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First of all, the disclaimer. This page is NOT meant to be a how-to article. This was my first electronics project and I had never soldered before in my life, so remember that before you take anything I say as advice. I'm simply sharing the schematics of the units that I built and have been using successfully for over a year without any glitches.
The relay unit's design comes from Michael Moncur's 12V relay launcher plans with the following modifications:
The list of features that I kept from the design:
The relay box has four interfaces: a female binding post for the local power supply, a female binding post for the ignitor clip cable, a momentary pushbutton for continuity checks, and a DB-9 socket (9-pin "computer" connector) for communication to the remote base unit.
The communication through the DB-9 currently uses only 4 pins. I chose the DB-9 over standard phone connectors for a couple of reasons. First, I could not find "project box" mountable phone sockets at a reasonable price. Second, the extra pins could be used to implement additional functions in the future.
Here's a rough walk-through of how the relay box works. Sending current through the Launch channel will close the switches in Relay 3. (All of the relays in the diagram are Normally Open.) This does two things. One, the warning Buzzer 1 sounds. This is a safety feature in case Relay 3 should ever fuse or the Launch channel is hot. If you hear the warning buzzer, don't hook up the ignitor clips. Secondly, a short is created across the igniter clips... whoosh! Pretty simple so far. Note that during a launch there is very high current running through one of the switches of Relay 3. Care was taken in selecting this component. The warning buzzer is optional and its removal greatly simplifies more than one aspect of the project (including the need to find a high-current DPST or DPDT relay), but it's one of the features that I really liked in the design.
Continuity can be checked at the pad by pressing Button 6. If there is continuity at the igniter, this sends current through Buzzer 2 (which should be audibly different from Buzzer 1). Continuity can be checked at the base unit by sending current through the Continuity Check channel. This closes Relay 4 as well as attempts to pass current through the switch of Relay 5. Closing Relay 4 has the exact same effect as pressing Button 6. Not only does this sound Buzzer 2 but is also closes Relay 5 (assuming continuity) which allows the Continuity Check current to pass back through the Continuity Return channel. This presumably sounds a buzzer or illuminates an LED at the base unit. Buzzer 2 and Relay 5 must draw very little current to prevent the igniter from firing during a continuity check. Bench tests during continuity checks indicate 13mA @ 9V and 17mA @ 12V.
One of the features of the Estes and Aerotech controllers that I didn't like was that a continuity check is engaged when the safety key is inserted. Not only is this an unnecessary power drain and annoyance on the ears (with the Aerotech controller) but extremely low current ignitors may be set off. Even with carefully chosen components I purposely designed the relay box so that a continuity check is optional for launch purposes.
This relay box can also be used in conjunction with non-relay controllers such as the Estes model. Since the launch sequence only requires two pins, a DB-9 could be wired with two external leads connected to the Launch and Common pins. Continuity can still be checked at the pad with Button 6. The only limitation is the amount of current the coil in Relay 3 can sustain.
| Qty. | Description | |
| 1 | 270-1805 | 6x3x2" project box. |
| 2 | 274-718 | Chassis-mount dual female binding posts. One is used as a connection to the external power supply. (NOTE: See the Addendum at the end of this document for updated information.)The other is used as a connection to the ignitor clips. The binding posts allow for banana plug connectors or bare wires. |
| 1 | 276-1537 | (DB-9) 9-position male connector (solder). This is used as the interface to the base unit for the Launch, Continuity Check, Continuity Return, and Common channels. |
| 1 | 275-644 | SPST momentary pushbutton switch. (Button 6 on the schematic.) Used to manually trigger the continuity check at the pad. |
| 2 | 275-233 | SPST reed relay. Used for the Continuity Check and Return relays (Relays 4 and 5 in the schematic), Because the Continuity Return relay must draw very little current so that the ignitor is not set off during a continuity check these are perfect, drawing only 11mA. |
| 1 | 275-218 | DPDT relay. This only needs to be a DPST relay, but it's all I could find that could handle high currents. This is the Launch relay (Relay 3 in the scematic) and one of the switches is in the high-current launch circuit. If you leave out the warning alarm (Buzzer 1 in the schematic) then I'd suggest the 30A SPST automotive relay (275-226) instead. |
| 1 | 273-075 | Panel piezo buzzer. Used as the warning alarm (Buzzer 1 in the schematic) because it can be wired to emit either a continuous tone or a pulsing tone. I chose the latter. |
| 1 | 273-065 | PC piezo. (10mA @ 12V) This is the continuity buzzer (Buzzer 2 in the schematic). This must draw very little current so that the ignitor is not set off during a continuity check. |
| 1 | 270-1217 | Heavy-duty inline fuse holder. Item 7 on the schematic. Obviously you'll need to insert a fuse for the circuit to work. I think I have 15A fuses in the relay units at the moment, but the only component that it is really protecting is one of the DPDT relay switches. |
For the base unit I got a bit ambitious right off the bat. It can control four pads with any combination of single or simultaneous launches. The base unit has a female binding post for an external power source and four DB-9 sockets for connection to the relay units. Each pad also has a separate arm switch, continuity button, green continuity LED, red arm LED, and DB-9 key socket.
