Light sensor circuit for a DC motor powered toy car

Thread Starter

popcalent

Joined Mar 17, 2018
128
Hi, all.

I found this toy car that is powered by a DC motor. All it does is just run straight. I am unable to identify the specifications of the motor since it doesn't say anything on it, but I'm attaching a picture of the toy, so perhaps someone can tell.

Anyway, I wanted to make the toy more interesting and I designed a circuit that turns on the motor when an LDR sense light. Of course, it won't be able to turn right or left and follow the light, since it has only one motor that controls the entire axle. I'm attaching a schematic of my circuit. I'm using a 9V battery to power it.

The circuit turns on the motor when the motor is not connected to the gear. The motor spins when it's connected to the gear but the wheels don't touch the ground (when I light the toy car). But, when the car is on the ground, the motor doesn't have enough power to spin the wheels. What changes can I make to my circuit to make it work?

Thank you!

Screenshot_2023-04-19-08-02-15-621_com.miui.gallery-edit.jpg

IMG_20230419_080305.jpg
 

Dodgydave

Joined Jun 22, 2012
11,320
It's probably the battery, not enough current , try using a Darlington pair with two transistors.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
6,765
The motor looks like the "coreless" brushed motor that was developed to vibrate a cell phone. It has low power and the tiny brushes soon wear out or burn out.

The 741 opamp is 54 years old and needs a supply that is 10V or higher. Yout 9V battery will quickly drop to 6V or less.
 

Audioguru again

Joined Oct 21, 2019
6,765
The motor continues to run when the output of the opamp goes low because the output of a very old 741 opamp does not go low enough to turn off the transistor. The output of "some" 741 opamps might go low enough to turn off a darlington transistor.
The outputs of modern "rail-to-rail" opamps go down to 0V.
 

KeithWalker

Joined Jul 10, 2017
3,147
Forget the negative comments. Let's get this working!
What is the voltage on the output of the 741 in the high and low states? You need a voltage divider between that and the transistor base that will give a voltage >0.7V in the high state and <<0.7V at the output to the transistor in the low state. Select resistors that can supply enough current to saturate the transistor.
 

Thread Starter

popcalent

Joined Mar 17, 2018
128
Forget the negative comments. Let's get this working!
What is the voltage on the output of the 741 in the high and low states? You need a voltage divider between that and the transistor base that will give a voltage >0.7V in the high state and <<0.7V at the output to the transistor in the low state. Select resistors that can supply enough current to saturate the transistor.
Thanks for the help.

If I use a 9V battery (mine is 7.47V) the output of the 741 is ~1.75V and ~4.15V. If I use a power supply at a steady 9V, the output is ~1.85 and ~4.98. None of the two makes the car move, but the whole thing works if the wheels don't touch ground. I tried the power supply at 12V, and the outputs are ~1.98V and ~6.00V, it still doesn't work.
 

Thread Starter

popcalent

Joined Mar 17, 2018
128
What happens if you replace the 22K resistor with 4.7K and connect a 1K resistor between the transistor base and emitter?
The voltage at the base is ~0.62V when the output of the 741 is high, and ~0.29V when the output is low. The behavior of the motor is the same.
 

ElectricSpidey

Joined Dec 2, 2017
2,839
Have you checked to make sure it's not the transistor?

Remove the base resistor connection from the 741 and connect it directly to positive.

Positive...Resistor...Base
 

Thread Starter

popcalent

Joined Mar 17, 2018
128
Have you checked to make sure it's not the transistor?

Remove the base resistor connection from the 741 and connect it directly to positive.

Positive...Resistor...Base
I just did, and the motor spins forcefully enough to move the car. So I guess the transistor is fine. At least, that possibility is now ruled out. Thanks for the suggestion!
 

KeithWalker

Joined Jul 10, 2017
3,147
The voltage at the base is ~0.62V when the output of the 741 is high, and ~0.29V when the output is low. The behavior of the motor is the same.
That's almost there. Reduce the value of the 4.7K to 2.2K. If that doesn't work, the 741 is not able to supply enough current to drive the transistor. You may have to add an amplifier stage.
 

LesJones

Joined Jan 8, 2017
4,217
The current taken by the LED and It's 220 ohm current limiting resistor will be draging down the output voltage of the 741. Also as you have already been told the output voltage of the 741 in it's low state is too high for the transistor to turn fully off. My first thought was to add two silicon diodes (Forwards biased. Cathode end to the base.) in series in series with the base resistor. This will subtract about 1.3 volts from the 741 output voltage. My next thought was to connect the LED and it's series resistor between the 741 output and the base of the transistor. The idea is that the LED would behave a bit like a 1.8 volt zener diode subreacting 1.8 volts from the 741 output. Remove the 22 k resistor.
The LED may have some leakage current befor it's forward voltage reaches 1.8 volts. Ir the transistor still does not turn off fully try adding a resistor between base and emitter of the transistor. (I suggest using the 22K resistor for that purpose. Befor this change repeat the high and low voltage measurements of the 741 with noting connected to it's output. (I am assuming the LED was still connected for the tests that you have done.)

Les.
 

Ford Prefect

Joined Jun 14, 2010
245
My suggestions:
1. Change the 1k resistor to a 10k resistor then you can adjust the 10k potentiometer/preset to the setting you like.
2. Change the 22k resistor to a resistor less than 1k - try a 220 ohm resistor. this will increase the current to the base of the transistor and switch the transistor hard on.
 
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