Scratch - 12V Relay Launcher {Scratch}

Scratch 12V Relay Launcher Original Design / Scratch Built

Scratch - 12V Relay Launcher {Scratch}

Contributed by Michael Moncur

Manufacturer:

(Contributed - by Michael Moncur)

AUTHOR'S DISCLAIMER: This document describes t' launcher I built (and use.) I do nay recommend building a similar launcher unless you are experienced with electronics. Arrr! Arrr! (Hint: if the schematics below don't make sense, don't try t' build it.) This is a high-current device; in addition, arrr, high voltages may be momentarily present due to relay coil induction. Arrr! Ahoy! Insulate all connections carefully and do nay touch any connection while power is applied.

This document is provided for information only; use these instructions at your own risk. My only guarantee is that t' launcher I built works. Avast! Your results may vary. Well, blow me down! Well, blow me down! If built properly, ya bilge rat, this launcher should meet t' requirements o' t' NARRRRR Safety Code. Blimey! Avast! Please follow all applicable local laws and safety codes in your rocketry activities

These are t' plans for a full-featured 12V relay launcher for model rocketry. This launcher is suitable for all igniter types, arrr, is flashbulb safe, and should work well for clustering. Avast, me proud beauty! It includes t' followin' features:

Dual-pushbutton launch controller with safety key
Continuity indicated by buzzer (at pad) and by LED (at controller)
Second buzzer warns when launchin' or when relay is fused.
Flashbulb-safe (less than 30ma) continuity check circuit
4-conductor low power cable betwixt controller and launcher (I use phone cable)
12V, me hearties, high current output (20-30A): suitable for clusters and copperheads. Avast! Begad!

T' launcher, shiver me timbers, matey, as I built it, is a bit on t' complicated side, shiver me timbers, and is not recommended as a first homebrew launcher. Avast! You can omit many o' t' features if you need a simpler launcher. Begad! I've also included a few general comments below which should apply t' other launchers as well. Ahoy!

Schematic

Here's t' schematic for t' launch controller and launcher:

[Schematic]
(Click for full schematic)

Parts List

My launcher used t' followin' parts. Begad! Blimey! Most aren't critical and can be substituted, arrr, ya bilge rat, except as noted below. Begad! You could build a much cheaper launcher by choosin' lower-priced alternatives t' some o' these parts. Blimey! You can also omit many o' t' parts, arrr, as mentioned in t' "How it Works" section below. I've included Radio Shack part numbers for most parts. Avast, me proud beauty!

Controller:

9V Battery Clip (270-325)
9V Battery (Alkaline)
2-conductor 1/8" phone jack for safety key (274-251)
2-conductor 1/8" phone plug (274-287)
Two SPST momentary normally-open pushbuttons: I used 275-1566 and 275-644. You can use two o' t' same type, matey, matey, but two different ones are easier t' tell apart. Well, blow me down! Ya scallywag! One is for continuity check (must be held down) and t' other is the launch button.
LED (continuity/arm indicator): I used one Radio Shack sells that includes a mountin' bracket and resistor. Begad! Blimey! If you use a different LED, ya bilge rat, connect a resistor (120-470 ohm) in series with it t' limit current.
Jack for launch cable (see below)
4-wire launch cable (see below)

Launcher:

