need help find suitable opto isolator (opto coupler)

Discussion in 'The Projects Forum' started by dward, Jun 30, 2006.

  1. dward

    Thread Starter New Member

    Jun 30, 2006
    I have a model rocket launch control system which uses LEDs on each pad to indicate continuity through the engine ignitor. I used 12v LEDs because they draw only 20ma and thus don't cause the ignitor to fire (50ma + needed to ignite them). The problem is that a) the LEDs are hard to see in the bright sunlight and b) I need to display continuity at two points - the launch pads and the main control panel. I figured that using an opto-isolator to power a couple of higher current 12volt indicator lamps would work, but I've not had much luck with figuring out which device to use (or how); most have a current rating for the input (LED) side of the device of 60ma - the one I happened to have in the junk box cheerfully fired the ignitor. Any suggestions for devices (or suggestions for how to find an appropriate device) or circuit ideas? Back when dinosaurs roamed the earth I built some 555 timer circuits, decade counters and the like, but I've never done anything with opto-isolators.
    Its a 12 volt DC system with a common ground but separate +Vcc wires for the continuity and launch power for each pad.
  2. beenthere

    Retired Moderator

    Apr 20, 2004

    If you get a couple of hi intensity blue leds, you will be able to see them even with only 10 mills through the devices. They will show up with only a bit of shading against the sun.
  3. dward

    Thread Starter New Member

    Jun 30, 2006
    Thanks for the suggestion, however I'm already using the brightest LEDs I can find that still only draw 20ma or less. The problem is that I need highly visible indicators (wide angle and bright) in two places, one at the launch pad, and one at the main controller which is currently (pun intended) 25 feet away, but with the planned expansion of the system, soon to be a 60+ ft cable run. With two LEDs in the circuit and short cable runs, I'm drawing 40+ma which is really close to our most common ignitor's no-fire current rating (given their variability, way too close), and I don't have the visibility (angle or brightness) I need. We also occasionally launch rockets using motors which employ much lower current ignitors, no-fire max current for those is 35ma.

    I thought the optoisolators would be the way to go, but not having a local store where I could pick up a selection to test with, I've ended up having to download one .pdf data sheet after another looking for something that would work; but the data sheets don't have application circuits/notes that would provide a clue to how well it would work in my particular circumstance, so I was hoping someone out there who uses/plays with the things might have some suggestions.

  4. hgmjr

    Retired Moderator

    Jan 28, 2005
    Greetings dward,

    Take a look at the datasheet for the PS2506-1-A from NEC/CEL in the link I have attached. The link is to the webpage so you will need to click on the PS2506-1-A to open the datasheet. It looks like they are available from Digikey. An advantage to using this part is that it is intended for AC input so you will not have to worry about the direction you connect up your DC firing signal.

    It looks like it has a decent current-transfer-ratio (CTR) with a minimum of 200% and a typical of 2000%.

  5. dward

    Thread Starter New Member

    Jun 30, 2006
    Thanks, I'll order a few to try out.
  6. Chris Wright

    Well-Known Member

    Jul 26, 2006
    It seemed to me that a simple switching transistor might be the answer instead and this looked like an interesting project with which to learn something, so I started tinkering around. I'm afraid I got a little carried away, and some of the features and comments in the circuit are a little tongue-in-cheek. [​IMG]

    I ran this circuit in a little java applet simulation and every thing seemed OK, but as a novice I'm not sure. I do know that it only takes a small current to forward bias the base of Q1/Q2 and the simulation gave this as only 1mA for each or 2mA through the igniter with R1/R2 = 10K, but i'm not sure the value of R1/R2 is correct. Doesn't the value of these resistors determine if Q1/Q2 goes to saturation? I'm not 100% sure how to figure this. I know it's probably not critical for this circuit, I'm just trying to learn.

    A question for "dward" , Does the igniter have a resistance?

    I'm sure there will be some glaring error I've overlooked, so poke away at the new boy! [​IMG]

  7. dward

    Thread Starter New Member

    Jun 30, 2006

    Thanks for your circuit, it looks interesting. My system has 10 to 20 launch pads, with a master enable fire switch along with each pad fire switch, so I'd have to see if I can modify it a bit.

    The ignitors do have resistance, I don't have the values, but most are either a single nichrome wire or have a short nichrome bridge wire on which the pyrogen is deposited; as the bridge wire heats to incandenscence, the pyrogen is ignited which in turn ignites the solid fuel rocket motor.

    Current draw can be heavy, although if everything works as it should, the duration is rather short as the ignitor self destructs; however much higher draws for longer periods can occur (especially for the bridge wire type ignitors) if, when they were installed into the motor, the lead wires get shorted together.

    Power is provided by jumpstart or truck batteries.
  8. thingmaker3

    Retired Moderator

    May 16, 2005