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Re: led/ldr


 
6/3/1999 10:41 PM
Terry
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Re: led/ldr
Dod ENvelope filter 440 is the schematic, is it completely necessary to have an led/ldr instead of a normal led?  
 
 
thanx
 
6/4/1999 6:07 AM
R.G.
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I looked at the schematic.  
 
Yes, it is completely necessary to have an LED/LDR instead of a normal LED.  
 
The variable resistance of the LED/LDR is what makes the frequency of the "wah" move.
 
6/7/1999 7:36 PM
Terry
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REally? Could this be the cause of a completely non-functioning cuircuit and bypass, because that's what mine does.  
I asked my uncle (an electrical engineer) and a few tech shop people about the led/ldr and none of them have heard of it.  
 
Could you answer a few more questions please?:  
 
1.In the schematic it has 2 opamp symbols, i have some LM358 low power dual opamps, do i only need one?  
 
2. How do i connect the opamps to the cuircuit, i have the manufacturer notes from motorola and it gives a pinout but i am still unsure on how to do this.  
 
3. IS there any way to "emulate" an led/ldr by using other parts olong w/ and led or something of the sort?  
 
4. Is there any special way i need to wire the pots for this, in the schematic it shows one pot connected to the led/ldr,to a 22k resistor and a 100k resistor, does the center prong go to led/ldr and the other's go to the corresponding sides? The other is connected to a 0.1uf cap and the ground, and there is a line coming from the other end of the pot.  
 
5.IS there anything i should check for when troubleshooting my pedal?  
 
 
THANK YOU
 
6/7/1999 9:20 PM
R.G.
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quote:
"Could this be the cause of a completely non-functioning cuircuit and bypass, because that's what mine does."
 
No, it would cause the frequency to be stuck at one place and never move, but it's not likely to be completely non functional.  
 
quote:
"I asked my uncle (an electrical engineer) and a few tech shop people about the led/ldr and none of them have heard of it. "
 
That doesn't surprize me much. I'm an engineer, 25 years experience, and I've never encountered them in my professional work, only my audio hobby/mania. They're a very old solution to some highly specific problems, not something that most engineers run into.  
 
quote:
"1.In the schematic it has 2 opamp symbols, i have some LM358 low power dual opamps, do i only need one?"
 
Yes. You only need one dual opamp, which contains two independent opamps inside it.  
 
quote:
"2. How do i connect the opamps to the cuircuit, i have the manufacturer notes from motorola and it gives a pinout but i am still unsure on how to do this."
 
For each opamp symbol, you relate the opamp symbol to one of the internal opamps, then connect the logical function of the pin in the schematic (for example, inverting or (-) input) to a pin number on the manufacturer's pinout. Since the pinout for all dual opamps is the same, I can tell you how this works out. Picking one of the opamp symbols, you associate the output pin to pin 1, the (+) input to pin 3 and the (-) input to pin 2. The other opamp symbol has output at pin 7, (+) input at pin 5, and (-) input at pin 6. Pin 4 is ground or the most negative power supply, pin 8 is the most positive power supply.  
 
quote:
"3. IS there any way to "emulate" an led/ldr by using other parts olong w/ and led or something of the sort?"
 
No really good ones, or there would be NO led/ldr modules. All the other solutions to having a floating voltage variable resistor suffer from some defects as well, as I think one of my just-earlier notes pointed out. The easiest thing is to order some LDR's from a mail order place like Mouser, and rig them up with an LED pointed at the sensitive face inside some kind of tube. The defects in the other solutions are enough to require significant redesign of circuits to use them. The one possible exception is the Qualtech H11F3 LED to photo FET analog optoisolator. This uses an LED and a photo FET to replace the LDR. They are more available and cheaper, but have a more restricted signal range. For audio work this is often enough.  
 
quote:
"4. Is there any special way i need to wire the pots for this, in the schematic it shows one pot connected to the led/ldr,to a 22k resistor and a 100k resistor, does the center prong go to led/ldr and the other's go to the corresponding sides? The other is connected to a 0.1uf cap and the ground, and there is a line coming from the other end of the pot."
 
You need to recognize the wiper symbol from the schematic and get it correctly hooked up, then connect the two outer lugs. If the mechanical rotation of the pot is backwards with your first try, just reverse the two outside lug connections.  
 
quote:
"5.IS there anything i should check for when troubleshooting my pedal?"
 
