photocell help PLEASE

[jared dunn]

so I bought a pack of cadmium sulfide photocells. want to use them to control motor speed based on light intensity. I tested them using my ohmmeter. Everything works fine. I tested my motor usin a 1.5 volt dc battery. however when i connect the cell in series with the motor, i cant get any power flow regardless of light intensity. can anyone tell me why? i thought that maybe if i used a NPN bipolar transistor and connected the cell to the base and the motor to the emmitter then maybe the amplified signal would drive the motor but NO. so then i switched the motor to the collector and still no current. WHYWHYWHY?????

 

[dknguyen]

I think it's because it's a photocell...and not a photo-resistor. It generates a small current based on light, rather than reducing resistance in the presence of more light.

About the NPN transistor, your battery might not have enough voltage to properly bias the NPN transistor to turn it on as a switch, or the photocell is not providing enough current, or you are connecting the other end of the photocell incorrectly (you ARE connecting both ends of the photocell right? Not just connecting one end to the base and leaving the other unconnected?)

 

[akg]

so I bought a pack of cadmium sulfide photocells. want to use them to control motor speed based on light intensity. I tested them using my ohmmeter. Everything works fine. I tested my motor usin a 1.5 volt dc battery. however when i connect the cell in series with the motor, i cant get any power flow regardless of light intensity. can anyone tell me why? i thought that maybe if i used a NPN bipolar transistor and connected the cell to the base and the motor to the emmitter then maybe the amplified signal would drive the motor but NO. so then i switched the motor to the collector and still no current. WHYWHYWHY?????

because ur not connecting it properly , from ur description i feel that u connected the battery , the photocell and motor in series , the resistance of the cds cell is in the range of 100K in dark and ~1k in full bright , still it cannot pass enough power to turn a motor.also ur connection with the npn is also not correct.
for ur objective , u'll need some more complex circuitary

 

[hjames]

Grab a multimeter and tell me the following:

1) What's the resistance of the CDS cell when it's light?

2) If you have a 1.5V supply, and you have the CDS cell running between the base of the NPN and the collector, emitter grounded, and 1.5V connected to the collector, what's the voltage across the CDS cell?

3) How much current is flowing through the CDS cell?

4) What's the "beta"/gain of this particular transistor? numbers of 50-200 are pretty common. How much current is supposed to be flowing through the collector?

5) How much current does the motor need to run? Is it enough?

 

[jared dunn]

okay i get what you are saying about photoresistors but that is what i ordered so i assumed thats what i got. and really if its resistance decreases with light, doesnt that make it a photo resistor. i mean it is a light sensitive resistor right?

 

[jared dunn]

also yes i am a "newb" but i do know that both terminals have to be connected for current flow. And as far as incorrectly connecting the trans i tried all possible connections for an NPN which is really only like 3. so thats not the prob. follow this with me; base connected to photocell and positve terminal in series. collector connected to motor. emmitter connected to ground.this setup works without the cell but when i add it i cannot get current flow no matter how bright the light.

 

[jared dunn]

i was thinking the same thing about the battery just no tbeing enough to achieve that breakover voltage i need to flip the transistor "switch".so perhaps you could tell me what kind of more complex circuitry will be required to get the photocell to activate my transistor

 

[jared dunn]

now im pulling these figures out of my ass cuse im not home right now but i assume a 1.5v AA battery will provide around 300MaH?the motor is from a mini servo from my old r/c plane. and the small current really gets it spinning with some gusto. if you are familiar with model airplane tech, the reciever and servo systems operate on 4.8 volts. the servos themselves run at about 3v but can still be used at as low as 2v. however i did try the setup with 3v still to no avail. if i remember right the photocell is at 1-2 kohms in light and like 200-300kohms in dark. the trans has a beta factor of 50(?) i failed to cehck the voltage with it connected. sorry. meter is old and analog and i should have exhausted all options before posting but i really appreciate you guys helping me.

 

[Nigel Goodwin]

Post a circuit diagram of what you've doen so far, so we can see what you're doing.

