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Deploy Timer 1.1

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WATER ROCKET - Deployment Timer
This section describes the details of a simple water rocket parachute deployment timer as designed by the Air Command team.

The timer is aimed at providing a simpler and cheaper alternative to the Flight Computer.  The components for the timer can be obtained for less that AUD$10.

Version 1.1
Timer showing relative scale.
Circuit diagram of V1.1
Wiring diagram for V1.1



The deployment timer is intended to be used on water rockets to provide a delay after launch to release the parachute. The design is based on a simple two stage timer. After launch is detected, the first timer waits the deploy delay time before triggering the second timer. The second timer determines for how long the deploy actuator (motor) runs.

The timing delays can be set for both the deploy and the motor by changing the value of the corresponding potentiometers. With the values shown, one can set the deploy delay between about 0.5 - 11 seconds, and the actuator delay from ~0.1 to ~2 seconds. If the delays need to be set outside these ranges, then the R3,C2 and R5,C4 components can be substituted. The Time delay of each timer is calculated by:

T (sec) = 1.1 x R x C

(R is in ohms and C is in farads)

To trigger the first timer one can either push the Launch Detect button momentarily, or it may remain held down. For a practical rocket, one should set up an arrangement that makes this connection instead of the switch when it launches. For example: spring loaded electrical contacts with a piece of paper or plastic between them. The piece of plastic is attached to the launcher and when the rocket is launched it gets pulled out from between the contacts and to make the necessary connection.

The power supply can be anything between 5V and 12V so it should work with a wide range of batteries. A 6V battery is recommended. The motor driver part of the circuit can drive a 3 - 6V DC motor at currents up to about 500mA for brief amounts of time. Higher currents will need to be supplied by larger batteries.

Circuit diagram

See the circuit diagram at left.

Wiring Diagram

The timer should be wired according to the wiring diagram shown on the left.


Because it is a fairly simple circuit, one needs to make sure the "Arm" button is in the OFF position when power is turned ON. This is because the timers can come up in any state and run through the deploy cycle once. After about a 10-12 second delay it is safe to turn the Arm button to the ON position. The Arm button should be kept in the OFF position to prevent accidental deploys while still filling and adjusting the rocket on the pad. Just before launch the Arm switch should be set to ON.

Setting Delays

Before launching the deploy delay and motor delay trimpots need to be adjusted for the correct timing. The motor delay should only need to be set once and will depend on the deployment system, motor and gearbox used. Enough time should be given to the motor to fully operate the mechanism it needs to do the deploy.

The deploy delay is likely to need changing between launches and therefore the trimpot should be accesible from the outside of the rocket perhaps through a small hole using a screwdriver. The best way to calculate the approximate deploy delay is to use one of the online simulators and enter the rockets parameters and launch pressures to establish an approximation how long the rocket will take to get to apogee. The delay can then be set based on this value. On subsequent launches this delay can be adjusted to suit. When setting this value, you will need a stop watch to measure the exact time.

  1. Set the Arm switch to the OFF position.
  2. Turn timer on
  3. Adjust the trimpot half way.
  4. Activate the launch detect switch, and using the stopwatch, time how long it takes before the LED turns on.
  5. Adjust the trimpot and repeat step 4. until the desired deploy delay time is set.

The timer can be safely turned off with the correct delays being set next time the timer is turned on.

Water Rocket Parachute deployment timer

It is possible that the trimpots can become misadjusted due to jarring on landing, so the time delays should be verified before each launch.

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