LDR Motor Circuit

[pizoman]

Hi everyone, I'm new to this forum.

I found this schematic online that's suppose to be able to turn off/on a simple toy DC motor that you'd find in those model kits with an LDR and potentiometer. I breadboarded the circuit and it doesn't seem to work. I have exactly the parts that are on the diagram and I'm using a 9volt battery to power it up. Can someone confirm that this circuit works or not? I'll post a wiring diagram of what I did soon.

Thanks a lot!

 

[Sid723]

Take a DC voltage meter and measure the voltage at pin 2, then pin 3 with respect to ground and let us know what you get.

 

[pizoman]

I tried to measure the DC voltage but the transister gets really hot and starts to emit this burning smell so I'm forced to cut the power off before getting the voltage.

 

[pizoman]

Here's a wiring diagram of what I did on my breadboard. Again the 108 transistor gets really hot.

 

[jackorocko]

so disconnect the motor and see what happens to the transistor. then while your at it, measure the voltages.

 

[pizoman]

so disconnect the motor and see what happens to the transistor. then while your at it, measure the voltages.

Still heats up. I tried replacing a green LED for the motor and the led powers up but instead of turning green it becomes red. So there is power running through that part of the circuit but probably too much, maybe I'm not wiring my potentiometer properly.

 

[davenn]

Well the transistor is probably dead now hence why it heats up even with the motor disconnected

I'm picking it couldnt handle the current that was being drawn by the particular motor you decided to use. Do you know the voltage and current ratings of your motor ?
When you find out how much current the motor draws then you need to choose a transistor to suit.
Without a series resistor with the LED you are likely to kill it too, the reason it changed colour is cuz it was drawing too much current, if it hasnt died already I would bet its lifespan will be very short

Dave

 

[davenn]

Disconnect any transistor, motor, LED from the output and whilst taking the LDR from dark to light observe the voltage at pins 3 and 6 of the Op-amp
You have wired the pot OK for the above test just set the pot at ~ half way in its travel

tell us the results

Dave

 

[pizoman]

Well the transistor is probably dead now hence why it heats up even with the motor disconnected

I'm picking it couldnt handle the current that was being drawn by the particular motor you decided to use. Do you know the voltage and current ratings of your motor ?
When you find out how much current the motor draws then you need to choose a transistor to suit.
Without a series resistor with the LED you are likely to kill it too, the reason it changed colour is cuz it was drawing too much current, if it hasnt died already I would bet its lifespan will be very short

Dave

I don't think it's the motor, as it's only one of those small DC toy motors that you'd find in a dollar store fan.

 

[pizoman]

I set the multimeter to 20 at the DCV option and stuck the red lead to the OP pin and the black lead to a negative port on the breadboard.

Assuming I did it correctly I got the values:

LDR in Light
Pin 3 : 5.4Volts
Pin 6: 7.9Volts

LDR in dark:
Pin 3: 1.6Volts
Pin 6: 1.72Volts

 

[pizoman]

The LDR and Potentiometer seems to be working fine. I tested it at varying resistance from the pot.

I think the transister might be at fault here. Looks like the BC108 operates at max 100mA and 45Volts max.

My motor operates at 12Volts but draws 0.4A at nominal and has a stall current of 4.6A.

So it might be that my transister cant handle the current. True? Any recommendations for one that can handle that current?

 

[davenn]

My motor operates at 12Volts but draws 0.4A at nominal and has a stall current of 4.6A.

So it might be that my transister cant handle the current. True? Any recommendations for one that can handle that current?

Yup thats what I suggested
OK ummmm .... a BD 135 or 137 or 139 All NPN and contineous collector current of 1.5A and a peak of 3A well above your requirements

The only difference between the 3 devices is the Collector/Emitter Voltage, all which are well above what you need


Dave

 

[pizoman]

That sounds good. I'll look for one of those tomorrow at the shop. Also, if my motor operates at 12 volts but I have a power supply at 6V (2Amps, and 10Watts) would I be able to power up the motor with those specifications?

 

[davenn]

its probably not going to run overly well and may well draw extra current

the circuit diagram doesnt say what the voltage supply was supposed to be
where did you get the cct ? was there any other info supplied ?

Dave

 

[pizoman]

Not sure what you mean by cct. I just found the schematic through google and the only information supplied was on the schematic. I wouldn't need the motor to run at peak performance but just to run.

 

[davenn]

cct shorthand for circuit ..... one of those other things you learn with experience

 

[pizoman]

I replaced the old transister with a new BD139 and the motor runs now (gets a little hot after a while). But now I can't seem to get the LDR to shut off the circuit when I cover it in the dark. I thought that part of the circuit was fine before when I tested it. Do you think it could be that the POT resistance isn't high enough or too high for the LDR?

 

[jackorocko]

Do you think it could be that the POT resistance isn't high enough or too high for the LDR?

the way you have it hooked up, the lower the pot. resistance the more the transistor should turn off

 

[pizoman]

the way you have it hooked up, the lower the pot. resistance the more the transistor should turn off

I tried turning the pot to a bare minimum and then cover up the LDR but to no effect, the motor still runs.

 

[jackorocko]

I tried turning the pot to a bare minimum and then cover up the LDR but to no effect, the motor still runs.

What is the output voltage of the op-amp with the pot. at zero and the LDR is dark?