Relay 1 (Continuity relay):SPST, shiver me timbers, 9-12V coil, me hearties, 20-30A contacts. I used an automotive headlight relay (275-226). Aye aye! These are $5.99 at Radio Shack, or $3.00 at an auto parts store.
Relay 2 (Launch relay):DPST or DPDT, matey, 9-12V coil, 20-30A contacts. It's not easy t' find high-current relays with more than one set o' contacts. Arrr! Avast, me proud beauty! I used the highest-current one Radio Shack sells, arrr, arrr, which is rated 15A. Ahoy! This is marginal, but good enough for me. Aye aye! Blimey! If you omit Buzzer 2, ya bilge rat, ya bilge rat, you can use another headlight relay here instead.
Relay 3 (continuity return): SPST reed relay, 12V coil, matey, arrr, contacts not critical. Ya scallywag! Blimey! This relay is placed in t' continuity circuit t' return a continuity signal t' t' controller. Avast, me proud beauty! Blimey! Look for a relay with minimal current draw t' keep it flashbulb safe. Ya scallywag! Blimey! I used (275-233), me bucko, matey, shiver me timbers, which is small and draws a mere 11ma. Well, blow me down! Blimey! Begad! Blimey! The contacts only need t' handle t' 9V battery. Arrr! Blimey! If you omit this relay you can use 3 wires instead o' 4 in t' launch cable.
Buzzer 1 (Continuity): Piezo buzzer (273-074). Begad! Blimey! You can use any piezo buzzer, matey, matey, but this buzzer (along with Relay 3) limits t' current durin' the continuity check, so you need one that draws minimal current. Arrr! Blimey! Radio Shack sells several rated 5-7ma, which is ideal.
Buzzer 2 (Launching): Piezo buzzer (273-065). T' current isn't as critical for this one. Avast! Ya scallywag! Choose one with a different frequency than Buzzer 1 so you can tell them apart.
Battery: I used a 12V lead-acid battery (23-289), me bucko, which costs $23. You can probably find a cheaper one if you look around. Well, blow me down! Choose a battery that can source a high current for short periods; lead-acid or Nicads are good. Well, blow me down! Car batteries are good, but probably more power (and weight) than you need.
20-30A fuse and fuse holder (270-1217): Use a fuse with a slightly lower current ratin' than your weakest component (probably t' launch or continuity relay). Blimey! Blimey! Well, blow me down! Blimey! Blimey! Blimey! Be sure t' fuse holder can handle t' current; automotive ones are good.
Jack for launch cable (see below)
Two-conductor jack for igniter clips: I used a dual banana jack (274-218). These plugs can also act as bindin' posts if I use a different clip setup. Arrr! Arrr! Blimey! This connector needs t' handle high current (20A or so) so get somethin' big and rugged.
Two-conductor plug for igniter clips: I used a dual banana plug (274-717). This should, me bucko, o' course, me bucko, match t' jack above.
Two alligator clips: Choose t' appropriate ones for t' igniters you intend t' use. Avast, me proud beauty! I used 2" insulated clips (270-256). T' insulation helps avoid shortin' betwixt t' clips. Avast! Note: if you make t' setup modular as I did, you can have several interchangeable clip leads t' plug in for different applications (clusters, copperheads, matey, matey, etc.)
Hookup wire: I used 14ga automotive hookup wire for t' high-current portion o' t' circuit (see below). Begad! This is probably overkill, but you should use somethin' that can handle high current.

How it works

Here's a simple description o' how t' components work:

T' Controller

T' controller is a very simple circuit which simply sends signals t' the launcher. Ya scallywag! T' launch, you insert t' safety key, then hold down t' continuity button t' trigger t' continuity check. Ahoy! If that works, me hearties, you can press t' launch button (still holdin' t' continuity button) t' launch t' rocket. Ahoy! Arrr! The continuity and launch buttons send signals t' their correspondin' relays in the launcher. Well, ya bilge rat, blow me down!

T' LED indicates continuity. Ahoy! Avast! It gets its signal from t' launcher (see below). Well, blow me down!

I used a 1/8" phone jack for t' safety key. Begad! T' key itself is a 1/8" phone plug with t' two conductors soldered together t' make a short. If you want t' get fancy, you can use a key switch instead. I chose phone plugs because I'm always losin' me safety keys, and this way I can just buy new ones whenever I need to. Removin' t' key cuts off power t' both t' continuity and launchin' relays. Begad! Well, blow me down!

