(Cheap) Light Activated DC Motors

MikeML

Joined Oct 2, 2009
5,444
David,

You will have to pick a LDR and stick with it for all 200 units. I suggest the PDV-P8104-ND, stocked by DigiKey at $0.42ea (100pcs). As shown on the data sheet, when illuminated, it has a resistance of between 27KΩ and 60KΩ.

I have designed a 6V Motor driver that will drive two of your motors in-parallel, which has hysteresis (meaning that the 330mA motor driver NFET does not have to be on a heatsink like the Darlington NPN you like would), and has low standby current (300uA), and is adjustable over a range of light levels and variance of the sensitivity of the LDR.

The second most expensive part in this circuit is the NFET. How cheap you can get these depends on what you can work with in how it is packaged. Unfortunately, the really cheap ones are surface mount. This list has ones you can use based on electrical specs, sorted by increasing price. The first through-hole part that would be good in this circuit would be this one. The rest of the parts are not the cost determining factor.

Before I post the circuit, let me know if I am on the right track, cost wise...
 
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Bernard

Joined Aug 7, 2008
5,784
How cheap is cheap?
Logic level FET $ .59, LDR $ .50, 3k ohm resistor $ .05.
LDR, ambient inside light from 4 40 W celling lights @ 6 ft. about 9k ohms, LED flashlight @ 6 in, 300 ohms.
As per dannyf, I'd keep the diode, anode to drain.
If interested I'll breadboard it & report.
 

Thread Starter

David Koonce

Joined Oct 1, 2015
8
How cheap is cheap?
Logic level FET $ .59, LDR $ .50, 3k ohm resistor $ .05.
LDR, ambient inside light from 4 40 W celling lights @ 6 ft. about 9k ohms, LED flashlight @ 6 in, 300 ohms.
As per dannyf, I'd keep the diode, anode to drain.
If interested I'll breadboard it & report.

I am very interested. I was unaware that my previous design was unable to do "hard switching" until you all enlightened me. Thank you.

As for the cost, I am looking to keep it under $2.00 for just the board components. Do you think that is achievable?
 

ronv

Joined Nov 12, 2008
3,770
I am very interested. I was unaware that my previous design was unable to do "hard switching" until you all enlightened me. Thank you.

As for the cost, I am looking to keep it under $2.00 for just the board components. Do you think that is achievable?
Maybe while we are waiting for Benard to check in you could measure the resistance of your LDR in ambient room light and with the flashlight shining on it.
 

Thread Starter

David Koonce

Joined Oct 1, 2015
8
Maybe while we are waiting for Benard to check in you could measure the resistance of your LDR in ambient room light and with the flashlight shining on it.

The only issue is that the room will always change. We will be in classrooms all across our state.
 

MikeML

Joined Oct 2, 2009
5,444
The only issue is that the room will always change. We will be in classrooms all across our state.
So you expect this circuit to work reliably in rather high ambient light? This makes this project much more difficult. I am seeing a circuit that actually measures the ambient light, calibrates itself, and then looks for an increase in light when you turn on the flashlight.

You may also have to use an IR filter lens, which blocks some of the light that comes from florescent lighting, and is more sensitive to IR, which is what comes from the incandescent lamp in a flashlight.
 

ronv

Joined Nov 12, 2008
3,770
The only issue is that the room will always change. We will be in classrooms all across our state.
Well, lets measure it in what you consider typical to see how much room we have. Can you control where it points? In other words, much harder if it is pointing at the room lights or a window.
Then also think about putting it in a dark tube so the flashlight must point into it. Do you control the flashlight?
 

MikeML

Joined Oct 2, 2009
5,444
I just went an grabbed a random CdS LDR out of my junk box. In a dark room it measures >3.3megΩ. In my daylight-lighted lab near my computer screen, it measures 3.6KΩ. In a more brightly lit part of the room closer to a window at my workbench, it measures 720Ω, and when I shine a flashlight directly on it from about a foot away (standard two D-cell w/ bulb; not a LED one), the resistance drops to about 350Ω.

Starting in the dark room, if I point the flashlight at the ldr from a distance of ~ 1foot, the resistance varies from ~350Ω to over 1KΩ as my hand shakes....

