Expression

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Expression

Aaron Laws
I have an Allen t12 (something like http://www.netinstruments.com/pics/39973.jpg). I think it's from the late 50s or early 60s. I'm trying to figure out how the expression pedal works. I see what appears to be a capacitor on the right (looking at it from the rear of the organ), and two wires coming out of the left. One of the wires goes to what appears to be an adjustable resistor.

I'm assuming the expression pedal acts as a rheostat (variable resistor or potentiometer), but I've put my ohm-meter all over this thing, and can't detect any variation in resistance anywhere as the pedal moves.

Does it actually act as a variable capacitor or something exotic like that? Any ideas on what I should be looking for so I can use it in my VPO? Thanks!

In Christ,
Aaron Laws

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Re: Expression

tbeck
Hi Aaron,

There are some very knowledgeable people on the organ forum. I think this question has been discussed there. Check it out. The members are very friendly and helpful.


Tom Beck

On Mon, Mar 6, 2017 at 4:28 PM, Aaron Laws <[hidden email]> wrote:
I have an Allen t12 (something like http://www.netinstruments.com/pics/39973.jpg). I think it's from the late 50s or early 60s. I'm trying to figure out how the expression pedal works. I see what appears to be a capacitor on the right (looking at it from the rear of the organ), and two wires coming out of the left. One of the wires goes to what appears to be an adjustable resistor.

I'm assuming the expression pedal acts as a rheostat (variable resistor or potentiometer), but I've put my ohm-meter all over this thing, and can't detect any variation in resistance anywhere as the pedal moves.

Does it actually act as a variable capacitor or something exotic like that? Any ideas on what I should be looking for so I can use it in my VPO? Thanks!

In Christ,
Aaron Laws

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Re: Expression

Leisesturm
In reply to this post by Aaron Laws
I didn't see your question in the forum suggested by the previous responder. As I understand it, the variable resistance is provided by a light sensitive resistor: a photocell. What the expression pedal controls is a variable width slot which allows more or less light (supplied by a small lamp) to strike the light sensitive area of the photo-resistor. It is a testament to their durability that yours still operates as designed.

For the purposes of a VPO I would remove the photocell assembly and use the expression pedal mechanism to control the expected 5k or 10k variable resistance expected by modern MIDI encoders.

<quote author="Aaron Laws">
I have an Allen t12 ... ...I'm assuming the expression pedal acts as a rheostat (variable resistor or
potentiometer), but I've put my ohm-meter all over this thing, and can't
detect any variation in resistance anywhere as the pedal moves.

Does it actually act as a variable capacitor or something exotic like that?
Any ideas on what I should be looking for so I can use it in my VPO? Thanks!


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Re: Expression

Aaron Laws
On Thu, Mar 9, 2017 at 2:29 PM, Leisesturm <[hidden email]> wrote:
I didn't see your question in the forum suggested by the previous responder.
As I understand it, the variable resistance is provided by a light sensitive
resistor: a photocell. What the expression pedal controls is a variable
width slot which allows more or less light (supplied by a small lamp) to
strike the light sensitive area of the photo-resistor. It is a testament to
their durability that yours still operates as designed.

For the purposes of a VPO I would remove the photocell assembly and use the
expression pedal mechanism to control the expected 5k or 10k variable
resistance expected by modern MIDI encoders.


I have an Allen t12 ... ...I'm assuming the expression pedal acts as a
rheostat (variable resistor or
potentiometer), but I've put my ohm-meter all over this thing, and can't
detect any variation in resistance anywhere as the pedal moves.

Does it actually act as a variable capacitor or something exotic like that?
Any ideas on what I should be looking for so I can use it in my VPO? Thanks!


I would never have guessed that. At first, I was going to respond simply with "I don't believe you." because it's so unexpected! It worked before I gutted the instrument.

I don't have a midi encoder; I've wired this one up "from scratch": https://www.youtube.com/watch?v=n-vsQvJ7Yoo so it should be simple to get a voltage divider or something that the ADC on teensy LC can read. Thanks a ton for shedding light on this ;-)

In Christ,
Aaron Laws

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Re: Expression

John Reimer
Administrator
Aaron Laws wrote
it should be simple to get a
voltage divider or something that the ADC on teensy LC can read.
Aaron,

If your swell pedal uses a lamp/LDR combination then of course it is no surprise that you could not get a variable reading, if the lamp power supply was not functioning. However, I would not dismiss the use of a lamp/LDR combination to use with some kind of MIDI encoder. When the swell pedal potentiometer on the analogue electronic organ in my local church began to give trouble (I actually built that organ back in the late 1970’s), I rigged up an LED and LDR to take its place, the LDR forming part of a resistive network which delivered a varying voltage to the circuitry. When midifying that organ just over a year ago, I found I could use the LED/LDR to feed a varying voltage to the MIDI encoder. It worked first go, without having to change the relevant MIDI messages in jOrgan.

John Reimer
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Re: Expression

Aaron Laws
On Fri, Mar 10, 2017 at 5:21 AM, John Reimer <[hidden email]> wrote:
Aaron Laws wrote
> it should be simple to get a
> voltage divider or something that the ADC on teensy LC can read.

Aaron,

If your swell pedal uses a lamp/LDR combination then of course it is no
surprise that you could not get a variable reading, if the lamp power supply
was not functioning. However, I would not dismiss the use of a lamp/LDR
combination to use with some kind of MIDI encoder. When the swell pedal
potentiometer on the analogue electronic organ in my local church began to
give trouble (I actually built that organ back in the late 1970’s), I rigged
up an LED and LDR to take its place, the LDR forming part of a resistive
network which delivered a varying voltage to the circuitry. When midifying
that organ just over a year ago, I found I could use the LED/LDR to feed a
varying voltage to the MIDI encoder. It worked first go, without having to
change the relevant MIDI messages in jOrgan.

