I want to build a model rocket flight computer, that ignites a model rocket motor using a standard model rocket igniter. My idea is that my Arduino Pro Mini(the 3.3v version) first checks continuity with the igniter and then fires it after a countdown. My Igniter needs 2A to fire and the manufacturer states, that continuity checks shouldn't use higher currents than 1-2ma. This entire system is powered by a 2s(7,4V-8.4v) lipo battery. Without the continuity check, I designed this small circuit:
But now I don't know how I can add continuity detection. I would appreciate any help with that. To safe cost, I would prefer a solution without a current sensor. I am a newbie with electronics, so a relatively simple solution would be best.
Use a second transistor stage having a suitable resistor in serie with the igniter. It will make the continuation current flow. Using a suitable omtage divider to read the voltage o er the igniter.
first, thank you for your fast answer.
Second, as I wrote in my question, the Microcontroller is the Arduino Pro Micro, the 3.3V variant. The voltage of my batterie is anywhere from 7.4v to 8.4v, so +VBAT is from 7.4v - 8.4v. After looking around for a bit, I chose this igniter: Firewire Initiator (6/pk) It has a safe continuity current of 40mA and an ignition current of 1A. It also has a resistance of 1 Ohm. I hope this helps
If you measure the voltage from GND and your mosfets DRAIN, there is only voltage there if the IGNITER circuit is OK. You can read this voltage with an analog input (with a voltage divider, so your analog input do not get damaged) and set your alarm accordingly. If OK, you can set the ArduinoPin high, to fire your rocket.
Thank you for your fast answer!
If I understand you correctly, is this what the circuit would look like? (I calculated that a 5.1k and 3.3k resistor would get me a maximal voltage of 3.3v)
Yes it will work, but I would calculate the voltage divider circuit, so you get f.ex max 1.5V to the arduino analog input, no need to max it out (safety margin).
Below is a typical model rocket launcher schematic.
Worth noting s how when continuity test is done we use a 2 K Ohm resistor to limit the current through the igniter. The idea here is to keep the current through the igniter below limits where the igniter gets hot. Once the user knows that continuity is OK pressing the Launch button bypasses the current limiting and the igniter gets full current and ignites the engine.
Working from the schematic we want to automate the process manually done using the 2 switches and the Launch button replacing the button with a MOSFET. One problem is your uC will only provide 3.3 volts to your MOSFET Gate. Rather than a MOSFET you may want to consider a transistor like the old 2N3055.
You can likely look at voltage drops across the 3 LEDs. PWR, ARM and READY. When all 3 conditions are met you start your 10 second countdown.
I would start by using a Google of Model Rocket Launcher Schematics as there are dozens of schematics out there. You can use a circuit as simple as the above or get as fancy as you wish. A Google of Model Rocket Launcher Arduino will yield circuits as used here in this example.
