This page describes our deployment timer. Its purpose is to provide a delay after launch to release the parachute at apogee. The design consists of two parts: The NE556 which represents the actual timer and the NE555 which provides the pwm-signal to drive a standard servo actuator.
Download the .sch file created with EAGLE.
The timer has two stages. The first stage is triggered by the launch detect switch. It waits a defined time before triggering the second stage. The second stage provides a pulse with a duration of approx. one second. This causes the servo actuator to go to its "open" position for about a second.
Con1 is connected to a 6V battery. Currently we use four Duracell 1.5 Volt cells of the type "AAAA".
Con2 is connected to a standard servo actuator.
The delay time can be adjusted via R6. The value of 47k allows a delay of up to 5 seconds.
S3 is the launch detect switch. It is default OPEN. Once on the launch pad it will be held in the closed position by a magnet mounted at the launcher. After launch S3 opens again. When using a G-Switch remove R3 completely, remove S3 from V_Bat and connect it to ground.
In order to avoid EMC problems design a centre point for ground and Vbat. Supply the timer with an individual branch, use another branch for the single 555 and a third branch for T2. Use a blocking capacitor for each branch in case of persistent interference.
Place C2 close to T2 because of current peaks caused by the servo.
Follow the switch on sequence: Close S1 first, wait until the red Led lights up, then close S2.