN
VN
VSS
C2
GND LED
Recommended Components Values
R1: 27k(epc135/137/138), 6.8k(epc134) and 4.7k (epc136/139) (bias resistor). Sensitivity can be reduced by the reduction of this resistor.
R2: dependent on the required LED current
C1: Usually not needed. May be up to 100 pF (refer to section 'Photodiode Capacitance').
C2: 33nF (DC input current filter capacitor)
C3: 100nF or greater (power supply filter capacitor)
D1: PIN photo diode, BP104 (OSRAM) or similar devices
D2: IR LED, TSML1000 (Vishay) or similar devices
Working Principle
The IR LED D2 emits light pulses which are sent towards the photo diode D1. If there is no obstacle between the two devices, the light pulse
generates an AC current (I
PD
) in the reverse biased photo diode D1 into the pin PD. I
PD
is proportional to the power of the light pulse. If I
PD
is
greater than the trigger threshold of the circuitry in the device, a pulse at the pin OUT is generated. If I
PD
is greater than the trigger threshold
for the light reserve (I
PDres
), a second pulse at the output is generated. The length of the output pulse is given by t
OUT
. Once a light pulse is gen-
erated by the IR LED, a next light pulse must not be generated until the recovery time t
REC
(max.).
Design Precautions
The sensitivity at pin PD is very high in order to achieve a long operation range of light barriers even without lenses in front of the IR LED
and/or the photo diode. Thus, the pin PD is very sensitive to EMI. Special care should be taken to keep the PCB track
at pin PD as short as possible (a few mm only!). This track should be kept away from the IR LED signal tracks and
from other sources which may induce unwanted signals. It is strongly recommended to cover the chip, the photodiode
and all passive components around the chip with a metal shield. A recommended part is shown in the following figure:
The pins at the bottom are to solder the shield to the PCB with electrical connection to VDD. The hole in the front is
the opening window for the photo diode. The back side of the PCB below the sensitive area (D1, C1, R1, epc13x)
shall be a polygon connected to VDD to shield the circuit from the back side. C1 must be of high mechanical stability
(no piezoelectric effect) in order to avoid unwanted signals by mechanical shock or vibration.
Recommended EMC
shield
Ambient Light
Photodiode DC current can be generated by ambient light, e.g. sun light. DC currents at pin PD do not generate an output signal. However, if
I
PDDC
is above the stated value, the input is saturated which blocks the detection of AC current pulses.
Photodiode Capacitance
If the photodiode capacitance is below the specified value, the system becomes more sensitive to power supply ripple voltage at higher fre -
quencies (>200kHz). This sensitivity can be reduced by a parallel capacitor to the photodiode. However, this measure reduces the detection
sensitivity. If the photo diode capacity is above the specified value, a lower detection sensitivity and a higher sensitivity spread results.
© 2011 ESPROS Photonics Corporation
Characteristics subject to change without notice
8
Datasheet epc13x digital - V2.1
www.espros.ch
EPC [ ESPROS PHOTONICS CORP ]
