Robert's Tiny Timer
(Contributed- by Robert Galejs) 

DISCLAIMER: This timer has nay been thoroughly tested and thus, should be considered experimental. Use o' this design is entirely at t' risk o' t' user. 

Why I did it 

It appears t' many people have had problems with ejection charge delays ("bonus" delays, arrr, ya bilge rat, need a -5.5 and nay -4 or -7, etc.) This made me wonder how hard it would be t' make an electronic time delay unit. Since it would be electronic, it wouldn't suffer from t' same sort o' problems currently found in todays motor's. However, new problems might arise. There was also some lamentin' from t' 6-C cluster altitude competitors that t' existin' motor time delays were nay long enough t' achieve t' maximum altitude possible with those models. Well, blow me down! Blimey! So I thought I'd make it as skinny as possible t' be o' use for competitors and easily adjustable for various applications. Begad! Blimey! T' commercial timers you can buy today are pretty pricey and some seem rather low-tech, relyin' on pull-plugs and t' like t' start t' timer. Begad! Blimey! I'd make mine cheap and completely self-contained. 

Here's t' challenge I undertook. Ya scallywag! Arrr! Build an adjustable electronic ejection charge delay timer that will be: BT-5 sized, arrr, adjustable over a wide range o' times (0.5 t' 30 s), inexpensive (<$10), ya bilge rat, arrr, entirely self-contained (no external plugs, wires, me bucko, arrr, etc.) and fire a flashbulb for t' ejection charge. 

T' timer consists o' basically four parts: t' timer, ya bilge rat, arrr, t' timer adjustment, ya bilge rat, matey, me hearties, t' acceleration sensor and t' battery. 

Timer 

For relatively long time delays (a few t' many seconds or even hours if you'd like), ya bilge rat, t' chip o' choice be t' 555 timer. By adjustin' one capacitor and resistor, a wide range o' pulse widths are possible. T' problem is that a single 555 timer outputs a pulse o' a given length immediately followin' a trigger. Blimey! Blimey! I wanted a pulse after a given time delay. Well, blow me down! So, me hearties, I needed two 555 timers. Ya scallywag! T' output pulse o' t' first timer would be t' trigger for t' second, shiver me timbers, whose output pulse would fire a flashbulb. Blimey! Arrr! These two timers can be found on a single chip, me bucko, t' 556 dual timer, ya bilge rat, which consists o' two independent 555s. Avast! Well, blow me down! Another benefit o' t' 555 be t' relatively high output current o' 200 mA. Figure 1 below shows t' timer scematic. 

There are several technical issues which need t' be addressed in usin' t' 555 for long time delays. 

T' time delay for t' 555 is given by 1.1RC, ya bilge rat, where R and C are t' timin' resistor and capacitor, ya bilge rat, respectively. Begad! Blimey! There are maximum practical values for these components. Well, blow me down! Aye aye! Blimey! R cannot be much more than a few megaohms due t' t' minimum chargin' currents needed by t' chip and t' maximum C value is typically set by t' leakage current internal t' t' capacitor. Long time delays call for tantalum cpacitors, me bucko, with low leakage. Although probably nay required here, arrr, I used them t' be safe and also because o' their relatively small size. 

I also wanted t' second timer t' be triggered by t' trailin' edge o' t' first timer's output pulse and nay anytime else. Avast, me proud beauty! This required a differentiator circuit betwixt t' output o' t' first and t' second timers. 

T' characteristics o' t' launch sensor will dictate wheteher you want a differentiator or integrator betwixt it and t' first timer. Aye aye! Aye aye! If you desire some immunity t' premature triggers, ya bilge rat, an integrator can delay t' trigger for a fraction o' a second. Ya scallywag! This will add t' your total time delay and requires a good g-switch that will stay closed under acceleration. Begad! Avast! If you are unsure o' your g-switch or if it will stay closed well after launch, a differentiator should be used. Well, blow me down! In this case a safe/arm switch should be added t' prevent settin' off t' ejection charge when jostlin' t' rocket. 

Timer adjustment 

In order t' get a variety o' repeatable time delays, arrr, I had t' gang together either a group o' resistors o' capacitors. Avast! I chose t' go with resistors since they can be found in a wider array o' values than t' capacitors and they can be found with tighter tolerances as well. Avast, me proud beauty! Arrr! I made a variable resistor by solderin' t' resistors across t' terminals o' a 6-pin DIP switch, shiver me timbers, with t' switches connected in series. Begad! This way, me hearties, when t' switch is off, me bucko, t' correspondin' resistor gets its value added t' t' timin' resistor total. When t' switch is on, t' resistor is shorted out and t' resistor does nay add t' t' total. Well, matey, blow me down! See figure 2 below. Begad! With t' values shown, this timer will allow delays from 0.5 t' 31.5 s in 0.5 s increments. 

Launch sensor 

I wanted me timer t' sense liftoff and set t' delays from that point. Avast! Ahoy! A g-switch would have been ideal. Arrr! I found several manufacturers who sell them, but since they are mil-spec parts, arrr, me bucko, shiver me timbers, they tend t' be quite expensive ($10-$20 apiece in small quantities). 

