This page shows a home made G-switch timer that I designed and built in 1996.  It was flown many times in my Tempest hybrid rocket as a back-up to the on-board altimeter in order to fire the parachute ejection charge. I would typically set it for a 17 second delay so that it would fire the ejection charge about 2 seconds after expected apogee.  That way the altimeter should fire first and right at apogee if all went well. If the altimeter failed for some reason then this timer would put out the parachute.

I eventually quit using this timer because it was specifically designed to fire a flash bulb to set off the ejection charge.  I stopped using flash bulbs because I preferred the higher reliability of purpose built igniters like the Daveyfire brand.  I never actually had a flash bulb fail me, but they seem rather fragile. They were never really designed to be able to handle high G accelerations or high vibration environments.  Regular igniters are much more rugged but typically require a little more current to fire them.  I suspect this timer would work with the sensitive N28B Daveyfire igniters or the BlackSky sensitive HiRMI brand, but I have never actually tested it.  The problem is, I built it with two small AAA batteries.  Therefore it provides less than 3 volts to the igniter and it will not be able to deliver a lot of current either.

The G-switch itself uses a cantilevered metal bar that pivots down as the rocket accelerates upward.  The metal bar actuates a small micro-switch that triggers a digital counter to begin the timing sequence.  The timer is designed so that it will not start unless the micro-switch has been activated for a duration of at least 0.5 seconds.  This prevents the timer from being triggered accidentally while still on the ground.  The desired delay time is set by six small switches that each add progressively more delay.  S1 = 0.5 sec, S2 = 1 sec, S3 = 2 sec, S4 = 4 sec, S5 = 8 sec, and S6 = 16 sec.  Therefore any delay time between 0.5 sec and 31.5 sec can be set by selecting the correct combination of switches.

To confirm the delay has been set correctly and that the device is operating properly, the user can simply hold down the G-switch for about 1 sec and then release it.  The timer will become activated and begin counting up.  Each second it will produce a short beep and when it matches the switch settings it will illuminate the "fire" LED.  The user can either time how long it actually took, or just count the number of one-second beeps. Of course all of this should be done prior to connecting the ejection charge!

The timer was built using a small prototyping circuit board and point-to-point wiring.  It is shown in the pictures below. 

There are two small toggle switches on the timer.  One is for power, the other is a safety switch that can be used to arm the unit once power is on and the rocket is ready to launch.  The safety switch also resets the timer logic so that it starts from zero.  I built the timer into my Tempest so that I could access these small toggle switches through a small hole that also served as the altimeter pressure port.  Once the rocket was on the launch pad I could switch-on the power switch and then the arming switch.

There are three small LEDs just to the right of the toggle switches.  The yellow LED indicates power is on.  The green LED indicates there is continuity through the flash bulb in order to confirm it is properly connected.  These two LEDs are what you want to see when it is armed and ready for launch.  The red LED indicates the timer has reached the final count and has fired the flash bulb. This is used for checking proper operation of the unit prior to connecting the bulb.  It will also be lit once the rocket has been recovered and in that case it indicates the timer did indeed get triggered by the acceleration of the rocket and eventually fired the charge.

The flash bulbs had wire leads that connected into a two terminal screw type connector.

The timer is built using four small inexpensive CMOS integrated circuits. This allows it to run on just 3V from two small AAA batteries.  U1 is a 74HC688 8-bit equality comparator.  It compares the switch settings to the counter output and will set pin U1-19 low when they match.  The actual counter is U2.  It is a 74HC4060 14-stage binary ripple counter with built-in oscillator.  The oscillator is set to run at about 16Hz by the two resistors and one capacitor that are connected to pins 9, 10, and 11 on U2.  This oscillator frequency provides the proper time scale factors for the six programming switches.  S1 = 0.5 sec, S2 = 1 sec, S3 = 2 sec, S4 = 4 sec, S5 = 8 sec, and S6 = 16 sec.  Therefore any time delay between 0.5 sec and 31.5 sec can be set by selecting the correct combination of switches.

U3 is a 74HC74 D-type flip-flop that is used as part of the triggering logic.  U3-12 will be low when the counter matches the switch settings.  Consequently, on the next oscillator clock edge from U2-9 the flip-flop output at U3-9 will go low. This is the event that activates the 2N4403 transistor that fires the flash bulb.  It also activates the 2N3906 transistor and illuminates the red "fire" LED.  A 2N4403 transistor was selected to fire the flash bulb because it has higher gain at higher current levels than the 2N3906.

The other half of U3 is a flip-flop that is used to ensure that the timer does not actually continue counting unless the G-switch has been activated for at least 0.5 seconds.  This is a safety feature that prevents the timer from triggering if the rocket is jostled or even tips over or falls down after the timer is armed. 

U4 is a 74HC00 quad 2-input NAND gate.  When the G-switch is activated, the input to a NAND gate at U4-2 will go low.  This forces the output at U4-3 to go high. If the arming switch is in the "arm" position then both inputs at U4-4 and U4-5 will be high. This forces U4-6 low and releases the clear input to the counter at U2-12.  This allows the counter U2 to begin counting up.  So long as the G-switch continues to be activated, the counter will continue to count up.  If the G-switch deactivates, even very briefly, the counter will be immediately reset via the NAND gate path from U4-3 and U4-6.  However, once the counter has counted for 0.5 seconds, then its output at U2-7 will transition high. This clocks the flip-flop at U3-3 and causes its output at U3-5 to go low.  This essentially represents a "point of no return" because once U3-5 goes low, it will no longer matter what the G-switch does.  The output at U4-3 will remain high and the counter will be allowed to continue to count even if the G-switch deactivates.  This is of course very desirable since the acceleration on the rocket will cease once the motor burns out and we want the counter to continue to count as the rocket coasts to apogee.

The counter output at U2-7 is also buffered by another U4 NAND gate and used to drive a small piezoelectric beeper. (Beeper is Radio Shack part number 273-074 and operates from 3-16 volts.)  This beeper is activated for 0.5 seconds every 1.0 seconds while the counter is counting.  It provides an easy way for the operator (me) to verify that the correct switch settings have be set.  All I have to do is count the beeps to determine how long the counter delay time is set for.  The beeper also provides a safety feature in that it warns that the timer has been activated and is about to fire the charge.  If for some reason the timer should be accidentally activated while the rocket was still on the ground, then the timer would start beeping and I would be warned to get back out of the way before it sets off the ejection charge.  Fortunately this has never happened.

Another NAND in U4 is used as part of a continuity check circuit to make sure that the flash bulb is properly connected and has good continuity through it.  If no flash bulb is connected, then the inputs at U4-9 and U4-10 are pulled low with a 100K resistor.  This causes the output at U4-8 to go high and turns-off the green "continuity" LED.  However, once a flash bulb has been connected, and if it has good continuity through it, then a 10K resistor will provide a low level of current (0.3ma) that will pass through the flash bulb (without setting it off) and put the inputs at U4-9 and U4-10 both high.  This forces the output at U4-8 low which turns-on the green "continuity" LED.