Not much signal-to-noise ratio to work with...

How far away from the sensor will the flashlight be?
 
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ian field

Joined Oct 27, 2012
6,536
I happened to have a lm311 set up on my breadboard so I added a Photo transistor and a IRF530 and worked fine.
The hysteresis with a P.T. instead of LDR was large enough.
The P.T. had the base lead, so it could be biased if needed.
Max.
Here's a hand trace of the circuit from an outdoor light fitting that switches off to save energy during daylight.

The photodiode looks very similar to a type used for remote control in TV front panels back around the 80s/90s. It has the red-tinged black opaque encapsulation that admits IR. But the switch evidently worked well enough as a daylight sensor.

The cadmium sulphide LDR is among the closest matches to human eye response, and far better than any silicon detector. But AFAIK: the Cd-S LDR isn't RoHS compliant, and the circuit I've attached looks to be evidence that manufacturers are responding to the situation.
 

Attachments

MaxHeadRoom

Joined Jul 18, 2013
28,622
The circuit I mocked up shown in Post #2 uses a P.T. (I do not have a LDR on hand at this moment) this is easily set to ignore well lit room lighting, and only activated by another beam/source, and the LM311 acts as a Schmidt trigger.
Max.
 

Bernard

Joined Aug 7, 2008
5,784
A circuit board and all parts about US $ 1.35. Add about 30 min. construction tine per board.
Board - One in. square cardboard with 5/32 copper foil strips. Foil from stained glass shop, full roll
36 yards @ $ 17. ?; might dicker for fraction roll?
I bought my LDRs & FDD6530A 's from Electronic Goldmine.
Substitute a cap. for diode- G20839 on sale, 1 uF @ 250V, 100 pc bag for $ 2.50- confirmed.
With private message for address I could send you my sample. No cost.


( shakey hands & did not want to start over)

LDR Teacher 00000.jpg LDR Teacher 00000.jpg LDR Teacher 00000.jpg
 

Thread Starter

David Koonce

Joined Oct 1, 2015
8
A circuit board and all parts about US $ 1.35. Add about 30 min. construction tine per board.
Board - One in. square cardboard with 5/32 copper foil strips. Foil from stained glass shop, full roll
36 yards @ $ 17. ?; might dicker for fraction roll?
I bought my LDRs & FDD6530A 's from Electronic Goldmine.
Substitute a cap. for diode- G20839 on sale, 1 uF @ 250V, 100 pc bag for $ 2.50- confirmed.
With private message for address I could send you my sample. No cost.


( shakey hands & did not want to start over)

View attachment 92416 View attachment 92416 View attachment 92416

Bernard,

This assembly will essentially "switch"? I am not familiar with FET's vs Transistors.
 

Bernard

Joined Aug 7, 2008
5,784
FET's are V controlled, transistors are current controlled. In this circuit they are almost interchangeable.
Using a D1326 darlington, equivalent to TIP120 in this application, changed 3K to 2.2k. Added a red LED in series with LDR, cathode to base.
Motor off with flashlight at 2 ft., on at 6 in.
Speed of switching depends on how fast flashlight is brought to bear on LDR, slowly withdrawing light causes motor speed to fall from full on gradually down to off.
 

ian field

Joined Oct 27, 2012
6,536
The circuit I mocked up shown in Post #2 uses a P.T. (I do not have a LDR on hand at this moment) this is easily set to ignore well lit room lighting, and only activated by another beam/source, and the LM311 acts as a Schmidt trigger.
Max.
Getting it to ignore incandescent lightbulbs could be tricky - they put out more IR than visible light. Even florescent lighting isn't devoid of IR.
 

floomdoggle

Joined Sep 1, 2008
217
Just to throw something out there, don't know if it would work, but how about one of those circuits that turn on/off those cheap solar powered garden lights?
 

#12

Joined Nov 30, 2010
18,224
how about one of those circuits that turn on/off those cheap solar powered garden lights?
That's pretty much what we are doing here, except the motors use more current than most decorative lights. If you have a bushel of the garden light controllers for free, we can work out a hack. Otherwise, the progress already accomplished here is rather good work.;)
 
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