Lots, much more that I can type here. I can only do a very brief introduction to trouble shooting.  
1. fire the circuit up, ensuring that all the wires are as correct as you can make them and that parts are plugged into the sockets, if any. No signal yet.  
2. With a voltmeter (get one, and learn how to use it if you're going to do much of this) measure the battery voltage *both* at the battery and on the circuit itself. Trace and fix wires until you have good supply voltage as measured *on the board itself*.  
3. Measure the supply voltages *on the pins of the IC's* themselves. Fix wiring and/or soldering mistakes until the power supply appears on pin 8 and ground appears on pin 4 of all dual opamps. For other chips, consult the pinout and verify that each chip has the proper voltage on its supply pins.  
4. Measure the voltage at the junction of the two 22K resistors from the + supply to ground. The voltage at their junction should be 1/2 the battery voltage. Measure the voltage at each signal pin of both opamps. They should be with a few millivolts of the voltage at the junction of the two 22K resistors. If they are not, you have a bad connection, wrong wiring, or a bad opamp, this last being highly unlikely.  
5. If all the above is correct, you will have signal coming through it.  
If you have a good opto, you should get wah action.  
 
There's a lot more to it - I've only just scratched the surface.
 
6/7/1999 10:10 PM
Terry
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THanx for your help, i'm such a beginner...:)  
 
I can't really order from any online stores for parts mainly because i'd only need the one and never use the others that i'd also have to buy for a minimal order....  
 
You were talking about the Qualtech H11f3 LED to PHOTO FET analog optoisolator, so that device would replace the led/ldr? I would not have to be perfect , just as long as i get some effect from it. Would some other kind of LED to PHOTO fet analog optoisolator work? or is there some kind of variable resistor which i can use ?  
 
 
THANk you
 
6/8/1999 2:46 AM
Joe Fuzz
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Terry: I built something like this (well, am in the process of finishing it actually) called a Mutron 3. It also uses one LED to control 2 photocells.  
 
I used some spare parts I bought at Radio Shack to build my LED/LDR. They sell a package of 5 photocells for around $2. The catalog number is 276-1657. I took all 5 and measured the resistance when it was completely dark and when it was light. I then used the two that were the closest in value when there was light (because the dark resistance didn't seem to matter as much as long as they were both in the Mega-ohm range when dark).  
 
RS sells a wide variety of LEDs but you probably already have some LEDs on your bench that you can use. The LED I used was RS catalog number 276-304; it was an LED I just happened to have laying around.  
 
I took my LED and put it in the breadboard. Then I took the 2 photocells and put one on either side of the LED. I then bent the legs of the photocells so that the photocells' light-sensitive faces were turned towards the LED. A photocell on each side with the LED in the center. That was it. You can encase this in cardboard or some electrical tape if you want.  
 
Just a word of advice, if you don't mind: Don't get discouraged! You've chosen a tough effect to build here. If this is your first one you should know that a lot of the others are *much* easier.
 
6/8/1999 7:18 AM
R.G.
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quote:
"I can't really order from any online stores for parts mainly because i'd only need the one and never use the others that i'd also have to buy for a minimal order...."
 
Then you're looking at the wrong places. First of all, there are other places than online stores. Although most mail order places are beginning to get on line sites, most of the real work is still done through catalogs. Second, many catalog places do not have minimums, or charge a small fee under a certain dollar amount. In particular, Mouser Electronics does not have a minimum order requirement. The places that do have a minimum have a total-order-amount minimum, not a minimum number of each piece.  
 
You may need to read the Effects Economics 101 section of the Guitar Effects FAQ at GEO. In it, I point out that the real costs of an effect are NOT in the electronics in most case. Most of the time the costs are in the box, the jacks, the controls, the switches and so on.  
You can get cheap boxes, but you're going to need jacks, switches and controls, and by the time you get those, you're likely to be at or over the minimum order at most mail order places. Also, the per-part prices at the distributors is usually much less than at Radio Shack, so you may come out cheaper.  
 
The QT H11F3 is a single LED/photoFET, you'd need to use two, with the LED's in series. and that would replace the LED/ LDR. If you're thinking about that, also give some thought to getting two LDR's and one LED and cobbling them together so the LDR sensitive faces see light from the LED. I think that can even be built from Radio Shack parts.  
 
We all start as beginners. What separates us along the way is the willingness to dig for information. I urge you not to fall into the trap that I've seen many people do with electronics - don't try to get all your information from the internet. The internet is very wide, but it isn't very deep. You can get pointers and tips from the web, but you simply can't get the depth of information you need from it. For that, you still need books. Go to a library, find the technical books section, and start digging at beginning electronics, schematics and how to read them, all kinds of electronics books. Get in the habit of digging out books whenever you need information. People who aren't willing to do that never get very deep into electronics.
 
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