# Will this circuit work? what am i missing?

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1. Apr 1, 2015

### sulicat

hi,
so i want to be able to run a motor depending on whether or not an IR detector senses IR light. so the basic idea at the start was to connect the motor with the detector and a resistor in series and therefore if there is IR the current will flow turning on the motor. I then learned that the current is limited by the low current device, ie:- the detector.

so will the correct way to go about this be to connect the detector and a transistor in series then connect a motor in parallel and make it depend on the transistor ?
here's a quick diagram of what i mean.

will this work ?

the math i used to determine the resistance of R1 is as follows.
needed voltage drop = 9 - 1.2 = 7.8
r = v/I = 7.8/0.001
r = 7800 ohms
again im not completely sure that this way of doing it is correct. you can probably tell I am very much a newbie....

also what kinda stuff will i have to consider for the transistor ? like is there a voltage limit ? does it provide considerable Resistance ? will it cause a voltage drop ? will the voltage from the inner circuit be added to it ?
the motor is a 9 v DC motor.

thank you

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2. Apr 2, 2015

### davenn

hi there

welcome to PF

No it wont work the way you want because the transistor is hard biased on all the time

here's a quick cct I put together for you
see how you go
diode across the motor a 1N4007
the variable resistor experiment with something ~ 4k7 to 10k .... it will adj the turn on sensitivity
make sure the transistor can handle the current required by the motor

cheers
Dave

3. Apr 2, 2015

### sulicat

Can you explain to me why you put a diode across the motor ? How does it affect current flow ?

4. Apr 2, 2015

### donpacino

a motor is basically an inductotor. When you shut the transistor off there will be a voltage spike due to the motor inductance. That diode will clamp the voltage and allow current to flow. Otherwise you can/will damage the transistor.

5. Apr 2, 2015

### sophiecentaur

When you interrupt the current through an inductor (including motors) you get a voltage spike that can damage transistors etc.. The diode will not conduct in normal operation because it is reverse biased but a voltage spike in the other polarity will make the diode conduct and dissipate the magnetic energy in the inductor when the transistor turns off. It's a very effective form of protection (and quite smart, imo).

6. Apr 2, 2015

### sulicat

Hmmmnn that is extremely interesting. Thank you for explaining. So the voltage spike that the motor will produce once turned off is AC?

7. Apr 3, 2015

### davenn

thanks Don and sophie for answering that Q in my absence

ummmm ... not sure if I would deem it AC or DC .... its just a large spike

one of the others may clarify it better

cheers
Dave

8. Apr 3, 2015

### donpacino

sulicat look at it this way. It is a DC system, due to the fact that it runs off DC voltage.
Most systems have 2 main modes of operation, steady state and transient. Transient operations occur when something is a system changes (like a motor shutting off).
It is a transient pulse caused by changing conditions. When that pulse goes away, the circuit will be in steady state again (but the motor will be off)