Photocell help

[Mr. D.]

I pulled this photocell from an old nightlight. The problem is when I opened the nightlight everything kinda just came out, so I didn't get to see where the prongs were touching.

Can anyone tell me what would be power in and power out and grounds?

Thanks

 

[Cjcrescent]

I don't know much about electronics in this type of use, but I believe that the answer is on the circuit board itself. I assume that the two holes through the circuit board itself holds the key. One goes through the board on the center "prong" of the board, and the other is located in the "glob" in the center of the board.

These appear to be the only two places where something appears to have been either attached, or at least touching the board. Just look on the back of the board for these holes. The third hole, since it isn't attached to anything, could just be a locator hole, that helped attach the board to the rest of the night light.

Again this is total guesswork on my part. I apologize if I'm wrong.

 

[Mr. D.]

I don't know much about electronics in this type of use, but I believe that the answer is on the circuit board itself. I assume that the two holes through the circuit board itself holds the key. One goes through the board on the center "prong" of the board, and the other is located in the "glob" in the center of the board.

These appear to be the only two places where something appears to have been either attached, or at least touching the board. Just look on the back of the board for these holes. The third hole, since it isn't attached to anything, could just be a locator hole, that helped attach the board to the rest of the night light.

Again this is total guesswork on my part. I apologize if I'm wrong.

Those are not actually holes. The prongs that go into the wall socket touched some place giving the circuit power, but I don't know where.

 

[waltr]

Your pics are pretty good and I can draw a schematic from them.
What I need now is the markings on the transistor (the three legged black device) and the colors on the resistor (value coding). The colors like to be Blue, Green, Orange which would be a 54k Ohm resistor.

Do you have a multi-meter to measure the resistance of the photocell?
The photocell looks to be a photo-sensitive resistive cell known as a photocell or Cds.
Once I have those I'll post a schematic of the circuit.

The first draft of this board likes to have only two connections which are the traces that Carey pointed out with holes.
The resistor and the photocell form a voltage divider to the Base from the Collector/Emitter. I am sure that there is additional circuit in the nightlight that biases this circuit and actually switches the lamp on/off. It would be useful to know what the rest of the circuits looks like.

 

[Iron Horseman]

Can anyone tell me what would be power in and power out and grounds?

The photo cell is going to be connected to the base of the transistor. This connection is shown in this photo as the thin conductor through the center. The resistor is a bias to keep the transistor (ON or OFF) when the photo sell is not activated. So yes Cresent is right the power has to go to the one top center tab thingy. The other side of the power has to go through the load (a relay?) and then to the big blob thing (lower right). The problem is without knowing if the transistor is a PNP or NPN there is no way of knowing which is postitive and which is negative.

Do you know if the circuit activated when light shines on the cell or when the light beam is broken? That will make a difference too as to whether the transistor is normally ON or normally OFF.

 

[Mr. D.]

The photo cell is going to be connected to the base of the transistor. This connection is shown in this photo as the thin conductor through the center. The resistor is a bias to keep the transistor (ON or OFF) when the photo sell is not activated. So yes Cresent is right the power has to go to the one top center tab thingy. The other side of the power has to go through the load (a relay?) and then to the big blob thing (lower right). The problem is without knowing if the transistor is a PNP or NPN there is no way of knowing which is postitive and which is negative.

Do you know if the circuit activated when light shines on the cell or when the light beam is broken? That will make a difference too as to whether the transistor is normally ON or normally OFF.

Everything you said is Greek to me. LOL But this came out of a night light so when the lights were on, the nightlight was off. when it was dark, the nightlight came on.

There was no relay or anything, just this circuit and the prongs that went into the wall.

Your pics are pretty good and I can draw a schematic from them.
What I need now is the markings on the transistor (the three legged black device) and the colors on the resistor (value coding). The colors like to be Blue, Green, Orange which would be a 54k Ohm resistor.

