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I finally developed a proximity sensor that can be attached with other identical ones to the same RCX input port, till a maximum of 3 sensors.

On the left you can see the second exemplar (I built 3 of them).

The limit of 3 sensors is due to current comsumption, to avoid RCX overload.

Every device is based on Sharp IS417F IR sensor, that returns high ot low voltage on object detection or absence.

Stacking 3 sensors on the same port let you always know which of them sees something.

How?

Every sensor has different output resistor, that scales the reading for RCX

(as passive sensor MUX on Michael Gasperi's page).

Here's the complete IRPD sensor, fitted in a box made by a 2x4 brick.

From the photo are visible the IR led and the hole through which sensor detects obstacles.

It is completely insensitive to ambient light variation, but only seeks modulated IR light reflected. 

For the packaging, I must thank Philo for his kind help: I developed this sensor basing on his prototype,

built in SMD.

Now, the schematic description.

To reduce space needed, I simplified diode bridge: the sensor is thus polarized and can be used one way only (no damage if reversed).

It is a basic implementation of the schematic included in IS471F datasheets.

The Sharp sensor requires few external components: it drives the led and reads reflected light back.

R2 is the resistor that sets the value read by RCX in case of detection (47k, 100k, 220k should work well).

The value of 10uF for C1 is enough.

Adjusting R1 value, you can set the range of the sensor: with 100 ohms, the maximum distance from a WHITE obstacle is about 13 cm.

September, 2005