Smoke Pump Controller

Obviously I have too much time on my hands.

While reading some posts on Van’s Air Force about wiring up smoke pump control switches, I thought there must be a better way than most of the ideas presented. I know if I were putting smoke in my airplane (and I fully intend to do so), I’d want an ARM switch and a pushbutton on the stick or throttle. What I would NOT want is to have to keep my finger (or thumb) on that switch all the time.

So, I designed a little circuit board with an ultra simple microcontroller to act as a multi-mode switch. It does everything I want it to do…

  • An indicator that uses a bi-color LED to show the ARMED / ON status
  • On/off toggle action with a simple “click” of the pushbutton
  • Constant ON mode while the pushbutton is held
  • Sufficient to control a 10 A pump with 100% duty cycle
  • Easy and intuitive to use
  • Simple and inexpensive to produce
  • Small and light weight, easy to mount wherever there’s space

The switching MOSFET was really the only part I was concerned about. I contacted Marvin at Smoke System Helper, who helped with testing to validate that it will drive a 7A pump constantly with no detectable rise in temperature. All the parts are rated for 10A or more, and the MOSFET has a ridiculously low Rds(on), less than 4 mOhm at the 5V gate drive voltage used. It’s rated for 160A at 30V, so it’s snoozing. The max current limit is actually imposed by the terminal block.

Overall I’m very pleased with the result. The case is not perfect — no mounting tabs — but really, the thing weighs so little that some double-sided foam tape will hold it in place, maybe with a zip tie for high vibration environments. My plan for the Celebrity is to tuck it behind the panel.

Score!

I haven’t been doing much on the biplane recently, but I did manage to score an altimeter for it.  I’ve been watching the for sale ads on line for decent deals.  Someone offered an altimeter that worked, but failed an IFR certification due to out of tolerance leakage at 15,000′.  Well…  if I ever do finish the bipe and fly it, it certainly won’t be at anything over 10K, let alone 15K.  So for fifty bucks it was a no-brainer.  It’s a pretty nice altimeter, not one of the super cheap imported junkers that’s being sold now.  It’s really more altimeter than I need… 

My “new” used Shinko Electric altimeter

So…  I have an altimeter, and a quad oil temp / oil pressure / CHT / EGT gauge for monitoring the engine.  I will still need an airspeed indicator.  Most that I see for sale have speed ranges not appropriate for the Celebrity — I really don’t need or want a 200 knot ASI.  Then I’ll need a compass, of course; a slip/skid ball; a tachometer.  Whether I need a fuel gauge or not depends on what I decide to do for fuel tanks.  I’m thinking no electrical fuel gauges, all float or sight type.

I could just ditch all of it and go with a single instrument from Dynon or Garmin, but I just think such a thing would look out of place in a classic biplane.  I’m not sure what I’ll do for the front cockpit, though.  It would be nice to at least have airspeed and altitude there for passengers, but I’m not sure I’ll be able to score a deal like this again.  There’s always UMA

Sensor scored!

An update on the oil pressure sending unit.  I had found exactly one of the correct sending units for sale, a NOS Honeywell part that was listed on eBay for a couple hundred bucks.  I didn’t buy it, but I was watching the listing.  They dropped the price, I made a lower offer and they accepted.  I ended up paying roughly what a normal oil pressure sending unit would have cost from a source like Aircraft Spruce.  And, it’s got the exact output that the Westach quad gauge needs.  Now I won’t have to build a conversion unit but can just wire this one up directly.  These transducers are no longer in production, and if you can find one you’ll pay 8-10 times what I got this one for.  If it ever fails, I’ll probably replace it with a cheap one and a converter board — now that I have that figured out.

Update 5/4/17:  Transducer arrived, NOS as advertised with a nice sealed electrical connector.  This thing will likely outlive me.

Some advance planning

A while ago, I picked up a used Westach quad gauge.  it looked like a great deal…  oil temperature, oil pressure, CHT & EGT in one round gauge.  What I found was that it’s an older gauge, though in excellent condition — almost like brand new.  The only problem is, it’s designed to use an oil pressure transducer that is no longer produced.  0 to 100 PSI, with an output of 0 to 50 mV.  You can get transducers like that, but they cost a few hundred bucks.

Fortunately, there are other transducers that are dirt cheap ($20 and below)   that can be adapted to work with the Westach gauge.  I found some that have a 0-5V output, and looking at the spec sheet they actually output 0.5 to 4.5V.  They also require a 5V supply, and I think the gauge supplies 12V nominal.  So…  we have two tasks.  Supply the transducer with 5V regulated DC, and convert the 5V ouptut to 50mV.  Easy!

Below is a schematic for a little interface board to do the job.  A common 78L05 regulator supplies power to the transducer.  A simple resistive voltage divider does the 100:1 voltage conversion.  I used a 100K Ohm fixed resistor, and a 2K Ohm trimmer.  That way you can calibrate the divider to account for resistor tolerances.  Apply 2.5V to the transducer side, and adjust the gauge side for 25 mV.  There will be some non-linearity near the high and low ends of the scale, but that’s OK in this case.  We don’t need an absolutely accurate oil pressure indication, more of a relatively good indication within a reasonable range.  once you get very low or very high, the exact numbers aren’t really important.

Really, it was a great deal.  A new similar gauge would cost over $800.  Individual small gauges to do the same jobs would cost several hundred and take up a lot more panel space.  I’m pretty happy with this one.