Will this simple photocell circuit work?

Steve Lawson,
Thanks for your response of earlier today. I have a further question, one that most electronics people wouldn't even have to ask, I'm sure. From what I gather, the three prongs of a transistor are called the base, the emitter, and the collector. Based on what you posted earlier today, I gather that the transistor base is the middle prong in the circuit picture you posted today. Correct? What I want to know is, is the emitter the left prong and the collector the right prong? I assume it's important for me to get that right. .......... Also, you mentioned once before that I might need to use a Darlington transistor or a MOSFET. It's no big deal for me to go to RadioShack -- should I just go ahead and buy one of those and forego the PN2222A? If so, can you give me specs on them ........... Finally, you mentioned earlier today that I might need to "tweak" the 47K-Ohm resistor. Would I accomplish that by buying resistors at about 40K, 35K, 30K, 50K, 55K and 60K? Or would another way to do that be to take out the 47K resistor and put in that same place a trimpot? What specifications would the trimpot have? Would it have a resistance that was adjustable from, say, 10K Ohms up to 300K Ohms, something like that?

And please, don't worry about offending me rergarding my "lack of experience." Your reference to my level of electronics knowledge (shall we call it unknowledge?) is extremely gentlemanly, totally unlike Colin, who seems to revel in making a point of his superiority to me in this field. I can't tell if you work as a teacher or not, Steve, but you definitely conduct yourself the way any student would appreciate.

Oddly, the text of your latest reply to this thread, emailed to me, is not present here, so I will work from the email:

bq). I have a further question...From
what I gather, the three prongs of a transistor are called the base, the
emitter, and the collector. Based on what you posted earlier today, I gather
that the transistor base is the middle prong in the circuit picture you
posted today. Correct? What I want to know is, is the emitter the left prong
and the collector the right prong? I assume it's important for me to get
that right.

If you are using a PN2222A, and the flat face of the transistor is facing you and the "prongs" (commonly call them "leads" or "pins") are extending downward, then the Emitter is the left most lead, and the collector is the right most lead.

Now, I challenge you to Google "Pn2222A" and find a datasheet. Look for a hit from the manufacturer of the transistor you have, or one of the major manufacturers if you can't discern the manufacturer: Fairchild, OnSemi, TI [Texas Instruments], Vishay, NXP, etc.

In the datasheet you will find a "pin-out" (at least, in most cases -- if not find a datasheet from a different manufacturer, the specs are pretty standard)

Another technique is to go to the Digi-key (or Mouser, or Jameco, etc.) website and search on the part number and then, in the listing select a part (preferable from the manufacturer of your part, but otherwise if needed) and then usually there is a link to a datasheet.

bq). Also, you mentioned once before that I might need
to use a Darlington transistor or a MOSFET.

Let's not get ahead of the game. Get it working with the PN2222A and LED, first, then we'll work from there.

bq). Finally, you
mentioned earlier today that I might need to "tweak" the 47K-Ohm resistor.
Would I accomplish that by buying resistors at about 40K, 35K, 30K, 50K, 55K
and 60K? Or would another way to do that be to take out the 47K resistor and
put in that same place a trimpot?

If you look closely at the new diagram, it's no longer a 47K. It's now a 470Ω (like the one that goes from the collector to the LED (i.e. in the "collector leg"). And, yes, the best way to tweak that value would be to use a trim pot. Use one that has a maximum resistance that is larger than 470Ω so you have values above and below. Something like 1K or 2K or even 5K. Then, connect the center pin and one of the outer pins in place of the 470Ω resistor -- in other words, remove the 470Ω resistor and put the trimmer in it's place, then turn the adjustor (probably need a small screw driver) to the center, power up your circuit and:

1. If the LED is on, then try covering the photo cell [LDR]. If
A. the LED goes out, then you're half way there. Affix it to the light on the phone and cover it such that no external light can get in (e.g. put it in a black tube that fits snugly over the phone light and seal off the back of the tube, so no light gets in from there), then get the light on the phone to turn on and
i. if the LED comes on, you're done
ii. if it doesn't then, turn the trimmer until it does -- again, done.
iii. if that doesn't turn the LED on, then tell me and we'll see if we can figure out why.
B. if the LED stays lit, then turn the trimmer until:
i. it goes out
ii. if it doesn't go out, then tell me and we'll troubleshoot.
2. if the LED is off, then try turning the trimmer until
A, it comes on. If it does then go to (1.)
B. if it doesn't come on then it troubleshootin' time (yee-haa!)
3. If none of the above (which probably means my decision tree is flawed , then, again, it's troubleshooin' time (glory-be!)

bq). What specifications would the trimpot
have? Would it have a resistance that was adjustable from, say, 10K Ohms up
to 300K Ohms, something like that?

trimpots (or "trimmers") generally adjust from nearly 0Ω to the rated value (the value often printed on the pot and the value used to order or select the pot, i.e. when you go to Radio Shack and say you want a "10K trimpot", the "10K" is the maximum value the pot will adjust to). Check these out:

* http://www.robotroom.com/Trimpots-2.html
* http://sound.westhost.com/pots.htm

Ahh... here it is. I've never encountered a thread this long, on this forum, such that it generated a second page My reply is reproduced, below:


bq). I have a further question...From
what I gather, the three prongs of a transistor are called the base, the
emitter, and the collector. Based on what you posted earlier today, I gather
that the transistor base is the middle prong in the circuit picture you
posted today. Correct? What I want to know is, is the emitter the left prong
and the collector the right prong? I assume it's important for me to get
that right.

