Simple deployment timer
This is a very simple electronic deployment timer I designed as a prototype.
A microcontroller offers a lot more flexibility but when all you need is a cheap,simple, reliable,
easy-to-build timer, this may work for you.
The main components in the schematic diagram are:
- A 4013 dual D flip-flop.
- An inertial switch (normal open type).
- A 9V.. 12V relay.
- A 9 V battery.
- A bottle of strong regional beer (not in the diagram).
The inertial switch was taken from a CD ROM drive. It is a dual normal open switch that
was used as an end stop indicator. I attached some weight to the moving contact arm of the
switch. This way, in rest the arm is in the middle and does not make contact with any of the
two contact points. During launch, the arm will move down and make contact with one of the
How does it work?
When you close the power switch of the 9 V battery pack, a positive pulse is generated
by a 10M cap and a 10k resistor. This pulse is applied to both flip-flops in the 4013:
both are reset: output Q is low.
At launch, the weight on the arm of the inertial switch will bounce up and down. One single
contact pulse will set the left flip-flop, causing its output Q to go high.
This high level will start charging the 22M capacitor through the resistor and the
potmeter. The potmeter setting determines the delay after launch. With this configuration
it is adjustable between about 1 second and 6 seconds (tolerance on the electrolytic cap).
When the level on the capacitor becomes high, the right flip-flop is set, causing its
output to go high. The MOSFET will conduct and current flows through the
relay solenoid. This activates the relay and hopefully a successful parachute deployment.
Re-arming the timer circuit is done by switching the power off and back on again. The green
LED acts as an indicator that the circuit is powered but also as a bleeder circuit for
the decoupling capacitor, allowing relatively short off time before it can be switched
on again in the reset condition.
While you are soldering all this stuff on a piece of experimenting printed circuit board,
do not forget to enjoy your favourite regional beer !!