Neovibe bulb voltage?

[sfr]

I'm in the process of building the Neovibe and having a hard time getting the appropriate 12V/25mA bulb.   I've found some previous threads on here discussing the same problem.  Seems like the current rating is important, as getting the circuit to drive a higher current bulb is difficult.  How important is the voltage rating, however?  Considering that the bulb is only being driven to an orange light rather than full brightness if I understand correctly, could a lower voltage bulb be used in this circuit?  I have found bulbs locally, rated for 25mA, but less than the 12V rating (1.5V and 6v if I remember correctly)  Could these work?  Maybe if I replaced the trim pot with something of a higher value?

[sfr]

Sorry to bump my own thread here.

I have the entire board populated except for the photocells and and the lamp.   Used 2N3904 transistors throughout, and an In an attempt to answer my own question (above) I wired a few of the outboard components - the speed pot, the depth pot, and the power connection.  I figured this would allow me to get voltages off of the pads where the lamp should go, and give me an idea of whether or not a lesser voltage lamp/bulb would work.

I connected the input to the 18V DC (.35A) connection on my Gator G-BUS 8 power supply.  With the circuit connected, I show 18.34V DC before the diode bridge, but only 16.9 V on the other side of the diode bridge.  (The text says I should see greater than 17V, but I'm unsure if I'm supposed to be measuring across the bridge or between the diode bridge + and ground.  Interestingly enough, my meter seems to jump to around 20-24V for a moment when I connect the meter, and then back to 16.9V)

Measuring between ground and the points with voltages,  At the R39/R40/R41 junction point, I get 10V.  (The schematic says I should get 11.7)    I get 1.4V at R34.  (Schematic says I should get 5.4).  I get lesser voltages in most places.   

But measuring across the two pads for the lamp, I'm seeing numbers between 16V and 20V, depending on the settings of the depth pot and the trimpot. The value tends to go up and down over around the range of 1 or 2 volts, the speed of this seems to change according to the setting of the speed dual pot, which makes sense.  But these voltages seem too high, if this thing is supposed to be driving a 12V lamp, and considering that it's higher than the numbers I'm seeing at across the two terminals of the diode bridge.

So now I'm just confused.   Is any of this normal?  I haven't wired up input, output, volume pot or chorus switch.  Nor the photocells or bulb.  But I figured I should be able to take usuable measurements off this without those things.

[sfr]

Never mind . . .

Doing the another pass of trouble shooting (which, admittedly, I should have done *before* posting, I apologize) I realized I used a 7805 instead of a 7815 voltage regulator.  So using a 5V voltage regulator instead of a 15V is probably going to create all my lower voltages all over the circuit.  However, does that explain the lower value at the other side of the diode bridge and the rather high voltages at the lamp/bulb pads? 

(I guess the high voltages at the lamp/bulb pads could be caused by the lack of current draw with no lamp present - but I don't understand how they could be higher than the voltages at the other side of the diode bridge.)

Also while I'm asking stupid questions, there's one point in the layout where I'm confused:



Point A is the - connection off the "out" of the diode bridge.  Points B and C are the negative leads on C24 and C25, respectively.  The green line represents where I think I may have a short. (The pad at C etched a little large.) Of course, since I the lead loops around, I can't test it with my meter I get continuity regardless.  Since that is the case, I figure it's a non-issue.  However, I figure it it was a non-issue, the trace from point A would just run straight to point C, so maybe this is important?   

[Aharon]

Yes but there are caps connected to those pads,at best you have less filtering at worst your voltages may in fact be screwed cause you may have some weird RC network happening there,heck you can end up with a voltage doubler..........I don't know enough to tell you more.

As far as the light bulb,I have a working NeoVibe,used a "grain of wheat" light bulb,I know it's 12V or possibly more,amperage unknown.
The only mod I had to do is change the 200R trim to a larger one as per RGs suggestion.He suggested a 500R,did not have it,put a 1K and it worked fine,now I could tune the brightness of the bulb to taste.Mind you it was working before but I could not tune the brightness to my liking,lacked range.

Hope it helps
Aharon

[RedHouse]

Some great work RG did on that PCB, but if it has an issue, it's most definately that some of the traces are just too close for the average DIY guy. I made one but had to clean it up in Photoshop before I could get it to etch and build properly, to leave enough space between some pads and traces.

I also use grain of wheat bulbs, I think mine are like 14v/50mA and come with 1" long leads.
(I got tired of messin round with those Radio Shack bulbs with wire leads soldered-on)

I use a 500-Ohm trimmer and JC's darlington mod on my Univibes.

