Introduction: Optical Potentiometer for a DIY Amplifier

This instructable needs a short foreword. The title states that the pot is for an amplifier and not for any amplifier, but for a DIY amplifier - this is because I developed information technology for such an amplifier, but you, of course, might find a better exercise for it :)

Even so, it has confident characteristics, which go far interesting for audio applications, and DIY amplifier might have enough place for it. Above, I used the verb "formulated" for a purpose - even though the approximation is quite a simple, I never adage IT on the Web nor elsewhere. I didn't do a deep patent search and I wouldn't constitute popeyed if somebody will point out that a similar theme was patented back in 19xx or so (an update - I found a commercially available crapper for an exorbitant amount, which uses the same principle, merely is dignified of a single resistance and a undivided photoresistor, so it can't put back an audio pot; another article describes a pot with a reflective stripe, which is also not what is needed). In any case, feel rid of to use it for non-commercial purposes. The others are welcome to contact me :)

What is a need for such a device? I guess, most of you make faced with a pot of some sound device, which started makind a hissing sound in the loudspeakers when you turn it and you know that some Day this pot leave start losing unitary channel then you'll have to replace IT. In the world of HiEnd audio, these pots have been replaced with the 24- Oregon 32-position rotary switches, which ensured a proper reach. Nowadays, the volume is programmed electronically and the modern amplifier has an encoder attached to a intensity knob, sol it's more of an stimulation device rather than a rattling potentiometer. Still, at that place're a lot of devices, which use a "median" audio pot to adjust the volume and this instructable is for the people design or repairing much devices.

Step 1: Explaining the Idea

The idea is as simple as the electronic scheme above. Envisage that we have a potentiometer, which is nerveless of two photoresistors Rph2 and Rph1. If we had a light source and if we could redistribute the flux between these photoresistors, we would change the Vout/Vin ratio in the Min(Rph1)/(min(Rph1)+Georgia home boy(Rph2)) ... max(Rph1)/(max(Rph1)+min(Rph2)) limits.

To adjust these limits and the candle, one can use the geostationary resistors R1 and R2, which will make up discussed below.

Footmark 2: Redistributing the Immix

One can ideate various schemes of fluxion redistribution between Rph1 and Rph2, simply since we are public speaking about an sound pot, it would follow sensible to bind this redistribution to the rotation of the volume thickening. The 1st idea, which comes to mind, is shown in the figure. The Rph1 for the left and right channels are mounted on one side of the PCB, two other photoresistors are mounted connected its backside and the PCB itself is mounted in the rotating enclosure, illuminated from one side. This apparatus ensures a smooth sharpen because of the cosine of the angle between the photoresistor's normal and the focal point to the light source. It's needless to say that for a given position the illumination of L and R photoresistors should be equal. This is achieved by using a disjointed glow or by a source placed at a sufficient distance - the choice is up to you.

Step 3: Choosing Your Trend

In audio equipment, the log pots are used Thomas More often than linear ones because the response of human auricle to the loudness of sound is logarithmic. Making a custom-kink corporation is difficult for an amateur unless he/she uses an aforementioned rotary substitution, but an optical pot provides you endless opportunities. Imagine, for example, a tinted film occupancy front of Rph1 and Rph2 - aside changing the density, one can fit any curve.

But, the setup described in this instructable aims to reproduce a exponent curve or some smooth curve, which would resemble a logarithmic same. The Public figure shows the results of simple calculations for a couple of pairs of (R1;R2) coupled with real-life photoresistors available at RadioSchack Oregon elsewhere. As unrivalled can see, the optical pot has two drawbacks w.r.t. electrical phenomenon one: (1) the level-headed is never fully cut and (2) the sperm-filled volume is ne'er reached. However, the curves give you an idea of what can comprise done to improve the situation - either the number of photoresitors has to atomic number 4 increased (check the curves with N=2) or a brighter source should be taken (check the curves with F=2). The sharp transition near 90 deg is non that sharp in the reality because the light is scattered and the simple model does not include the spreading operating room the shadow of the enclosure bulwark. In the real setup, the pot is characterized by a S-curve, which is non a power one, but which is idyllic for the spike.

Step 4: The Instructable Itself

If you want to regurgitate this variation of the optical pot, you will need:

(1) a image PCB of about 3x5 cm

(2) 4 photoresistors, which change their values from 10K to 0.3K under pattern conditions (lilliputian flashlight)

(3) a set of 1-5-10-20-50-100-200K resistors (you can opt a pair from the curves above, but you mightiness also want to experiment a bit)

(4) a length of one-inch PVC pipe

(5) a solder, a multimeter, and a saw.

(6) wires, screws, het glue

(7) a flashlight or any other source of light (I secondhand the light ballast lamp of my parafeed subway amplifier)

Once you've collected these items, the rest is straight - swing the PCB to fit the PVC pipe, solder the photoresistors and regular resistors according to the circuit diagram, cut two windows in the PVC pipe, order the PCB inside the pipe, solder the pot wires, mount the pipe and connect information technology to the volume knob (I did information technology using a flexible hose to have a fluent rotation), backing the light and point it at the photoresistors.

IT'd atomic number 4 a good idea to anticipate a theory of replacing R1 and R2 without unmounting the whole organisation. If you find that the auditory sensation is excessively loud in the leftmost position of the pot, decrease R1 or increase the bioluminescent flux.

If you did everything right, you'll have a nice command of your music and the pot's hissing, clicks and otherwise sounds will be at rest forever. The pot itself does not introduce any coloring to the sound, and there's no XT from 50-60Hz light sources, either. The latter is explained by a "running mean" nature of photoresistor - information technology takes a while ahead the alteration of the stimulant flux turns to a change of the resistance. This creates a weensy wait of volume change, which is a feature and not a hemipteron. The loudness increases smoothly and the overall impression of this pot is quite good. Hope IT will be your encase, too.


That's it, thanks for reading :)

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