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Vintage threads from the first ten years

LDR emulation (LED and photocell)

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6/18/1999 3:29 AM
Terry	LDR emulation (LED and photocell) I am making a dod envelope filter 440 and i cannot get a hold of LDR's so i am going to emulate one with an LED and 2 photocells wrapped in tape... Since the schematic shows that there is a +/- for the led/ldr how do i know which end is which? Also how would i hookup these up? one photocell,LED,one photocell in series? THANX

6/18/1999 1:15 PM

Hello, Terry, and welcome back.

First of all "LDR" means "Light Dependent Resistor" so an LDR is equal to "photocell". You meant "LED/LDR optoisolator module" I think.

There are two ways to find polarity on an LED. Usually, the leads have been cut so that one is longer than the other. The longer lead is the + one, and the shorter the - in most cases, just like with radial aluminum electrolytic capacitors. The second is more direct - Just use a 1K to 4.7K resistor and a 9V battery in series with the LED. The LED will light up one way, not the other way. The way it lights is the correct polarity.

You hook them up physically by taping them together so the light from the LED falls equally on the two photocell/LDR sensitive surfaces. You hook them up electrically just like it shows in the schematic - the LED to the places shown for its leads, and the photocells/LDR's to other places. The LED is not connected to the photocells electrically.

6/18/1999 4:06 PM
Terry	Ok, i think i'm beginning to understand, but i am still confused about how to hook them up..... Does the LED only connect electrically to the cuircuit.. and/or where do the photocells get connected electrically? The schematic shows the LED/LDR as a single unit \|\| photocell \|\| LED connected in series? \|\| photocell THANX

6/18/1999 8:17 PM

Re: LDR emulation (R.G.)

Which schematic are you looking at?

Physically, the LED/LDR module is one unit. Electrically, the separate pieces are wired to different places.

The LED is wired to one section of the circuit. The wires from the two photocells are hooked to a different part of the circuit. Although the LED and LDR's are in the same physical lump, the wires from each part inside the lump go different places in the overall circuit. They all have to be connected to the right places to work.

6/21/1999 0:16 AM
Terry	Dod evelope filter 440 is the schematic, do you think you could tell me where to connect the led/ldr devices? thanx

6/22/1999 4:09 PM
Terry	Can someone PLEASE tell me how to hook up the photocells and led's?? THANX

6/22/1999 5:38 PM

Terry, I don't understand what you don't understand, and no, that's not sarcasm.

The photocell/LED module is enclosed in three dotted line sections on the schematic from GEO and from ualberta. You hook up the LED as shown on the LED dotted-box, each photocell as shown in the two other dotted boxes. The LED has to be hooked up in the correct polarity, the photocells are non-polar. You have to connect the electrical leads at the points in the schematic as shown, and "connect" the LED's light to the sensitive faces of the photocells.

Which part of that is not clear?

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