Do you have a multi-meter to measure the resistance of the photocell?
The photocell looks to be a photo-sensitive resistive cell known as a photocell or Cds.
Once I have those I'll post a schematic of the circuit.

The first draft of this board likes to have only two connections which are the traces that Carey pointed out with holes.
The resistor and the photocell form a voltage divider to the Base from the Collector/Emitter. I am sure that there is additional circuit in the nightlight that biases this circuit and actually switches the lamp on/off. It would be useful to know what the rest of the circuits looks like.

The transistor says S705
Under that it says T B7 (or 87).

The resistor colors are Gold, Yellow, Silver, Blue

I do not have a multi-meter.

What I am needing is if someone could mark the points on the photo where I need to solder wires from the AC terminal on the CAB then to the accessory.

Thanks for any help you can give.

 

[waltr]

I was hoping to determine the transistor type from the markings but no luck so far.
The resistor colors are Blue, Gray, Yellow, Gold which is a 680k Ohm.

If you had an multi-meter than a measurement would tell us whether the transistor is a NPN or PNP.

If there really isn't any other circuit then the three terminal device would be something other than a bi-polar transistor.
You probably have the third circuit called "AC Relay" in this link:
http://www.ladyada.net/media/sensors/APP_PhotocellIntroduction.pdf

The 3-term device is a DIAC/TRIAC. Here is a data sheet for a likely device:
http://www.nxp.com/documents/data_sheet/ACT108-600D.pdf

This would make sense since for a night light since it only takes three components. However these will only work for the AC at the voltage and load it was designed for (the night light bulb & 120VAC).

 

[Iron Horseman]

This came out of a night light so when the lights were on, the nightlight was off. when it was dark, the nightlight came on.

There was no relay or anything, just this circuit and the prongs that went into the wall.

So the "load" would be the light. Is it a neon glow, regular incandesant bulb, LED, or other?

I was hoping to determine the transistor type from the markings but no luck so far.
The resistor colors are Blue, Gray, Yellow, Gold which is a 680k Ohm.

If there really isn't any other circuit then the three terminal device would be something other than a bi-polar transistor.
You probably have the third circuit called "AC Relay" in this link:
http://www.ladyada.net/media/sensors/APP_PhotocellIntroduction.pdf

Looking at those circuits I agree. That would still mean the center top tab goes to one power and the blob at lower right goes to the light creating device.

 

[Mr. D.]

I was hoping to determine the transistor type from the markings but no luck so far.
The resistor colors are Blue, Gray, Yellow, Gold which is a 680k Ohm.

If you had an multi-meter than a measurement would tell us whether the transistor is a NPN or PNP.

If there really isn't any other circuit then the three terminal device would be something other than a bi-polar transistor.
You probably have the third circuit called "AC Relay" in this link:
http://www.ladyada.net/media/sensors/APP_PhotocellIntroduction.pdf

The 3-term device is a DIAC/TRIAC. Here is a data sheet for a likely device:
http://www.nxp.com/documents/data_sheet/ACT108-600D.pdf

This would make sense since for a night light since it only takes three components. However these will only work for the AC at the voltage and load it was designed for (the night light bulb & 120VAC).

So using it for a simple crossing with LED's could work? Or am I wasting my time and just need to buy kits made for model railroading?

So the "load" would be the light. Is it a neon glow, regular incandesant bulb, LED, or other?

regular incandesant bulb

 

[waltr]

This circuit as is will not work at low voltages or to light an LED.

If it was cheap enough the photocell could be removed and used with another circuit at low voltage DC.

Since you have almost no electronics experience I suggest buy ready build circuits designed for what you want. Or a kit of parts if you want to start learning about building and using electronics.

I also recommend you buy an inexpensive DMM (Digital Multi Meter) sometimes called a DVM (Ditital Volt Meter). These can be had for under $20 and are invaluable for any work on electric or electronic circuits.