Electronics question

[Jeff Haas]

Hi Jonny,

I met you guys at the Maker Faire, I think I talked to you about my magic prop and showed you pics I had on my digital camera.  The sticking point I ran into is that the Prop-1 doesn't do sound effects, and the larger boards may not do that many either, and are larger than my project boxes will probably hold.

I had an idea and I don't know if this will work...but what if there was a way to hook up one knob visible on the outside of the project box and make the pot connected to it control two different circuits?  I have an analog circuit that creates the correct sounds and it's a pretty small board; then I think I could use the Prop-1 to control the lighting.

Is there a type of pot that controls two independent circuits at the same time?  Twist one knob, have each circuit read its section of the pot without interfering with the other circuit?  Or is this unimportant...can two circuits both share the same pot (which I suspect is a bad idea)?

Jeff

[Jeff Haas]

OK, I found a website that explained things in more detail...am I looking for a dual-gang pot?

Jeff

[JonnyMac]

Yes, Jeff, what you're looking for is a ganged potentiometer -- they are commonly used in stereo systems.

[Jeff Haas]

Thanks!  Now I have to find a ganged pot that will work with the existing analog sound board. 

I figure that reading the value of a pot and then responding to how it changes is pretty straightforward for the Prop-1.  However, does the value of the pot matter?  Or can you deal with whatever values are put out in the code?

[menehune]

Ganged pots have six terminals instead of the usual three.

[JonnyMac]

For using the POT command, 10K is the idea pot value.

[Jeff Haas]

menehune, that's exactly what I need...basically, two independent pots in one package.

Once I figure out how to make a 10k pot work with my existing analog audio board, then I can go forward with the Prop-1.

[Jeff Haas]

I've done some additional work on this, and have a few questions.

First, the existing sound board uses a 56k pot.  I can find dual-gang pots that are 50K, I'm going to see if the sound board will work OK with that value.

If it does, can the Prop-1 work with a 50K pot?

Second, I want the Prop-1 to drive a bunch of LEDs.  How many can it drive, assuming that they're in a Knight Rider type of display (among others)?  And, given that I want these to be seen from the widest possible angle on stage, which type of LEDs should I get?  This website seems to have good prices and selection on all types: Buy LEDs Online  What specs should I look for to pick the right type that will work with the Prop-1?

Thanks!  As I go along I should be able to post a schematic of the sound board and describe exactly what I want the Prop-1 to do.

Jeff

[menehune]

If the audiance is close to the display, you should get a LEd with a wide view angle.  If the audiance is far away you may be able to get by with a narrower viewing angle.
The further away the viewers are, the bigger the led needs to be for viewing.
Are the LEDs going to be in a shadowy area or a bright area?  A brighter area requires a brighter LED.  LEDs displayed against a black background or mounted inside a shadow box will be easier to see.

You can check the ULN2x03A Max Current thread for a chart that shows how much current the Prop1 can supply.  200mA per channel will drive 20 10mA LEDs (with appropriate current limiting resistors) per channel.

[Jeff Haas]

The audience will be unpredictable distances away, depending on the gig.  This is the deal with doing shows where ever people ask you to!  Sometimes it's in a living room, and sometimes it's on a stage for 350 people.

The LEDs will be mounted in a project box, so I can either just drill holes in the box and use some of those plastic holders to keep them in place.  Or, I've also played with some of those chrome LED reflectors, they act like the reflector in a flashlight.

Also, this will be a hand-held prop, so does the Prop-1 work off batteries?  It doesn't have to run long each show, so rechargeables are fine. Ideally a 9v.

[menehune]

The prop should be able to run off batteries.  There is an on-board regulator which reduces the voltage supplied to the processor to 5v, but I don't remember if the ULN chip runs off the 5v or V+ supply voltage.

I did check my Prop1 and the V+ screw terminal is wired directly to position 2 on the power switch which is wired directly to the power input jack.  So you may be able to use a 9v transistor battery if you recalculate the voltage drop across your LED current limiting resistor.  Due to the low current capacity of 9v batteries, your prop may not run for very long.  I have not tried running the prop at 9v.

It may be better to get 2 "4 AA battery holders" and 1 "2 AA battery holder" and make your own 12V rechargable battery back from AA rechargeables.  Or you could get a 12V 7AH rechargable battery.

Edit: Sorry, I missed the "Hand Held" part of your post.  The prop should be fine running from 9V rechargables if you test that the completed prop does not exceed the power available from the battery. 

[menehune]

I just did some checks on my Prop1.

At 12vdc, I measured an "idle" current of 29mA, power switch in pos 2, program running, FC4 powered off, no ULN chip.
With the FC4 on, I measured a current draw of 112mA, no uln chip, program running, no FC4 120Vac power applied.

So yes, the prop could run from a 9V, 200mA battery.  You would need to check how much power the LEDs require and how long it takes the 9v battery to drop to below the Prop's cutoff voltage-probably around 6.5v so the prop's onboard voltage regulator is still working.

The major component of power draw will probably be the LEDs.  More LEDs on = more current drawn = shorter run time.

Use 9V NiMh batteries if possible.  9V NiMh batteries provide 8.4 or 9.6V whereas 9V NiCd batteries only provide 7.2Vdc.  Higher battery pack voltage (should) equal longer run time.

[Jeff Haas]

Thanks for taking the trouble to check!

I'm thinking that 8 or 10 LEDs are enough for the effect I want.  They will probably run for about 5 mins total per performance.  The entire routine is about 7 1/2 mins long, so the prop itself will be on for less than that.

Gotta drop by the electronics surplus stores tomorrow and pick up a few more parts, then I can breadboard the sound board and try out a couple of things.

I still need to know if the Prop-1 can work with a 50k pot, since that's one of the few values that dual-gang pots come in.  Hope to hear from Jonny when he's back in town.

[JonnyMac]

Sorry, I thought I answered this before I dashed out of the hotel this morning -- I'm presently on a 10-hour break between trips... oi.

Yes, you can use a 50K pot with the POT command.  You may want to drop the cap for down to 0.047 or 0.001 when you do this.  In any case, you still need to calibrate POT function with the Run >> Pot Scaling... dialog.

[Jeff Haas]

Thanks, Jonny!  That solves another problem!

The challenge with this prop is that it's got the two boards in it, one for sound and the Prop-1, and they need to do things in sync at the same time, interactively.  The one thing I'm having trouble finding is a DPST momentary-on switch - which would turn on two independent circuits at the same time.  I may take the angle that one guy who restores arcade machines did...he built his own.  Still working on how to do it for this thing.