If you are using a PN2222A, and the flat face of the transistor is facing you and the "prongs" (commmonly called "leads" or "pins") are extending downward, then the Emitter is the left most lead, and the collector is the right most lead.

Now, I challenge you to Google "Pn2222A" and find a datasheet. Look for a hit from the manufacturer of the transistor you have, or one of the major manufacturers if you can't discern the manufacturer: Fairchild, OnSemi, TI [Texas Instruments], Vishay, NXP, etc.

In the datasheet you will find a "pin-out" (at least, in most cases -- if not find a datasheet from a different manufacturer, the specs are pretty standard)

Another technique is to go to the Digi-key (or Mouser, or Jameco, etc.) website and search on the part number and then, in the listing select a part (preferable from the manufacturer of your part, but otherwise if needed) and then usually there is a link to a datasheet.

bq). Also, you mentioned once before that I might need
to use a Darlington transistor or a MOSFET.

Let's not get ahead of the game. Get it working with the PN2222A and LED, first, then we'll work from there.

bq). Finally, you
mentioned earlier today that I might need to "tweak" the 47K-Ohm resistor.
Would I accomplish that by buying resistors at about 40K, 35K, 30K, 50K, 55K
and 60K? Or would another way to do that be to take out the 47K resistor and
put in that same place a trimpot?

If you look closely at the new diagram, it's no longer a 47K. It's now a 470Ω (like the one that goes from the collector to the LED (i.e. in the "collector leg")). And, yes, the best way to tweak that value would be to use a trimpot. Use one that has a maximum resistance that is larger than 470Ω so you have values above and below. Something like 1K or 2K or even 5K. Then, connect the center pin and one of the outer pins in place of the 470Ω resistor -- in other words, remove the 470Ω resistor and put the trimmer in it's place, then turn the adjustor (probably need a small screw driver) to the center, power up your circuit and:

1. If the LED is on, then try covering the photo cell [LDR]. If
A. the LED goes out, then you're half way there. Affix it to the light on the phone and cover it such that no external light can get in (e.g. put it in a black tube that fits snugly over the phone light and seal off the back of the tube, so no light gets in from there), then get the light on the phone to turn on and
i. if the LED comes on, you're done
ii. if it doesn't then, turn the trimmer until it does -- again, done.
iii. if that doesn't turn the LED on, then tell me and we'll see if we can figure out why.
B. if the LED stays lit, then turn the trimmer until:
i. it goes out
ii. if it doesn't go out, then tell me and we'll troubleshoot.
2. if the LED is off, then try turning the trimmer until
A, it comes on. If it does then go to (1.)
B. if it doesn't come on then it troubleshootin' time (yee-haa!)
3. If none of the above (which probably means my decision tree is flawed , then, again, it's troubleshooin' time (glory-be!)

bq). What specifications would the trimpot
have? Would it have a resistance that was adjustable from, say, 10K Ohms up
to 300K Ohms, something like that?

trimpots (or "trimmers") generally adjust from nearly 0Ω to the rated value (the value often printed on the pot and the value used to order or select the pot, i.e. when you go to Radio Shack and say you want a "10K trimpot", the "10K" is the maximum value the pot will adjust to). Check these out:

* http://www.robotroom.com/Trimpots-2.html
* http://sound.westhost.com/pots.htm

Steve,
Let me say again how much I appreciate your help and your genial attitude. I'll get the things I need to buy this week, and then I should be able to "crank her up," as the car people say.

Holy crab cakes! I should have thought of this before:

Rig your photocell over the phone light so no external light can leak in (i.e., the way it will be in the final build), and then use an ohm meter to measure it's resistance:

A. When the phone light is off
B. When the phone light is on

And post the results here.

Will do, just as soon as I get it all put together.

EUREKA!! EUREKA!! Steve, I had a delay because of other stuff in life taking precedence, but I finally got all the stuff together, including a 10K trimpot. Got it all plugged into the breadboard, and the LED didn't come on. I turned it all the way counterclockwise, and nothing. Then as I was turning clockwise, THE LED TURNED ON!! I backed the trimpot screw up a ways, and the LED went off. I sat there for a couple minutes just turning the trimpot screw backward and forward, watching the light go on and off and on and off. FANTASTIC! Now I need to put it into a package I can take to work, which I think will be quite easy.
Thanks a million times, Steve, for your patience, advice, and great attitude.

