On the prior page, a rocket-motor-igniting relay circuit provides high current from a collocated battery. The signal to activate the relay can come from manual switches or from a microcontroller. The microcontroller circuit is as follows...
Rocket ignition system schematic part 3: Microcontroller
IC1 This simple 8-pin microcontroller (Atmel AVR ATtiny45) provides optional features to the rocket ignition system. The microcontroller monitors the state of the interlock switch (SW2) as well as the arm (SW3) and launch (SW4) buttons. It enforces a strict sequence of order and timing (unlock, arm for at least 5 seconds, and then launch). During the countdown, the buzzer (LS11) sounds to warn anyone near the launchpad.
The microcontroller does not monitor continuity because the operator may have turned off the continuity switch as part of his/her launch process.
C4 0.1 µF small, fast capacitor to prevent power supply glitches from affecting the microcontroller
LS11 A buzzer (not a plain speaker). Can be connected to regulated (5 V) or battery voltage (9 to 35 V).
Q11 2222A NPN transistor that connects the buzzer to GND to turn it on.
R11 Current-limiting resistor to prevent too much current from flowing into the buzzer transistor (Q11).
SW3 Arm button. This must be held down throughout the countdown and while the launch button has been pressed.
SW4 Launch button. This must be a pushbutton or switch that automatically returns to the off position when released, per the Model Rocket Safety Code.
Q10 2907A PNP transistor that connects LED2 to power when continuity (GND signal from igniter) is detected.
R10 This schematic symbol is a repeat from the relay circuit presented earlier. (In other words, there is only one R10 in the circuit.) However, it is being included in this schematic to demonstrate that it also protects Q10 from receiving too much current.
R9 When continuity is not detected, this pulldown resistor provides a default GND signal. So, if you want to connect other circuitry to the collector of Q10, you’ll get 5V when continuity is detected and GND when it is not.
R2 Current-limiting resistor protects LED2 from receiving too much power
LED2 Continuity indicator. This means an unbroken igniter is connected and SW10 is switched on.
R3 Current-limiting resistor protects LED3 from receiving too much power
LED3 Unlock indicator. When the operator has unlocked the ignition system, 5 V is supplied to this LED.
R8 This pulldown resistor provides a default value of GND. That way, the microcontroller can detect locked (GND) from unlocked (5V).
R4 Current-limiting resistor protects LED4 from receiving too much power
LED4 Ready indicator. The microcontroller blinks this LED faster and faster as the launch sequence is completed. If the LED is not lit, it means the operator needs to lock the keyswitch and let go of the arm and launch buttons. After doing so, then the operator can restart the launch sequence.
Even though the microcontroller adds functionality (buzzer, safety sequence), remember that the remote igniter works even without the microcontroller. LED2 (continuity indicator) and LED3 (unlock indicator) still work, and a switch (or pair) connecting 5V to R7 will activate the relay to launch the rocket.
That’s it for the circuit. Let’s see how the rocket ignition system is laid out on a PCB.