# (Cheap) Light Activated DC Motors

Discussion in 'The Projects Forum' started by David Koonce, Oct 1, 2015.

1. ### David Koonce Thread Starter New Member

Oct 1, 2015
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I am an educator for a science museum. I have the most circuit experience of anyone on staff and that is not saying much.

We do outreaches to local schools and we are trying to create a engineering outreach.
We want to create a simple set of DC motors that are light activated.

I have some very simple knowledge of how circuits work, however I do not know how to do the calculations for the different components.

I have included a drawing of what my limited knowledge tells me the circuit might look like.

Can anyone help me? Tell me if I am doing it wrong and help me pick the right resistors and transistors?

I have purchased the geared motors that we need and I can provide any specs that are needed.

Jul 18, 2013
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3. ### ronv AAC Fanatic!

Nov 12, 2008
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Yes, the motor specs will be needed to see how big the motor transistors need to be. You show 2 motors in parallel. You want them to run when there is light?

4. ### David Koonce Thread Starter New Member

Oct 1, 2015
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Here is the data for the selected motor.
• Operating Voltage: 6 V
• Free-run speed at 6 V: 100 RPM
• Free-run current at 6 V: 160 mA
• Stall current at 6 V: 200 mA
• Stall torque at 6 V: 3.2 kg.cm
• Gear ratio: 1:150
• Weight: 40 g
https://www.openimpulse.com/blog/products-page/dc-gearmotors/ja12-n20-dc-gearmotor-100-rpm-6-v/

To answer your second question, yes I would like for the motors to turn on when their is a direct light from a flashlight.

Thanks.

5. ### Bernard AAC Fanatic!

Aug 7, 2008
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Using parts on hand, this might work:
LM393 as a common single supply comparator. Ground unused input pins to prevent oscillations.
Common light dependent resistors, LDR, have a resistance around 1 meg dark, down to 1k light.
If desired light level gives LDR 10k, then select R1 as 10k. If light brightens slightly, comp. pin 3 will be slightly more + than pin 2, making output + by way of pull-up R4. FET shown is large surface mount, only LL power FET in my stock.

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Jul 18, 2013
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Using the link I showed for the LM311 or LM393 you could either use a IRF630 to drive the pair of motors or may get away with two 2n7000's, one for each motor.
Max.

Nov 12, 2008
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9. ### David Koonce Thread Starter New Member

Oct 1, 2015
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Thank you, Mike.

I will have to make 200 of these and I do not have a large budget.

I am simply asking if my circuit will work. Yes? No? Why/Why Not?

This will be presented to 5th graders for an engineering class that takes 45 minutes.

I really am appreciative of all the responses, However I am only seeing a few viable responses for my original thread.

I also originally stated that I have very limited knowledge.

10. ### ronv AAC Fanatic!

Nov 12, 2008
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Sorry. Is the resistor with light shining on it a light dependent resistor (LDR)? Have you picked one?
The transistor would need to be a darlington transistor or FET.
It will kind of fade on and fade off. Is that ok?
The diode is probably not required.

11. ### tracecom AAC Fanatic!

Apr 16, 2010
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As drawn, your posted circuit will not work as you hope. You need to include the specifications of the LDR, how much light will fall on it, and the specifics of the transistor (including whether it's an NPN or PNP, what it's current capabilities are, and how much current it will take to drive it into saturation.) I don't know what the purpose of the diode is, but if it's meant to protect the transistor, it's in the wrong place. What is going to be the source of the 6V for the motors?

The reason that you are receiving suggestions to use an op-amp is to provide a controllable switch point. Bernard posted a complete circuit that looks to me as if it will work. What don't you like about it? More input from you will improve the responses you get.

ETA: I was typing while ron posted, so my comments may be redundant.

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12. ### #12 Expert

Nov 30, 2010
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No. Light dependent resistors (and photo-transistors) are designed as detectors, not for carrying much current. You can't get 160 to 320 ma out of a bipolar transistor by having an LDR tell it to come on. Swap that to a mosfet and you have a chance to do it in one stage, but mosfets die from static electricity (clumsy handling). Students = Murphy's Law.

So, we talk about a Darlington transistor. Lots of gain and pretty much immune to novices. That leaves the biasing circuit for the LDR to Darlington interface. Choose an LDR.

See post #11. Tracecom just beat me to the same information.

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13. ### David Koonce Thread Starter New Member

Oct 1, 2015
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Thank you RONV,

I will try to answer your questions to the best of my ability.

1) I have tried both LDR's and Photo-transistors in the place of the "resistor with the light shining on it"
I do not know which one would be better or why to pick one over the other.

2) I am currently using too small of a transistor. It cannot handle to amps that I am pushing. I am looking to upgrade to this resistor possibly.
https://www.fairchildsemi.com/datasheets/TI/TIP120.pdf

3) The fading is an issue that I am having with my prototype and I cannot have fading. I need the motor to be completely off when there is not a direct flashlight being pointed at the LDR or Photo-Transistor, or whatever is used in its place.

4) I will remove the diode then saving money is my friend right now!

Thank you for your direct response.

14. ### #12 Expert

Nov 30, 2010
16,321
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1) Just pick one so we can stop guessing.
2) The TIP 120 looks good. It has a gain of about 3000 in your current range. If you can get 0.001 amp through the LDR, you have it whipped.
3) Move the diode so it is in parallel with the motors but facing so it will not allow normal current. That keeps the motor inductance from kicking the brains out of the transistor.
4) Snap-action switching can not be done with 3 parts. That's why everybody keeps handing you designs with an amplifier in them. Pick one: Fast, hard, switching or cheap and simple.

Personally, I can see an educational advantage to the (cheap) variable quality circuit. It will let people see that switching can happen in degrees. Then you can explain that hard, fast switching requires more complication.

@MikeML This looks like a job for the TL431

Last edited: Oct 1, 2015
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15. ### ronv AAC Fanatic!

Nov 12, 2008
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That one should work. If you use the LDR a resistor of 100 or 200K ohm from base to ground will probably turn it off. Try to shield it from ambient light. - like maybe in a black tube so you need to shine the light directly into it.

16. ### ian field Distinguished Member

Oct 27, 2012
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Most of those op-amp circuits would be better for a little positive feedback (Schmitt-trigger) so they snap on or off instead of lingering half way and dissipating in the transistors.

The TS's circuit with the LDR could hit the buffers, AFAIK: the cadmium-sulphide LDR doesn't pass RoHS. Although its a nearer match to human eye response than any of the silicon photo sensors.

A while back I bought a couple of bags of LDRs out of China - that was pretty much get a few in stock while I still could.

Jul 18, 2013
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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.

18. ### MikeML AAC Fanatic!

Oct 2, 2009
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The snap-action (hysteresis) is the most important attribute of a good circuit being discussed here.

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19. ### ronv AAC Fanatic!

Nov 12, 2008
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Bernard, Maybe you could find a red LED in that box and reverse bias it to the 6 volts then to the gate. Maybe a 1 meg pull down to turn it off. I'm thinking cheap now.

20. ### dannyf Well-Known Member

Sep 13, 2015
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Your circuit will work,subject to a few caveats

one, the light changes are significantly. Otherwise. The motor may not turn on or off completely.

two light sensors don't have good consistency so you will need a way to adjust them in circuit.

three, you may need to pick a transistor or mosfet for hard turn on or turn off.

lastly, your circuit doesn't need that diode but can benefit from a flywheeel diode.

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