Here is the updated, MOSFET-based circuit, using the basic design of
@sghioto with ideas from
@jderimig and
@brockrwood .
I will probably edit the title of this thread as it now contains both a Darlington Pair based circuit and the MOSFET-based circuit.
Nope. Just saw an error in the schematic. Back to the mechanical pencil...
OK, here it is.
Design compromise: I have the red LED, LED1 connected across the positive and negative rails. It is connected to the positive rail after the fuse and the big, master power switch (SW1). I want to be able to see LED1 glowing if the master power switch, SW1, is feeding battery power to the system. Basically, if battery is hooked up to the system, I want to know it. Resistor R3, 2.2K ohms, limits current through the "Power On" LED1, to about 6mA.
There is also another LED, a green one, LED2. This one is also current limited by a 2.2K ohm resistor, R4. This one, however, is connected to the positive rail THROUGH the igniter (about .711 ohm). It also has a push button in series with it, SW4. When power is on at the master switch, and an igniter has been clipped to the ignition wires, you push SW4 briefly to see if there is "continuity" through the igniter. If the green LED glows, then you have the igniter wired properly and you are ready for launch.
This presents an issue: When you press SW4 to check "continuity", you still have the "power on" LED glowing. You now have two LED's wired in parallel (and two current limiting resistors wired in parallel). I am really nervous about an any portion of the circuit that will draw current through the igniter before it is time to launch. So I have increased the value of the current limiting resistors on LED1 and LED2 to 2.2K ohms. This should make no more than about 6mA flow through either LED. Yes, that might make the LED's a bit dim. I might have to hold my hand over the LED's, in bright sunshine, to see if they are glowing.
Am I being too nervous here? I know it takes about 1 amp to set off an Estes-type igniter. Still, the idea of 10mA to 20mA flowing through the igniter, just to check it, makes me nervous. So I am going very conservative with the current limiting resistors on the LED's.
Also, hooking up LED's in parallel is trickier than hooking them up in series.
One solution: Put a push button in series with the "power on" LED, LED1. That way, you can "check to see if power is on" by just pushing the button. Once you have satisfied yourself that power is on to the system, you can release the button.
View attachment mosfet_based_12V_launch_controller.jpg
And another thing: Yes, I could have dispensed with the long wire in the schematic showing the connection to the negative terminal of the battery. Instead, I could have just used ground symbols where needed. But, the point of this controller is to separate the "controller box" and its switches from the rest of the circuit (the "pad box" and its innards and the battery connected to the pad box and the wires going to the igniter from the pad box). The idea is to be able to stand a considerable distance away from the pad (20 feet) while holding a light controller box that is connected to the pad box with light gauge wire. I wanted to show that in the schematic.