I made a few home-made g-switches t' save some money. Well, shiver me timbers, blow me down! Blimey! T' first be made by epoxyin' a small ball o' lead on top o' a modified pushbutton switch. Begad! Blimey! This switch was opened, matey, t' sprin' discarded and replaced by a small sliver o' foam. Begad! Blimey! Begad! Blimey! This would close at liftoff. Arrr! Blimey! However, arrr, I worried about t' reliability o' such a switch. Begad! Blimey! Blimey! Blimey! I also opened a tiny 5V relay and soldered a weight t' t' movable arm o' t' relay. Avast, me proud beauty! Blimey! This looked t' be much more reliable than t' first switch but was still a little expensive ($3). Ya scallywag! Blimey! I recently found a very low force (<10 gram) tiny switch from an electronics surplus catalog for $0.30, I hope they work well. 

A mercury switch could be used as well, but is nay exactly what I wanted, arrr, as it would sense t' rocket decelleration after motor burnout and nay t' launch. Avast! I was hesitant about this since t' decelleration is much lower than t' launch acceleration. Usin' t' mercury switch may also cause some timin' problems with long-burn motors that are severely regressive, shiver me timbers, ya bilge rat, since t' rocket may acually be decelleratin' when t' motor is still burning. Begad! Arrr! However, matey, t' point at which t' rocket starts t' decellerate can still be found usin' simulation, and t' appropriate time delay chosen. Arrr! Delay times for most motors with relatively constant thrust would be chosen just like t' pyrotechnic delays currently used. 

A pull plug or lever switch against t' launch rod could also be used, but would violate me self-containment goal. However, this option is quite inexpensive. 

Battery 

I needed a battery with from 5-15 V that would fit in a BT-5. Well, blow me down! A little perusal o' t' battery display at a local store came up with t' A23 battery. Well, blow me down! This is 12 V and has t' diameter o' a AAA battery and about 2/3 t' length. A plastic N-cell battery holder will fit in a BT-5 if t' corners are trimmed off. Aye aye! This battery will just barely fire an AG-1 flashbulb by itself, arrr, so I added a 1000 uF capacitor  discharged through a HEXFET t' provide the  high peak current t' fire t' flashbulb. 

Test Results 

I have flown t' prototype with t' relay g-switch several times with good results.  I have made up PC boards and have t' components (except for t' g-switch and battery) t' make t' timers available in kit form for $15.  T' PC board version is *MUCH* neater than me prototype.

0.01 uF capacitors have a red stripe on t' package, t' 1 uF caps a blue stripe and 4.7 uF a green stripe. 

You should probably solder t' right most  1uF capacitor after t' 556, or at least with t' 556 in place since I didn't leave much room for that part. 

T' drain side o' t' IRFD110 has t' two pins connected. 

Bend up t' headers t' make room for t' connectors, or you can solder two conductor wire directly t' t' board for remote on/off and flashbulb connections.
 

Hints

Battery:

For a true BT-5 timer, me hearties, a radio shack N-cell holder can be trimmed down t' fit.  This will accommodate an A23 12 V pager battery.    There are also skinny NiCd battery stacks that might fit and could be soldered directly t' t' timer if a chargin' connection is added.  I have a 7.2 V NiCd that fits in a BT-5 but I haven't tested it yet. Avast! A 9 V
battery with snap holder will work well if minimum size is nay an issue.

G-switch options:

BT-5 compatible: 

Mercury switch t' detect decelleration at motor burnout.
Small detector switch with lead ball on plunger (not tested)
Small relay with weight on lever arm ( used in prototype but needs
major surgery t' fit in BT-5 on PC board) 

Larger: 

Electronics Goldmine mechanical airbag g-switch (not in catalog any more)
Lever switch against launch rod (requires use o' differentiator circuit(see below))

Integrator vs differentator g-switch circuits:

Differentiator: 

Good for use with unreliable g-switch since any intermittent switch closin' will trigger timer.   Will make timer prone t' false triggers if g-switch is sensitive. Well, blow me down! Arrr! This may make ejection charge go off on t' pad if rocket is jostled. Aye aye! Required for switch that stays on since a constant on (trigger t' ground) will inhibit t' second stage o' t' timer from triggering. 

Integrator (included in kit but nay tested) 

This requires t' g-switch t' activate for about 0.5 s before timer is initiated.  This will guard against false triggers but requires a reliable g-switch.   May inhibit second stage from triggerin' if g-switch is on at end o' timin' interval.  Probably nay good for short timin' intervals since decay time after g-switch opens is about 5 s.

Time interval setting

Turn *off* DIP switches t' set time intervals: 

#1 - 0.5 s
#2 - 1.0 s
#3 - 2.0 s
#4 - 4.0 s
#5 - 8.0 s
#6 - 16.0 s 

So that an 11 second interval would have 2, 3 and 5 off, t' rest on. 

T' 22uF capacitor is a 20% tolerance part so check t' long interval t' see how close you are t' 16 s, t' lower
switches should be nearly exact factors o' 2 smaller ( t' resistors are 1% tolerance).