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. Well, blow me down! Avast, me hearties, me proud beauty! (Hint: if the schematics below don't make sense, matey, don't try t' build it.) This is a high-current device; in addition, me hearties, arrr, high voltages may be momentarily present due to relay coil induction. Blimey! Insulate all connections carefully and do nay touch any connection while power is applied. Ya scallywag!

This document is provided for information only; use these instructions at your own risk. My only guarantee is that t' launcher I built works. Arrr! Your results may vary. If built properly, matey, shiver me timbers, this launcher should meet t' requirements o' t' NARRRRR Safety Code. Begad! Ahoy! 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, is flashbulb safe, ya bilge rat, me hearties, and should work well for clustering. Avast! Arrr! Blimey! 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.

T' launcher, as I built it, is a bit on t' complicated side, matey, shiver me timbers, and is not recommended as a first homebrew launcher. You can omit many o' t' features if you need a simpler launcher. Avast! 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. Most aren't critical and can be substituted, arrr, except as noted below. You could build a much cheaper launcher by choosin' lower-priced alternatives t' some o' these parts. Avast! Blimey! Begad! Blimey! You can also omit many o' t' parts, as mentioned in t' "How it Works" section below. I've included Radio Shack part numbers for most parts. Avast, me proud beauty! Blimey!

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, me bucko, but two different ones are easier t' tell apart. 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. If you use a different LED, 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, matey, 9-12V coil, 20-30A contacts. Ahoy! I used an automotive headlight relay (275-226). Well, blow me down! Ya scallywag! These are $5.99 at Radio Shack, or $3.00 at an auto parts store.
Relay 2 (Launch relay):DPST or DPDT, arrr, 9-12V coil, ya bilge rat, ya bilge rat, me bucko, 20-30A contacts. It's not easy t' find high-current relays with more than one set o' contacts. Begad! I used the highest-current one Radio Shack sells, shiver me timbers, which is rated 15A. Avast! This is marginal, but good enough for me. If you omit Buzzer 2, ya bilge rat, you can use another headlight relay here instead.
Relay 3 (continuity return): SPST reed relay, arrr, 12V coil, arrr, shiver me timbers, contacts not critical. Well, blow me down! This relay is placed in t' continuity circuit t' return a continuity signal t' t' controller. Well, blow me down! Well, matey, blow me down! Look for a relay with minimal current draw t' keep it flashbulb safe. I used (275-233), matey, which is small and draws a mere 11ma. Blimey! Ahoy! The contacts only need t' handle t' 9V battery. Begad! If you omit this relay you can use 3 wires instead o' 4 in t' launch cable.
Buzzer 1 (Continuity): Piezo buzzer (273-074). Avast, me proud beauty! Begad! You can use any piezo buzzer, me bucko, but this buzzer (along with Relay 3) limits t' current durin' the continuity check, so you need one that draws minimal current. Avast! Aye aye! Radio Shack sells several rated 5-7ma, shiver me timbers, which is ideal.
Buzzer 2 (Launching): Piezo buzzer (273-065). Avast, arrr, me proud beauty! T' current isn't as critical for this one. Avast! 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), which costs $23. You can probably find a cheaper one if you look around. Ahoy! Choose a battery that can source a high current for short periods; lead-acid or Nicads are good. Avast, me proud beauty! Blimey! Car batteries are good, shiver me timbers, arrr, me bucko, but probably more power (and weight) than you need. Aye aye! Blimey!
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). Ahoy! 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. Ya scallywag! 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 hearties, me hearties, match t' jack above.
Two alligator clips: Choose t' appropriate ones for t' igniters you intend t' use. I used 2" insulated clips (270-256). Avast, me proud beauty! T' insulation helps avoid shortin' betwixt t' clips. 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, etc.)
Hookup wire: I used 14ga automotive hookup wire for t' high-current portion o' t' circuit (see below). Ya scallywag! Aye aye! This is probably overkill, me bucko, 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. T' launch, you insert t' safety key, arrr, matey, then hold down t' continuity button t' trigger t' continuity check. Avast, me proud beauty! If that works, me hearties, you can press t' launch button (still holdin' t' continuity button) t' launch t' rocket. Begad! The continuity and launch buttons send signals t' their correspondin' relays in the launcher. Begad! Arrr!

T' LED indicates continuity. Avast! Blimey! It gets its signal from t' launcher (see below).

I used a 1/8" phone jack for t' safety key. Ya scallywag! 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, matey, you can use a key switch instead. Blimey! Aye aye! 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. Begad! Removin' t' key cuts off power t' both t' continuity and launchin' relays. Blimey!

T' Launcher

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

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

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, matey, effectively removin' them from t' circuit. Ya scallywag! This allows t' full current t' travel through the igniter, shiver me timbers, me hearties, launchin' t' rocket. Avast!

Buzzer 2 is also activated by Relay 2. This buzzer indicates that a launch is in progress, me hearties, which doesn't seem very useful--and isn't. I included this buzzer as a safety feature. Avast, me proud beauty! 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. This tells you that t' rocket will launch as soon as t' continuity circuit engages. Avast, me proud beauty! Ahoy! If this buzzer is on when you're settin' up at t' pad, don't hook up t' igniter. Disconnect the battery and diagnose t' problem. Ya scallywag! Ya scallywag!

Construction

Here are some brief pointers on buildin' this thing. Blimey! Aye aye! Once again, me hearties, 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). Avast! Blimey! I used a small plastic box (270-220) for t' controller and a larger box (270-231) for the launcher. I kept t' battery separate with a cable t' connect t' it. Blimey! Blimey! Depending on your battery, me hearties, matey, you might want t' choose a box that can hold t' battery as well as t' launcher components. Begad! Blimey!

If your battery is inside t' launcher box, you should add an on/off switch t' t' launcher. Avast! I added one anyway, just for convenience. Ya scallywag! Blimey! I used a lighted rocker switch (275-712), ya bilge rat, which also acts as a "power on" indicator. Arrr! Begad!

T' Controller

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

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, Buzzer 1, Buzzer 2). Arrr! Everythin' else will be in t' circuit durin' t' actual launch, shiver me timbers, and needs t' be connected with heavy-guage wire. Avast, me proud beauty! 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. Aye aye! Ahoy! This is convenient because I can buy it in various lengths with t' connectors already attached. Begad! I currently use a 25 foot cable. Begad!

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

Conclusion

I hope this helps. Begad! Arrr! If you have any questions or comments, me hearties, arrr, matey, feel free to email me

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