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light sensers
By BrianK, Section Controls Posted on Tue Jun 14, 2005 at 04:49:40 PM MST
voltage?

I would like to know if the light sensers that are used in the outside lights that
come on at night care about what kind of power comes to it. I am thinking about pulling the 120v light out and rewiring the lamp for 12v.
'

light sensers | 13 comments (13 topical)

Re: light sensers (3.00 / 0) (#1)
by joelhacker on Tue Jun 14, 2005 at 11:47:41 AM MST

You'll have to ask Dr. Dongle about the actual sensor
itself, but I do know that the control logic use triacs
to control the on/off and they don't handle DC voltages.

Re: light sensers (3.00 / 0) (#2)
by Norm on Tue Jun 14, 2005 at 12:49:10 PM MST

Put in a 120 volt relay in place of the 120v light
...this then switches on 12vdc current to a 12 vdc light.
( :>) Norm.
( :>) Norm
[ Parent ]

Re: light sensers (3.00 / 0) (#3)
by drdongle on Tue Jun 14, 2005 at 04:41:33 PM MST

Most of these sensors are vary simple, just a Cadmium Sulfide photo cell in series with the coil of a relay. During the day the photocell is a low resistance and the relay is activated, disconnecting the power to the lamp. At night the resistance of the cell increases and the relay turns off activating the lamp.
It may be possible to gut the original relay out of the unit and replace it with a 12 volt unit. If you do so make sure that the relays contacts can handle the current the new lamp requires, automotive relays are a good choice for this.
You could also make a simple one transistor switch which would do the same job.
Using a 2n3055 (or a 2n3772 for higher power)power transistor connect the Collector lead to one side of the 12 volt lamp. Connect the other side of the lamp to 12V+, connect the Emitter lead to the 12V-. Connect one side of the photo cell to 12- and the other side to the Base lead, then connect a resistor ( start with 10 K ohms, this may require some experimentation) to the Base and the other end to the 12V+.
Heat sinking of the transistor is recommended as it passes the full current required by the lamp. This circuit keeps the transistor turned off during the day and turns it on at night, so it uses less power than the the original.

Carpe Vigor, Dr.D
[ Parent ]

Re: light sensers (3.00 / 0) (#4)
by RobD on Tue Jun 14, 2005 at 08:53:10 PM MST

Dr.,
I wouldn't use a 3055. I'd use a IRF510 or better mosfet.

Re: light sensers (3.00 / 0) (#5)
by drdongle on Wed Jun 15, 2005 at 04:58:27 AM MST

I'm old school and still think mostly in terms of bipolar devices, yes a fet would be even better.
Carpe Vigor, Dr.D

Re: light sensers (3.00 / 0) (#6)
by Ungrounded Lightning Rod on Wed Jun 15, 2005 at 07:17:09 PM MST

If it's using bipolar (or even if not) I'd want a little positive feedback to give it some hysteresis and "snap" it between on and off, so the final transistor didn't spend a lot of time unsaturated. (Think "Schmidt Trigger".) Power dissipated in the output transistor is E*I where I is current and E is voltage drop. So it dissipates a LOT more power when it's running the lamp at, say, 2 amps with 6 volts across the transistor, than when it's running with 4 amps and a half volt.

I can see how to do that with three resistors, two NPN transistors (say 2n2222 for the input stage) and the cadmium cell. (Both emitters grounded, both bases with resistor to +12, small-transistor base also has cadmium cell to ground, resistor to big transistor's collector. Big transistor base also hooked to small transistor collector. Resistive load (lamp) from big transistor collector to +12.

I think you can get away with a resistor less using a PNP / NPN pair,wired up vuagely like a split-up Triac.

And I'm SURE there's several ways to do it with an NE555, among the thousands of cute low-component-count applications for that ubiquitous el-cheapo chip. (It has a voltage divider, schmidt trigger, reference generator, and a moderately decent output transistor, all in a six-leg bug for pennies.)

[ Parent ]

Re: light sensers (3.00 / 0) (#8)
by commanda on Thu Jun 16, 2005 at 02:59:56 AM MST

Hate to nit-pick, but 555's have 8 legs.

Amanda

[ Parent ]

Re: light sensers (3.00 / 0) (#9)
by drdongle on Thu Jun 16, 2005 at 04:59:55 AM MST

While the circuit you described would work fine I don't see how it introduces any Hysteresis. You just introduced a buffer transistor that could provide a more positive turn off of the switch transistor. I kept my circuit simple as I had no idea how electronically profecent the original poster was. A 555 timer would be a great choice for introducing Hysteresis to the circuit.
Carpe Vigor, Dr.D
[ Parent ]

Re: light sensers (3.00 / 0) (#11)
by Ungrounded Lightning Rod on Thu Jun 16, 2005 at 10:48:03 AM MST

The resistor from the output transistor collector to the input transistor base provides the positive feedback.

You have to balance the values that one, the input's base pullup, and the photoresistor to get the behavior right. Too much and it "sticks" too much - like not turning off until the sun hits the photocell directly, or not turning off (or on) at all. Too little and it might as well not be there.

[ Parent ]

Re: light sensers (3.00 / 0) (#7)
by Nando on Wed Jun 15, 2005 at 08:34:01 PM MST

Brian:

You did not say that you have DC or AC volts.

It seems that is 120 V AC.
To replace the 120 Volts lamp with a 12 Volts lamp, one needs to pay attention to the wiring guage.

I believe that the wiring is too thin for 12 volts lamps, WHAT Wattage are you thinking in using ?.

The light SENSOR can be of different types from Silicon to cadmium sulfide.

The circuit is variable from relays to electronic circuits (TRIACs), please do not assume a single type design, I have found several types and I have designed several types for high production.

Why are you changing it to 12 Volts ?.

It may be better to get a simple DC/AC converter ( 115 Volts AC), I bought one on sale for 9 dollars, 170 watts.

Regards

Nando

Re: light sensers (3.00 / 0) (#10)
by ghurd on Thu Jun 16, 2005 at 07:49:23 AM MST

I make a very simple one, just a mosfet, resistor and CDS cell.
It would works with LEDs and I don't see why it wouldn't work with a relay.

Positive to the resistor and relay coil +.
Relay coil negative to the mosfet drain.
Resistor to the gate and CDS.
CDS and source to negative.

The resistor needs to be adjusted to the CDS cell, but 150K ohms is a good start.
The last couple used a IRF510, and CDS cell with <1K light, 8M dark...
just because they were handy and well stocked.

G-
Ghurd.info

Re: light sensers (3.00 / 0) (#12)
by BrianK on Thu Jun 16, 2005 at 11:08:48 AM MST

Thanks for all the input The idea was just that an idea. I watch that thing turn on at night, and it just got me to thinkin. I do have a small solar setup to run some lights in my home. And i am slowly trying to wean my family off the grid power its hard to do they are in love with all that power.

Re: light sensers (3.00 / 0) (#13)
by ghurd on Fri Jul 22, 2005 at 02:52:00 PM MST

Do you still want a diagram of a 12V On/Off light sensor?
G-
Ghurd.info
[ Parent ]

light sensers | 13 comments (13 topical)

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