Torpedo Activation Circuit

On a small scale machine, it is not always practical to activate components using mechanical/contact means. In the case of a torpedo launching from small AUV, it is ideal not to have any physical connections such as wires or connectors between the vessel and the projectile. A reliable light source and a light dependent resistor (LDR) can easily replace a momentary contact switch. The figure below illustrates the simplicity of the circuit:

For the given circuit, the trigger is connected to a voltage divider. In the dark, the LDR is measured to be at least 33k Ohms, providing a minimum of .75Vs to the trigger. As the LDR is exposed to light, its resistance drops, increasing the voltage going into the trigger. For a 10k Ohm pull up resistor, the resistance of the LDR should drop to around 5k Ohms to bring the voltage down below 1/3Vs, activating the 555 timer. At this point, the output, which powers the transistors controlling the motor to the torpedo, is driven high to Vs until the reset pin is pulled below .7Vs.

To stop the torpedo, the reset pin is connected to a momentary switch. This switch is connected to the tip of the torpedo. When the torpedo runs into an obstacle such as a wall, the reset pin will drop down to 0V, shutting off the output, and consequently, the motors.

A 555 timer in this configuration is known as a bistable circuit. It maintains its current output state until interrupted by the reset or trigger pins.

Image source: http://www.kpsec.freeuk.com/555timer.htm#bistable
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