[Aharon]

The PCB is definately a hard one to make and populate,I consider myself an experienced builder and I had to use all my wits on this one.
The Neo is not one to be tackled as your first or second build.

When etching boards like this one make sure you have fresh etchant and for pete's sake do not use the dark setting on the printer!!!!!!!!!or you'll smudge the close traces together.
Aharon

[R.G.]

A light bulb is kind of a light-emitting resistor. It's resistance is not constant, but increases with increasing current. At its designed voltage/current point, it puts out the designed amount of light. The light output both increases and gets whiter as the filament gets hotter. It gets dimmer and redder as the filament cools. The total amount light output is a nonlinear function of the applied voltage x current or power in the bulb.

The difference between two bulbs both rated at the same current but different voltages is that the light output is approximately linear with power. At the rated power, each is the same temperature, so the light out is the same color. What is different is that the higher voltage bulb has more filament and more light output.

If you drive it from a voltage source (that is, it can eat all the current it wants at a given voltage) an incandescent lamp's life is inversely proportional to the thirteenth power of the  applied voltage. So even tiny changes in the applied voltage can drastically change the life of the lamp.

However, the 'vibe circuit drives the bulb from an almost constant current if the bulb is a low voltage one. So a 1.5V 25ma bulb will be 1/8th the total light output of a 12V 25ma bulb at the same current, and only drop 1/8 of the voltage. This is almost true for all voltages and really true at designed current/power.

LDR's have a HUGE range of resistance, so even an 8:1 difference it total light is probably OK.

The time response of a filament is dependent on its mass, emissivity, and things like what gas fills the bulb and whether or not the glass enclosure reflects or absorbs and reradiates the IR part of the filament's heat emission. But smaller always changes temperature faster than larger for like materials, so the 1.5V bulb will be faster at heating up and cooling off.

Which leads me to observe... hey, if what we want is an agile, fast light variance so the LDRs can change quickly, use several 1.5V 25ma bulbs. If what we want is slower, use a higher voltage bulb. That lets you tailor the speed response of the circuit because bulb cool-down time is a major part of the 'vibe's change in sound as you turn up the speed knob.

What's the perfect thing to use?

Unless you define "perfect" to mean "just like the originals", there isn't one. Experiment and make it work like you like.

[smallbearelec]

I have gotten made some samples of a 1.5 Volt/25 ma. bulb with a standard E-10 screw base. Two are on their way to Aharon for testing and two to J D Sleep. If they work well, I will buy bulk and have stock to post for New Year's.

SD

[powerplayj]

I ran into the issue of which bulb as well although my issue was too many choices rather than too few.  I will say that anything with a current over 25mA (say 50mA) will be hard to set with the trimmer as the bulb may not glow at all at the low intensity setting.  Even with the recommended 25mA, 12V bulb I encountered a problem similar to what RG talks about with the time response of the filament.  I assume this must be causing the "throbbing" symptom I am experiencing as changing the wet/dry mix has no affect on the result.  Even with the right parts this thing seems to need some tuning.  Don't know where you are located but I found the 12V, 25mA bulb at radio shack.

[alfafalfa]

Thanks Aharon for your reply,
I'm going to try what you suggested but the lamp only starts glowing when there is no resistor but a simple wire.

I have gotten made some samples of a 1.5 Volt/25 ma. bulb with a standard E-10 screw base. Two are on their way to Aharon for testing and two to J D Sleep. If they work well, I will buy bulk and have stock to post for New Year's.

Did you experiment with these ? 
If not what lamp are you using now ?

Alf

[Aharon]

I did the test for Steve when he sent me the bulbs and it worked exactly the same as with my original.
Aharon

[alfafalfa]

If you used the 1.5 volt / 25 mA lamp how did you get the voltage down  to 1.5 volt and are still able to have the right voltages for the rest of the circuit ?

Alf

[Aharon]

I did nothing to change the voltages,I just plugged it right in.........I did not measure any voltages cause mine worked right off the bat but I'm guessing  the bulb does not see the full voltage ever or if it does it does for very brief periods.........just my guess,I haven't looked at the schem in months.
Aharon

[RedHouse]

...if what we want is an agile, fast light variance so the LDRs can change quickly, use several 1.5V 25ma bulbs..

Havent looked at this post since posting, sory I didn't.

I have had a quite similar thought on that in the past but never pursued it, glad to hear confirmation on that RG.