 

[hjames]

now im pulling these figures out of my ass cuse im not home right now but i assume a 1.5v AA battery will provide around 300MaH?the motor is from a mini servo from my old r/c plane. and the small current really gets it spinning with some gusto. if you are familiar with model airplane tech, the reciever and servo systems operate on 4.8 volts. the servos themselves run at about 3v but can still be used at as low as 2v. however i did try the setup with 3v still to no avail. if i remember right the photocell is at 1-2 kohms in light and like 200-300kohms in dark. the trans has a beta factor of 50(?) i failed to cehck the voltage with it connected. sorry. meter is old and analog and i should have exhausted all options before posting but i really appreciate you guys helping me.

Debugging circuits is a ludicrously important skill - at some point when solder meets parts, circuits will magically not work and the only thing you can do is start measuring and figure out what isn't fitting into the picture. In this case, I was asking a lot of small questions, trying to steer you to possible problem spots - If the CDS resistance is 2K, there is a (1.5-.7)/2 = ~.4mA base current, gain of 50 => Ic = Ib*{beta} = ~20mA, plus the transistor isn't anywhere near saturation, so Vce is probably something large... If you measure the circuit while it is running, you'll get much more info - you will probably see the voltages on the nodes moving, and from that you can figure out what's happening.


BTW, nothing wrong with analog multimeters. For experimentation, having a moving meter is much more responsive than an LCD with a 1Hz display rate.

 

[jared dunn]

finally SUCCESS! thanks for all your help guys but i think my problem stems from my soldering skills. You see, i must have damaged each trans i used by keeping the heat on too long. i tried a new st of transistors and used minimal heat and solder and viola, Success. so i guess my problem was more in technique than planning. if curious i decided to mount a set of rudders on an Estes model rocket. You guessed it; a sun-seeker. Should be interesting. I would probabaly have more luck with JFETs but for the sake of economy i am using bipolars as i probably wont be retrieving this one. I plan to aim it away from the sun because, to be completely satisfied, it must make a turn and then level out towards the sun. My First Project. How cute.

 

[dknguyen]

How did you manage to mount a moving find to guide the rocket? Are you using a normal motor or a servo? I've been wanting to do that but I have a feeling it's a bit dangerous and illegalish where I am.

Also...wouldn't you want it to avoid the sun so you could see it go up?

 

[jared dunn]

well the fins aer more unconventional than you are thinknig. Think of them more as flaps that lay flat against the sides and deploy to slow that side of the rocket. the fins that are already on the vehicle ar fixed. they provide the stability. the flaps provide steering. the reason for flying it towards the sun is that it will be the brightest detectable light source. Also i plan to stand with the rocket to the east of me and the sun to the west. also by aiming it away from the sun i will be able to watch it turn and then level out and head towards the sun. that is the only way i will be satisfied that it was successful. I mounted a servo horn to the commutater itself snd will run a simple metal ro that will push on its flap.When the motor is not recieving input, the force of the wind will return the flap to its resting point against the body tube. the flaps have small fins on them as well that are fixed in place on the flaps. these will increase stability even when flaps are at full throw. As far as legality and safety( definately an issue) well i guess i just hope their are no laws against this here in the states. Could be dangerous but of course i will NOT launch without testig my equipment first. i will have to get vid of it cause if it works i will want you to see it. I just would be so proud to know that all these years of studying electronics has paid off and although its no big deal because man has been to the moon, it would be a HUGE accomplishment for me. i am just not very content building crystal radios and night lights you know?

 

[jared dunn]

what kind of prijects do you do? i need ideas but i CANT use ICs. I am still veeeeerrrrryyyyy inexperienced with them . I know some basic logic functions but if i used logic i would do it with a couple transistors and diodes

 

[RODALCO]

If you are new to the field and want to start using IC's I recommend that you use IC sockets. You can solder those in place on the cct board and when all is ok insert the IC. ensure that it is the correct way around.
There is a little notch at the left hand side and counting the pins will be from the bottom LHS. (as sketched below )

16 15 14 13 12 11 10 09
____________________

_
_I IC

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01 02 03 04 05 06 07 08
a Ne 555 is a forgiving IC and a simple led changeover light is a nice start to build to get the hang of it.
That one is only a 8 pin IC.

 

[Hero999]

**broken link removed**