John Reimer

Thanks for the encouragement. Is there any advantage to using a light-dependent resistor rather than a rheostat? I guess these things are usually passive? (no carrier voltage required?) In other words, I should be able to put my ohm-meter on the correct leads, then shine a flashlight in the right place and get a reading?

In Christ,
Aaron Laws

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Re: Expression

BrianS
Aaron,

I the days of analogue organs, the swell pedal had to be LDR and light, in order to eliminate physical wear on an a  rheostat type of volume control.

In the days of midi messages, there will not be any noise if using a rheostat or potensiometer , but the midi missages might go mad if the rheostat is worn out or dirty.

Brian.
Regards,

BrianS
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Re: Expression

Leisesturm
In reply to this post by Aaron Laws
The 'advantage' would likely be the fact that the LDR system is already wired in place in your console. A LDR is a 'passive' device just like a rheostat (variable resistor), neither one generates a voltage or current, they modify an existing voltage/current. In the Allen console, the LDR and the activation lamp are separated by only fractions of an inch. The lamp probably appears fairly dim from a human point of view, but the close proximity, and the lack of any spill due to the assembly being sealed in some kind of capsule means the LDR 'sees' and needs a fairly intense luminous flux to make any significant change in resistance.

I wouldn't know the actual mechanics of measuring electrical components ("I'm just a simple country musician, Jim") but to measure a resistance, your volt/ohmmeter has to be placing some kind of known 'carrier voltage" on the LDR so the meter can determine its resistance to said voltage. A flashlight may, or may not be, bright enough or focused enough to do a test, and the light sensitive area of the LDR may, or may not be, accessible to ambient light. <shrug>

A verbose way of saying that IMO the best way to test your LDR might simply be to wire it up to a MIDI encoder and see what you get.



<quote author="Aaron Laws">

Thanks for the encouragement. Is there any advantage to using a
light-dependent resistor rather than a rheostat? I guess these things are
usually passive? (no carrier voltage required?) In other words, I should be
able to put my ohm-meter on the correct leads, then shine a flashlight in
the right place and get a reading?

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Re: Expression

Marc-Paul
Greetings:  I have an Allen 705 with a couple of those LDR systems... they are almost impossible to make work with midi... the amps are modified to work with their peculiarities.  A flat slider pot is pretty easy to slave into the console.  I slaved a Yamaha volume pedal that outputs more normal resistance and it works good.
Cheers
Marc-Paul



-----Original Message-----
From: Leisesturm <[hidden email]>
To: jorgan-user <[hidden email]>
Sent: Fri, Mar 10, 2017 11:27 am
Subject: Re: [jOrgan-user] Expression

The 'advantage' would likely be the fact that the LDR system is already wired
in place in your console. A LDR is a 'passive' device just like a rheostat
(variable resistor), neither one generates a voltage or current, they modify
an existing voltage/current. In the Allen console, the LDR and the
activation lamp are separated by only fractions of an inch. The lamp
probably appears fairly dim from a human point of view, but the close
proximity, and the lack of any spill due to the assembly being sealed in
some kind of capsule means the LDR 'sees' and needs a fairly intense
luminous flux to make any significant change in resistance.

I wouldn't know the actual mechanics of measuring electrical components
("I'm just a simple country musician, Jim") but to measure a resistance,
your volt/ohmmeter has to be placing some kind of known 'carrier voltage" on
the LDR so the meter can determine its resistance to said voltage. A
flashlight may, or may not be, bright enough or focused enough to do a test,
and the light sensitive area of the LDR may, or may not be, accessible to
ambient light. <shrug>

A verbose way of saying that IMO the best way to test your LDR might simply
be to wire it up to a MIDI encoder and see what you get.





Thanks for the encouragement. Is there any advantage to using a
light-dependent resistor rather than a rheostat? I guess these things are
usually passive? (no carrier voltage required?) In other words, I should be
able to put my ohm-meter on the correct leads, then shine a flashlight in
the right place and get a reading?





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Re: Expression

Aaron Laws
In reply to this post by Leisesturm
On Fri, Mar 10, 2017 at 12:20 PM, Leisesturm <[hidden email]> wrote:
The 'advantage' would likely be the fact that the LDR system is already wired
in place in your console. A LDR is a 'passive' device just like a rheostat
(variable resistor), neither one generates a voltage or current, they modify
an existing voltage/current. In the Allen console, the LDR and the
activation lamp are separated by only fractions of an inch. The lamp
probably appears fairly dim from a human point of view, but the close
proximity, and the lack of any spill due to the assembly being sealed in
some kind of capsule means the LDR 'sees' and needs a fairly intense
luminous flux to make any significant change in resistance.

I wouldn't know the actual mechanics of measuring electrical components
("I'm just a simple country musician, Jim") but to measure a resistance,
your volt/ohmmeter has to be placing some kind of known 'carrier voltage" on
the LDR so the meter can determine its resistance to said voltage. A
flashlight may, or may not be, bright enough or focused enough to do a test,
and the light sensitive area of the LDR may, or may not be, accessible to
ambient light. <shrug>

A verbose way of saying that IMO the best way to test your LDR might simply
be to wire it up to a MIDI encoder and see what you get.

This all seems sound, but the question to me is: how do I get the light on in there? How many volts? AC or DC? Which leads? The original wiring for the organ is long gone. I realize it's not likely for you to know the answers to these questions! Thank you very much for your help!

In Christ,
Aaron Laws

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