I'm making a parachute deployment device for a rocket. The program is relatively simple - I continually sample pressure measurements (from a barometer) until the measurements are increasing (meaning that the rocket is falling), and then I send current to an igniter that lights a charge that pushes the parachute out. However, the igniter requires more current than the arduino outputs from its digital pins, so instead of wiring the "fire" pin directly to the igniter, I decided to wire it through the base-emitter junction of an NPN transistor, and then put the igniter in the collector-emitter loop WITH a connection to the 9V battery (the SAME ONE that is powering the arduino). Basically, the base-emitter loop has a wire running from the "fire" pin on the arduino to the base pin on the transistor, and then the emitter is connected to the arduino's ground through a 400 ohm resistor. For the collector emitter loop, one end of the igniter is connected to the emitter, the other end of the igniter goes to the negative terminal of the battery (the same battery that is plugged into the arduino) through a resistor, and the positive terminal of the battery is connected to the collector. I was testing the program with a light bulb instead of an igniter and I ran into an issue. When plugging the cord from the battery into the auxiliary power port, initially, only the inside pin of the port makes contact with the inside of the connector (the positive terminal of the battery), and then as you plug it all the way in, the outer face of the plug (the negative terminal of the battery) finally makes contact with the other terminal inside the port (arduino ground). During the plug-in process, when just the inside pin of the port is touching the inside of the connector, the lightbulb lights up dimly, and continues to stay lit up as long as I leave the plug partially into the port. The program doesn't start. If I plug the connector in the rest of the way, the lightbulb goes off and the program begins to run correctly. Obviously this can't happen when I'm using it in a rocket, because it would set off the ejection charge as soon as I plugged it in. Why exactly does this happen? Does it have to do with the way that the port is designed? What is the center pin on the port connected to?
I don't know what is happening with the 9V plug, but on start-up all of the pins go to their default of an Input pin. It could also be the case that the built-in pull up resistors are activated in this status, but I don't know for sure. Try adding something along the lines of a 5K pull-down resistor to the output pin you care about and see if it solves the issue.
Post a diagram and a piece of the code! I think you have a lack of pull-down/up power
Cheers!
Back in the day a simple mechanical/electrical contact setup initiated the parachute deployment and similar actions when the rocket started to tip over. The project is interesting, but unless the rocket is large, the extra electronics may pose a weight penalty.
What you omitted to say, though it should not need to be said, is that as you have it, the connection from the emitter of the transistor is direct from the battery and and does not pass through the Arduino board.
Thus by connecting only the 9V to the Arduino, and not the ground, the Arduino connects the 9V through the port pin to the base of the transistor - and switches it on because the Arduino is essentially, a conductor.
The connection between the Arduino ground and the emitter of the transistor must be soldered, not via any sort of plug.
Incidentally, you should not be using a UNO, but a Pro Mini.
JohnLincoln:
I think you have your 400? resistor in the wrong place.Instead of having it between the emitter of the transistor and ground, you should have it between the Arduino output pin and the base of the transistor.
Can you post a circuit diagram/schematic of your circuit? I find it easier to understand than just a text description.
Am I right in thinking that the battery is already connected to the igniter circuitry when you plug the battery connector into the Arduino?
I think that when you are slowly inserting the connector and the centre positive pin makes contact, you already have a "weak" connection to the Arduino ground through the 400? resistor and the wiring between the battery and igniter. That would explain why the light in your test circuit was on dimly, the 400? limiting the current.
The solution may be to have a switch that connects the battery to the Arduino and igniter simultaneously. (if moving the 400? isn't the answer).
But why would current be flowing out of the ground when I plugged in the positive terminal?
Hi, your ignitier should be between the transistor collector and batt positive, the emitter goes to gnd. The resistor between the arduino digital output and transistor base can be about 1K.
Please a CAD or photograph of a hand drawn circuit diagram, and a picture of your project will be vital, i we are to give you accurate advice.
Hope to help.
Leave the plug in and use a switch.
Tom...... 