The first schematic of the rocket ignition system regulates the voltage of the remote rocket ignition system. The ignition system is remote, in that the controller is located many meters away in the hands of the operator, while the battery is near the launchpad to deliver maximum current. We are now going to look at the second schematic that shows how to activate a relay in order to supply battery power to the rocket igniter.
Rocket ignition system schematic part 2 Relay Igniter
Igniter This entire project exists to light the igniter at the right time, as well as to avoid firing the igniter all other times. The igniter fires when it receives enough electrical energy. In this circuit, the only way energy can flow though the igniter is through the relay (K5).
D7 Vishay Schottky barrier rectifier (fast diode) #SB560-E3/54 to protect against reverse voltage when the igniter vaporizes. I put this in the circuit to provide a path for current flowing in the opposite direction. However, I’m not sure it serves any purpose in practice.
K5 The T90S5D12-5 relay is controlled by its coil.
The normally-open and normally-closed pins never connect to each other. This avoids having the full battery voltage (9-35V) damage the regulated voltage (5V) circuitry.
The downsides of a relay are:
The advantages of a relay are:
SW10 An optional switch or pushbutton that can connect or disconnect the continuity checking from the igniter. The switch exists for rocket-launching organizations whose rules require the continuity check to be only on demand. If that isn’t a requirement for you, you can replace this switch by soldering a wire in place.
R10 A 33 kΩ resistor that severely restricts the amount of current that can flow during continuity checking. Even if everything else had no resistance or voltage drop, at 5 V, no more than a sixth of a milliamp could flow. Even if 35 V somehow got connected, only 1 milliamp would flow.
SW2 A DPST (double pole, double throw) keyswitch such as Y201132C203NQ. This is a great safety feature required by the Model Rocket Safety Code. When the switch is off, no power can flow through the relay coil. It doesn’t matter if the microcontroller goes crazy and tries to launch - nothing will happen. The second pole of the keyswitch allows the remainder of the circuit to detect when the keyswitch is unlocked, without the risk of an electrical signal flowing into the relay coil like a single pole switch would cause.
D5 Vishay TransZorb transient voltage suppressor (P6KE6.8CA-E3/1) that in connected across the relay coil to protect the relay coil and the rest of the circuit from inductive spikes. This modern type of protection is recommended by the relay manufacturer, as voltages below 6.8 can fully power the coil, whereas voltage above that are suppressed.
R5 Current-limiting resistor to prevent too much power from damaging LED5.
LED5 Launch indicator. Lights when the coil is turned on.
Q7 2222A NPN transistor that provides GND to the relay coil when enabled. When not enabled, the transistor electrically disconnects that end of the relay coil.
R6 This pulldown resistor provides a default value of off to the NPN transistor. When the microcontroller is powering up or when the launch button is disconnected, the transistor is held in an off state.
R7 Current-limiting resistor to protect Q7 from too much current. This has a lower resistance than R6, and thus can override R6 in providing an on signal to the transistor.
In summary, the rocket motor igniter cannot receive enough power to ignite unless the relay is activated. The relay can’t be activated unless the coil is energized. The coil can’t be energized unless it receives BOTH a high voltage (5V) from the safety keyswitch AND a low voltage (0V) from the transistor.
The transistor can either be controlled manually by one or two switches in series. Or, better still, a microcontroller can control the transistor after it has observed the operator follow a safe process. Let’s see the schematic for the rocket launcher microcontroller.