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| top down view of LDR and various schematic symbols for LDR |
Ive personally noticed that they are exceptionally common in 'circuit bent' devices/instruments offered for sale on various sites, particularly esuckBay. I believe this is because to the uneducated/initiated, adding 'optical theremin' or 'optical trigger' or 'magic eye' etc to the description makes whatever they are selling appear more impresive. Normally its simply someone replacing a potentiometer or fixed resistor with a LDR. With drilling and mounting, its probably something that took them all of 2 minutes to add. Not particularly impressive if you know what they've really done...but if you know, then you probably arent buying their INCREDIBLY overpriced junk anyway. I mean, 800$..EIGHT HUNDRED..for an old Alesis Hr-16 they've stuck some RCA jacks connected to the sound ROM & added a pitch bend via bodgy clock oscillator? crazy. And it screws everyone else cause now your average retard selling his hr-16 looks at what a search for 'hr-16' on eBay returns and thinks his old drum machine is worth hundreds of dollars! This screws us all because then that guy puts a 300$ reserve on his HR when he sells it and so forth..basically skyrocketing the price of the machines on the used market. Bogus. Apologies for the digression.
Anyways, we'll cover a few ways to add them to devices/instruments you are contructing as part of the 'easy bits' that I'm demonstrating on the blog. The idea being if you have enough little tricky bits you can add to a instrument you constructing or bending, you can essentially make a total instrument..mixing and matching different ones for differing outcomes.
A FEW THINGS TO KEEP IN MIND IN RE LDR'S:
-Luckily- CDs and most of the other less common formulations of LDRs operate primarily in response to similar wavelength spectrum as the human eye can discern. In other words, they respond to the same kind of light we can see. So lights we see or see as brighter/darker-they will respond to in response. (lighter-less resistance; darker-more resistance)
-They're cheap: the standard CDs cells (unless they are limited in your country by environmental regulation) can cost as little as .01 USD (a penny), and generally average about .15 USD (fifteen cents)
-They DO respond to temperature: so the amount of resistance varied by the LDR in circuit can vary pretty heavily depending upon temperature variance. So, if youre using an instrument that employs one in a recording situation, you might want to keep the Air Conditioning in mind.
-They have LAG aka Latency: meaning it can take up to 1 second (a lot of time in some situations, less in others) for them to respond to changes in light impacting the cadmium sulfide ribbon. Most LDRs average quite a bit less, to the point of being difficult to easily discern primarily, but a few can take longer.
Just keep those things in mind when making the decision to add one to an instrument or circuit you are working on.
1. Replacing existing potentiometer or fixed resistor with LDR:
a) normally LDRs have 2 leads: they are NOT polarized (neither is positive, and doesnt matter which you use in any given situation), so, to replace a fixed resistor in any given circuit or device etc then all you have to do is simply remove the resistor and
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| various sizes and values of LDR |
put the two leads for the LDR in the places where the resistor's leads formerly were. simple.
b) when replacing a potentiometer, its a bit more complicated as the potentiometer has 3 potential connections (lugs, leads, etc). However, it is also a variable resistance passive component-just like the LDR-so its not TOO complicated. Simply determine which of the 3 connections on the potentiometer/trimmer/variable resistor is the 'wiper' (usually the middle connection). One of the LDR leads will replace this. Then, determine which of the other leads replacement will result in the performance variance you desire via experimentation or investigation of circuit/schematic. (this means, if you want the LDR to affect the resistance between the wiper and ground, then replace the lead leading to ground and the wiper, if the other connection ..etc)
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| typical application of LDR replacing potentiometer |
c) Easy applications: often in circuit bending cheap toys or instruments, resistors will be used for clocks and you can replace them with the LDR via method A. This means that the amount of light striking the LDR will then determine the pitch alteration of the bent device.
2.Using as a control interface with musician/artist/device user. This is where you will most commonly see LDRs in circuit bent devices and things like guitar pedals on Ebay. What happens (im thinking), probably, is that some guy thinks "Well, Ive barely done anything to this toy that I bought for 3$ and I want to charge 700$ for it, but I've already stuck a cheap RCA panel on it (aka 'modular patch bay')...I know what I'll do..I'll stick a LDR in somewhere and put 'Optical control, Light Control, and/or Theremin' in the Ebay description field on the listing!!". There are lots of options here, most of the bogus ones you'll easily see on those cheap bent Ebay things and guitar pedals are similar to the illustration above left, but you can also :
- use other means to inhibit or project light onto the LDR: I've seen and done things like stuck salvaged computer cooling fans in between the light and the LDR, thus creating a "wobbly" or "tremelo" effect . There are lots of other ideas I'm not going to bother mentioning here, but really let your creativity loose, you know?
-use things like timers, pulses, and oscillators to control the light's emission: you can do things like the below right illustrated circuit which employs a lm1458 to create a repeating fading LED light so that when it hits the LDR there is a drawn-out response creating a 'sustain', or 'tail' effect. I've also combined this with the fan idea and created a 'sorta-reverb' effect on the synth VCO I was controlling.
THAT'S ALL FOR NOW, I've gotta run but we'll return to LDRs later.
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| image from GetLofi's 'get your fade on' article, link below |
http://getlofi.com/get-your-fade-on/
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