Circuit for Infra-Red Link
==========================

What this file contains
-----------------------

The two GIFs contained within this archive contain circuits
relating to an 'infra-red link'. These circuits are not
primarily designed for data transfer, but as a means to
detect if the beam has been broken. The circuits could be
used in a simple 'burglar alarm' circuit, or as a means to
check if (eg) a motorised buggy had passed a certain point.
It could also be used to check if an object had been placed
underneath it - the reflection of the IR beam could be
picked up by the detector.

I did not design these circuits, and I acknowledge the
copyright of the person who did.

TRANS.GIF contains the circuit for the transmitter stage,
which is quite simple. A 555 IC is used to pulse a IR LED.
In the circuit diagram, the pulse frequency is variable
(using RV1), but about 5kHz is recommended. The reason why
pulses (rather than a continuous beam) are used is as
follows. The receiver stage is constructed so that it
responds best to ('high'-frequency) pulses and not a
continuous beam. In this way interference from ambient light
is minimised.

REC.GIF is the receiver stage. It consists of a 741 op-amp
and an arrangement of capacitors such that only AC pulses
are allowed to be detected. In this way, constant ambient IR
light is not (erroneously) detected. This circuit actually
drives a relay, which is activated when the IR beam is
detected. However, the relay is (of course) not actually
necessary.


Construction
------------

There are no PCB layouts in this archive: it seems that the
circuits are very simple and are more likely to be used as
building-blocks for bigger projects. Hence I haven't
included any. Construction should be fairly simple.

Both circuits can be successfully powered from one of those
small (PP3) 9V batteries. The range of the beam/detector
setup is (at most) about 3 metres.

It may be a good idea to put some rubber sleeving around the
IR detector LED on the receiver to make the receiver more
directional.

Parts list
----------

*For the transmitter*
R1, R3: 470R 0.25W
R2: 2k2 0.25W
RV1: 4k7 preset
C1: 47nF polyester
C2: 10uF 16V electrolytic
IC1: 555 timer
LED1: CQY99 (or equiv) IR transmitter

*For the receiver*
R4, R7, R8: 47k 0.25W
R5, R6, R9: 100k 0.25W
R10: 5k6 0.25W
R11: 100R 0.25W
C2: 10nF polyester
C3: 100nF polyester
C4, C5: 2.2uF 16V electrolytic
C6: 100uF 16V electrolytic
IC1: 741 op-amp
Q1: BC237 (or other similar NPN transistor)
D3, D4, D5: 1N4148 signal diode
LED1: TIL78 (or equiv) IR detector
RLA: 12V relay

Hopefully this circuit should work when you build it.....

Any suggestions or queries, please feel free to email me.

--Dan Evans.
dan.evans@sjc.ox.ac.uk
http://sable.ox.ac.uk/~sjoh0014/
