Need To Drive a Meter "For Show Purposes Only"

[VE4ANC]

Hi Folks:

During the summers, I fly waterbombers against forest fires. During the "down times" when we're waiting for the phone to ring with a dispatch call, I'm working on an electronics project to to fill my time and be of benefit to my fellow bomber pilots in the group.

I'm building a flight training device (sometimes called a flight simulator) and am in the process of interfacing some real Convair 580 cockpit parts to a PC via a commercially manufactured USB interface.

Things are proceeding very well, and by the end of the summer, it will be in a state where it can be used for pilot training in SOPs .. Standard Operating Procedures Calls, and emergency actions.

I've been away from the basics for a long time and need some help with a homebrew project which might tickle the fancy of a few of you.

The aircraft uses 24 volts on battery, and 28.5 volts DC when engines are running. I need to simulate "action" on the load meter.....especially in the "test" position for an anti-ice system. These load meters are 500 mv full scale, which equates to 1.25 on the Load Meter. They are normally used with a shunt to measure current on the output of the Alternators/generators.

Pressing the "test" function on the Pitot Heat system gives approximately 1/3 scale deflection if the system is serviceable.

That's the action I need to get done, and it would also be really slick if I could simulate an appropriate increase in loadmeter when a particular service is turned on say ... the landing lights. As long as I'm dreaming, it may as well be in clor ... also, it would be good to have this controlled by a pot throughout full scale meter range.

So how do I calculate and generate the stable voltages required to drive the meter directly?

Thanks for the assistance on this, and I really appreciate your taking the time to help out.

73 de
Lee Smith
VE4ANC and also VE4WLS

[K7JEM]

That wouldn't be very hard to do at all. What you need to do is determine all the different things you want that will make the meter deflect, then run resistors to those voltage points. You could make each resistor a variable pot if you wanted each thing calibrated, or just a fixed resistor of a "close enough" value otherwise.

What meter readings do you want, then calculate the series resistor values.

Joe

[VE4ANC]

'Ya know, the penny just dropped for me ... one of those "of course, why didn't I see it before?" moments ...all I need here is to wire the meter as a voltmeter with multiplier resistors.

The internal resistance of the meter should be around 500 ohms and 1 ma for the .5 volt fsd. From there, its just calculate the individual resistors and run them in series.

Thanks for clearing the mental logjam guys.

73 de
Lee
VE4ANC and VE4WLS

[M0DSZ]

If you are picking up several sources of DC it might be a help to put a small diode in series with each source.

I don't suppose you want to go to the trouble of making a "virtual earth" mixer with an op-amp? This would mean that setting up individual pots wouldn't affect any of the others.

[G7JTB]

If you are into micro controllers and simple programming a pwm output could be used. Scale the series resistor to give FSD at supply voltage then meter deflection will be proportional to pwm value. Different inputs could then be programmed to give whatever deflection, (pwm value) you want.

Ron