Awesome! Glad to hear you achieved some success. I can relate to " I sat there for a couple minutes just turning the trimpot screw backward and forward, watching the light go on and off and on and off" -- kinda magical, isn't ;)

Now, the really big question: does the phone light turn the LED on and off?

And, also, don't we still need to scale that up to the brighter light?

Photo cell which you mentioned is not a photo cell but it is LDR. your idea may not work because, the LDR connected in series cannot handle several milli amps of current to light lamp and it burns or gets damaged. You may connect DC simple single transistor amplifier. in collector circuit connect a lamp. I am giving you the circuit in attached file.

Assemble a circuit as shown it works.

Look for attached picture.

Shrikant,
Thanks for your suggestion. If you have read much of my posts here in this thread, you'll know I'm quite inept with electronics. Bearing that in mind, please accept that I'm not quite sure how to construct the circuit you drew. First of all, I would need exact specifications of each item if I had to go out and buy them. ......... Main thing is that I have now gotten the device to work (see posts on Page 2) using Steve Lawson's direction. It does indeed work. Are you saying that one or more of my components will soon "blow out" if I use it for an hour or two (or more)? If it doesn't blow out, I am very happy with Steve's design.

I'll test it on the phone's LED soon ................... As far as scaling up to a brighter light, that would be good, but maybe not necessary after all. I was thinking that if I got a parabolic reflector (like in a small flashlight) I could aim the LED's light at where my head normally is when I'm sitting at my desk, and that would be sufficient. .......... If I did want to go to a brighter light than this LED, what kind of changes of the device do you reckon would be needed?

He _is_ using a transistor - but your circuit assumes a lot about that lamp -- do you know what current it requires?

The SL100 has a minimum beta of 40 and a max Ic of 500ma. What if the lamp requires several amps, how is this circuit going to drive it. At Hfe 40, 500ma on the collector will require 12.5ma at the base. I doubt that much current could be supplied to the base directly from the LDR. If the SL100 happens to have an Hfe of 300 (the max specified), then perhaps, since only 1.7ma will be required on the base, but it's not wise to ignore the minimum specification on a datasheet.

So, it would be wise to acquire the proper specifications before offering a design.

That will depend on what kind of lamp you choose to use. I will need to know it's rated voltage and wattage or voltage and current specifications or a part number might do it, if it's a standard part.

Steve,
You said in your most recent post, "That will depend on what kind of lamp you choose to use." Perhaps you'd care to provide the specs that you believe would be wise to use? Maybe you could even choose a specific lamp sold by RadioShack? Their website is:
http://www.radioshack.com/home/index.jsp

How about one of those 10mm SuperBtight LEDs, like:

* http://www.radioshack.com/product/index.jsp?productId=12580020

That will only require 20ma, so it can be driven with what you currently have, and it has a 40° viewing angle, so it should be fairly eye catching. Put it place of the current LED.

Other than that, Radio Shack doesn't offer much in that department.

Steve,
I will try that 10mm SuperBright LED, as you suggested. Thanks.

Fine if circuit is working. If it works for 10 minutes it will work for hours, don't worry. If there is any difficulty please contact.

To those interested:
Yesterday I finished constructing the device I've sought help and advice on. I brought it to the office and it works exactly as I had hoped. Let me say that this would have been impossible without the advice that Steve Lawson provided. That he did so in such an affable and patient manner is VERY much appreciated.

Steve, perhaps I can get one of my friends to show me how to post a picture of the device so you can get a look. It's pretty cool looking, I must say. The photocell sits in the "pocket" of a black suction cup, held in place by a small chromed cylinder. All that is affixed to a box I made of cherry wood (dimensions of 1-1/2 inch wide, 4 inches high, and 7 inches long), said box containing all the electrical components. All of that gets placed onto the far right surface of the phone, being held in place by simple gravity. I put the LED into the junked carcass of an old fountain pen I had lying around. It sits in a surround that rests on top of my computer's monitor.

As hoped, even the dim light of the phone's LED is enough to set off the bright red LED of the device, which is more than sufficient for my purposes -- one cannot sit at my desk and fail to notice that red LED when it lights up.

My partners in crime at work were all very impressed when they saw the device today. Steve, you were the brains that made all this possible. I'll just add that I think this episode provides an excellent example of the benefits provided by forums of this type. That is, people who are deficient in knowledge in a given area seek the help of those who do know, and such people are kind enough to share what they know in the spirit of teaching ........... Maybe there is hope for mankind.

Steve, where do you post from? I'm in Wisconsin, US of A.

Cool -- glad I could help.

A photo would be cool!

I'm in sunny [wish it would rain more] So. Calif. [also, US of A]