Yes, each pad has its own key which is a DB-9 connector with a unique pair of pins shorted. Thus, even inserting the wrong key into a key socket will still prevent a launch. If you have your key at the pad you can be confident in a safe hook-up. The keys also have a unique pair of "detector" pins shorted for the purposes of lighting the red arm LED when the arm switch is thrown. Thus, no matter what combination of inserted keys and arm switches are enabled, the red LED's will always tell you exactly what relays will be energized when you press the launch button.
The "P" numbers in the diagram below describe which pad and pin the endpoint is connected to. P1-1 through P4-1 are connected to the Launch channel on each of the four pads. P1-2 through P4-2 are connected to the Continuity Check channels. P1-3 through P4-3 are connected to the Continuity Return channels. And finally, the P#-4 is the buss connecting all of the Common channels together.
You may notice that the power source for the base unit is marked as 12 volts. This is only because I have several duct-taped pairs of 6V lantern batteries for various uses. Ideally you only need enough juice to trigger Relay #3 in the relay box for launch. I intend to build a small hand-held single-pad base unit for my relays that contains an internal 9V battery.
I'm stilll compiling the parts list for the base unit.
Accessories include the power cables, ignitor clip cables, and communications cables.
| Qty. | Description | |
| 5 | 270-343 | 1 pair 30 Amp heavy-duty car battery clips. One pair for every external power source which, in my case, is five: one for the base unit and one for each of the four relay units. Works great with car batteries and spring-topped lantern batteries. |
| 9 | 274-717 | Dual inline banana plugs. One pair for every external power source and one pair for every ingnitor-clip lead for a grand total of 9 in my case. |
| 2 | 278-567 | 25' spool 2-conductor 18-guage cable. Used for the 9 power source connections and ignitor clip leads. I made each about 3 feet so I needed a couple of spools. |
| 1 | 270-373 | Copper-plated, smooth-ended, micro clips. Comes in a bag of 10. You'll need a pair for every relay unit to be used as ignitor clips. Any small clips will do, but smooth-ended is best if you use Coppertails. |
| 8 | 276-1538 | (DB-9) 9-pin female connector (solder). One of these goes at each end of each of the data cables connecting the base unit to the relay units. |
| 8 | 276-1539 | (DB-9) 9-pin connector hood. Protects the 9-pin female connector. |
| 4 | 278-872 | 50' 4-strand telephone cable. Used as the data cables between the base unit and relay units. |
*Part numbers are from Radio Shack.
I have recently completed a new relay unit that has an internal power source. For single-motor Estes launches there's not always the need to lug around an external 12V source and so I've included an internal 9V battery. However, keeping with the theme of flexibility, I can still plug in an external 12V source for some extra juice when it's called for. To do this I replaced the female binding posts with closed circuit 1/4" phone jacks (274-255). The internal 9V is wired to the normally-closed contact. Inserting a 1/4" phone plug (274-266) opens the 9V contact and allows the external source to provide the power. Note that inserting an unwired phone jack effectively acts as a disarming switch. I have successfully completed bench tests and am eagerly awaiting field tests. Here's what the schematic looks like:
Matt Stum - NAR#73280 Level 2
