Justice Unlimited Short Range Transmitter Gadget

What is the Short Range Transmitter?

The Short Range Transmitter is a circuit which emits pulses of infra-red (IR) light that the DRUID can see. The Short Range Transmitters were placed near many of the clues during Justice Unlimited so that teams could find them more easily.

The Transmitter worked quite well at night, allowing teams to "see" hidden clues up to 50 feet away. However in daylight the transmitter did not work very well and was not helpful.

How it works

The trasmitter is a simple circuit using a PIC16F628 microcontroller. The microcontroller turns one or more IR LED's on and off very quickly. The detector on the DRUID is looking for a 40kHz IR signal, so the Transmitter turns the LED on and off 40,000 times per second.

The Transmitter actually sends 10 bursts of 40kHz IR per second. Each burst lasts about 300 microseconds. So most of the time there is no signal being sent at all. This saves power

The Circuit

Here is the Transmitter circuit schematic

and here is the Printed Circuir Board layout

The reason for the funny "L" shape design is that this board is using the extra space around the Bat Blinker circuit board. The Bat Blinker circuit gets cut from the inside of this one.

The circuit is set up to drive 1 LED per transistor and to use transistor packs. When we built the circuits we actually used discrete transistors (cheeper) and drove between 2 and 4 LED's per transistor in series (to save power and to use fewer transistors and resistors). We built different versions of the transmitter using 2, 3, 6, and 10 LED's per transmitter to cover an angular field of 60, 90, 180, and 360 degrees respectively. (The LEDs emit light most brightly in a 30 degree cone.)

You can read more about the Printed Circuit Board design process for Justice Unlimited here. Also, here is the parts list for the transmitter with costs and suppliers.


The software for the transmitter is the simplest of all the circuits in Justice Unlimited. It just turns the LED on and off for 300usec, pauses, and repeats.

You can read more about PIC Microcontrollers here.


The parts to make the transmitters cost us about $6.50 per transmitter. You can see the parts list with suppliers and cost here


Development Page

For a complete list of files see the Transmitter Development Page.

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This file Copyright (C) 2004 by Nathan (Acorn) Pooley
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