# Trying to find a suitable photocell replacement

#### montag1138

Joined Aug 28, 2021
25
I'm trying to recreate a 50 year old lighting effect circuit, and I'm having a very difficult time with a photocell. I'm hoping someone can help me figure out what numbers I should be looking for.

The original component is a Clairex CL5M4 CdSe photoresistor (which is coupled with 5 LEDs which flash at "random" intervals.) The basic component data is readily available online, but I cannot figure out how to apply that to a DigiKey or Mouser search result.

The important stats are:
Illuminated resistance 1.5k @ 2ft-candles (≈ 21 lumen)​
Min dark resistance 400k @ 2ft-candles (≈ 21 lumen)​
Max voltage 250V​

1: Photocell resistance is often given @ 10 lumen. Should the above numbers be divided by half or multiplied by two? (My thinking is that the light would appear brighter as the cell moves closer to the source, so multiply by two)

2: In most datasheets, illuminated resistance is given as a range, ie. "0.5 ~ 17kOhms @ 10 lumen". I don't understand how this range applies to the single resistance value given for the original component.

3: Finally my choices become very limited when I select 250V. I know to choose a component rated higher than the highest possible voltage, but I don't actually know what voltage is being applied to the photocell. How can I use a multimeter to measure the voltage applied to the photocell? (I'm worried about damaging an irreplaceable 50 year old device.)

Any advice would be greatly appreciated!

Here's a video of the original circuit in action if anyone is curious. The photocell and LED array are on the left inside a black plastic canister. (I'm sure that today there's a better way of achieving the random resistance created by the photocell/LED combination, but I want to try recreating it as designed first.)

#### montag1138

Joined Aug 28, 2021
25
Just an update to this question: The responses I received in a different forum said to try a handful of different LDRs (light dependent resistors) as they aren't very precise and the circuit has two potentiometers to tweak the performance. One recommendation was a 5528.

#### bertus

Joined Apr 5, 2008
22,277
Hello,

The important stats are:
Illuminated resistance 1.5k @ 2ft-candles (≈ 21 lumen)
Min dark resistance 400k @ 2ft-candles (≈ 21 lumen)
Max voltage 250V
Keep in mind that the 400k is reached after 5 seconds according to the posted datasheet.

Bertus

#### montag1138

Joined Aug 28, 2021
25
I'm afraid my choices are very limited. 400k at 5 seconds isn't an option, and the LDRs which are in that neighborhood seem to be too far out of spec.

This is the reason I asked how to measure the voltage being applied to the photocell. If I look only at LDRs rated at 250 volts, I have only 2 choices. Selecting 150 volts returns 25 results... Knowing the voltage would give me several more options to choose from. Unfortunately across 5 different forums, none of my three questions have been answered. The advice to pick a bunch and try them is all I have to go on.

#### BobTPH

Joined Jun 5, 2013
8,958
It looks to me like the photocell is controlling a neon light, which means is probably has line voltage across it, so the high voltage rating is needed.

I am confused by your comment about LEDs. Are there LEDs in the old device?

And by recreating, do you mean you are using a bulb like the one in the video? If so, where did you get such a thing?

Bob

#### ElectricSpidey

Joined Dec 2, 2017
2,777
You are trying to recreate a device that you don't understand how it works?

Do you have a schematic?

You asked how to measure the voltage across the photo cell, if you have a working unit just use a multimeter and place the probes across the cell, just be sure the meter is in the proper range setting.

#### montag1138

Joined Aug 28, 2021
25
Hi Bob-
The circuit board is mine; it's a discarded fire simulator from Disneyland. I wired it up and put it in a project box.

The bulb shown in the video is incandescent (I was surprised to discover that it was the only incandescent bulb in my house! Fluorescent bulbs don't work with the device, and all my other lightbulbs are non-dimmable.)

There are indeed LEDs in the device: On the left side is a black canister which contains the photocell and five LEDs. The LEDs flash in a pseudo-random sequence, increasing and decreasing the resistance in the photocell, causing the output voltage to fluctuate, which goes out to the light bulb or whatever is plugged into it. I know that today it's a very inefficient way to accomplish the effect but my goal is to recreate it, partly because it's very old, partly because I would like more variety (anything plugged into it will have the same pattern--imagine several old fashioned lanterns unconvincingly flashing in unison) and mostly because it's a fun challenge.

I'm hoping that I can use a photocell rated at 150 volts. If it must be 250 volts my choices are very few. Line voltage goes through a few resistors and capacitors before reaching the photocell, and I'm not sure how that affects the overall voltage. Checking it with a multimeter would tell me if I can use one rated lower than 150 volts, but I don't know how to measure it... if there's a way to mess it up I would probably find it.

#### montag1138

Joined Aug 28, 2021
25
Hi ElectricSpidey-
So just set it at 200 volts AC and touch the multimeter to each side (I assume polarity doesn't matter)?

Yes I have the schematic, I'll attach it. I understand mostly how it works, I just don't know how to shop for LDRs/photocells.

#### ElectricSpidey

Joined Dec 2, 2017
2,777
I see what looks like a Triac just below the transformer, is there a voltage rating marked on that.

I looked up the Triac, and it looks like it's rated @ 200 volts.

I would measure the cell using the maximum AC voltage setting on your meter at the maximum brightness setting and the base at minimum then look for the highest readings...it will vary. (actually I would select all the different settings)

The Triac dimming circuit looks pretty typical.

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