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!

 

It's probably the battery, not enough current , try using a Darlington pair with two transistors.

 

It's probably the battery, not enough current.

The motor works just fine when I connect it directly to the battery.

 

There is not enough current driving the base of the transistor. Reduce the value of the 22K resistor to 1K.

 

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.

 

There is not enough current driving the base of the transistor. Reduce the value of the 22K resistor to 1K.

I tried different values. Anything less than 22K will have the motor run no matter the output of the comparator.

 

Or use a Darlington pair with two transistors.

 

Or use a Darlington pair with two transistors.

I thought about that, but I'm afraid that I'll get the same thing that happens with a lower Rb, the motor will always be running.

I'll try when I get home.

 

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.

 

I thought about that, but I'm afraid that I'll get the same thing that happens with a lower Rb, the motor will always be running.

I'll try when I get home.

You need a better op amp, ideally rail to rail.

 

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.

 

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.

 

What happens if you replace the 22K resistor with 4.7K and connect a 1K resistor between the transistor base and emitter?

 

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.

 

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

 

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!

 

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.

 

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.

 

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.

 

Another problem revealed was the dead 9V battery.