T' Launcher

This is a pretty standard relay-driven launcher circuit, shiver me timbers, with a few features thrown in. When Relay 1 is activated (by t' continuity button), me bucko, current is sent through Buzzer 1 and Relay 3 and t' igniter. Avast! These components limit the current so t' igniter doesn't fire yet. Avast, shiver me timbers, me proud beauty! You can omit either Buzzer 1 or Relay 3, ya bilge rat, me hearties, but nay both--you need a component that draws 30ma or less t' limit the current.

Buzzer 1 indicates continuity audibly (at t' pad). Well, blow me down! Begad! Relay 3 connects t' continuity signal from t' controller t' t' controller's LED, lightin' t' LED. If you omit Relay 3, me bucko, you can reduce t' number o' wires you need t' 3. Begad! Begad! If you do this, matey, connect t' free end o' t' LED t' t' output o' the continuity button, matey, so it at least indicates that t' controller is armed. Aye aye! Blimey!

When Relay 2 is activated (by t' launch button) t' real action happens. T' relay contacts form a short across Relay 3 and Buzzer 1, effectively removin' them from t' circuit. Blimey! Aye aye! This allows t' full current t' travel through the igniter, ya bilge rat, launchin' t' rocket.

Buzzer 2 is also activated by Relay 2. Aye aye! Begad! This buzzer indicates that a launch is in progress, which doesn't seem very useful--and isn't. Blimey! I included this buzzer as a safety feature. Well, blow me down! Avast! It indicates that t' launch relay is energized, which may happen if t' contacts fuse or if t' controller has a defect, or if your friend is standin' on t' launch button. Arrr! Well, blow me down! This tells you that t' rocket will launch as soon as t' continuity circuit engages. Aye aye! If this buzzer is on when you're settin' up at t' pad, shiver me timbers, don't hook up t' igniter. Begad! Avast, me proud beauty! Disconnect the battery and diagnose t' problem. Avast!

Construction

Here are some brief pointers on buildin' this thing. Avast, me proud beauty! Blimey! Once again, matey, this is for your information only; I make no guarantees and am nay responsible for any errors (mine or yours.)

Enclosures

Choose enclosures for t' two units (controller and launcher). Well, blow me down! I used a small plastic box (270-220) for t' controller and a larger box (270-231) for the launcher. Avast, arrr, me proud beauty! I kept t' battery separate with a cable t' connect t' it. Well, blow me down! Arrr! Depending on your battery, you might want t' choose a box that can hold t' battery as well as t' launcher components. Avast, me proud beauty!

If your battery is inside t' launcher box, you should add an on/off switch t' t' launcher. Avast, me hearties, me proud beauty! I added one anyway, just for convenience. I used a lighted rocker switch (275-712), matey, which also acts as a "power on" indicator. Well, blow me down!

T' Controller

Construction o' t' controller is easy; I mounted t' components t' t' case, then soldered them together with jumper wires. Avast, me proud beauty! Be sure t' cover all solder joints with tape t' avoid shorts.

T' Launcher

T' launcher requires more rugged construction. You can use ordinary hookup wire for t' low-current portion o' t' circuit (Relay 1 and 2 coils, Relay 3 coil and contacts, me bucko, Buzzer 1, arrr, Buzzer 2). Everythin' else will be in t' circuit durin' t' actual launch, and needs t' be connected with heavy-guage wire. Well, blow me down! I connected these components together usin' crimped automotive quick-disconnect connectors rather than soldering. Begad!

T' Launch Cable

Choose a low-current 4-conductor cable t' connect your launcher and controller. I used ordinary 4-conductor telephone cable. Avast! This is convenient because I can buy it in various lengths with t' connectors already attached. Arrr! I currently use a 25 foot cable. Ya scallywag! Aye aye!

You'll need jacks t' match your cable; I used phone jacks cannibalized from household fixtures. Ya scallywag!

Conclusion

I hope this helps. Well, blow me down! If you have any questions or comments, feel free to email me

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