National University of Ireland, Galway, Dept of Electronic Engineering

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Circuits Designed

Opto-isolator Circuit :

The Gumstix generated the PWM signals used to power both motors. The outputs for the PWM signals came from the General Purpose Input Output (GPIO) pins 16 and 17. These were set to output a PWM signal rather than operate as GPIO. To allow the Gumstix's PWM signals to control the motors they had to be connected to the motors through an opto-isolator circuit.

The opto-isolator circuit was used to protect the Gumstix from 'surges' or 'spikes' in voltage or current. It was found that the Gumstix outputted a PWM signal of a maximum three volts. However more than three volts was required for the 4N25 chip in the opto-Isolator circuit to activate.

To achieve a non-distorted five volt PWM signal inverters were used. The PWM signal from the Gumstix was fed through two invertors which resulted in a five volt PWM signal with the same duty cycle as inputted. This allowed for both forward and backward movement


It was found that the same inverter could not be used to invert both PWM signals. If the same inverter was used one of the PWM signals was distorted. This distortion did not allow for the full range of movements designed for the robot. Two opto-isolator inverter circuits were used to create two clean five volt PWM signals.

Power Circuit :

As the robot was designed to be mobile, batteries were used to drive the motors, power the Gumstix and activate the inverters. All components required five volts. Three separate battery sources were used

Power Circuit
Power Circuit

A ten volt battery was split between the two motors. As the batteries driving the motors were quickly exhausted, rechargeable batteries were used. The voltage driving the inverters came from two separate nine volt batteries. In the case where one battery was shared between the two inverters the resulting PWM signals were distorted. This was not ideal as it did not allow for the full range of desired robot movements.

The voltage regulators used ensured no excess voltages were supplied to any component.

Gumstix Power Circuit :

The Gumstix was sensitive to changes in voltages while it was operating. Any unexpected change in voltages or feed-back would cause the Gumstix to 'blow' and render it useless. The opto-isolator was used to help prevent this. A separate circuit was also designed to isolate the Gumstix from the other components. It was built to specifically deal with powering the Gumstix and safely connecting the two PWM signals and ground signal to the rest of the circuits.

A six volt voltage regulator was used to power the Gumstix. Although five volts were recommended it was found that the Gumstix required six volts to run efficiently.

Ultrasonic Circuit :

Ultra-sonic was to be used as a locating capability in the swarm. Ultra-sonic is cyclic sound pressure. It has a frequency greater than the upper limit of human hearing.

If the swarm consisted of only two robots the ultrasonic could be used to ensure the robots do not come within a fixed distance of each other. With more than two robots the ultrasonic could be used to achieve formation movement, and random dispersion.

For the locating capability to work both infrared and ultra-sonic would be required. An infrared and ultra-sonic pulse would be sent simultaneously. The delay between both impacts was then to be measured. The infrared beam travels at the speed of light, instantly activating the IR-receiver. The ultra-sonic receiver must wait for the sound pulse, which is slow relative to the speed of light. The time it takes the ultra-sonic receiver to activate was then to be counted. This counted time could then be used to calculate the robots position in the swarm.

This requires an ultra-sonic transmitter and an ultra-sonic receiver. A circuit diagram of an ultra-sonic transmitter which could be used is shown.

Ultra-Sonic Circuit
Ultra-Sonic Circuit