I have a little 2 to 5 volt hobby motor that I would like to control with a photoresistor aka photocell. What is the simplest way to control the circuit? Is there a photo resistor with enough range to be the only element inthe battery motor series circuit? So that I can just turn off the motor by the photoresistor resistance going high enough? Or do I need a voltage divider with the motor in parallel with the larger resistor, so that the photoresistor can take voltage away form the larger resistor as the photoresistor resistance approaches the larger resistor? This is for a project for my kids school, so I want it to be really simple. Does a motor have a resistance when on or off?
How to control a motor with a photoresistor
dc motorphotoresistor
Related Solutions
The indicators go off because your output is connected directly to the base of your power transistor, so it will be sinking a lot of current from the 555 output, and since the transistor base to emitter voltage is around 0.7V, it will cause the 555 output to sag too low to turn the LED on.
You need to put a resistor in series with the base of your transistor, to prevent it loading the output of the 555 too much and it's voltage dropping - I don't know what current your motor needs, or the gain of your transistor, but try 100Ω first. This will limit the current to (9V - 0.7V) / 100 = 83mA, which combined with the LED current is within the 200mA output spec of the 555, so it should easily be able to light the LED as well.
The 83mA will give you plenty of current at the base of your power transistor, so even if it's gain is very low, it only needs a gain of 25 to sink over 2A.
Transistor
Okay, we now have discovered that the schematic shows an NPN transistor but you are using a TIP42, which is a PNP transistor. A PNP is like an opposite version of an NPN, and to use it the voltages must be reversed in comparison. Look at the symbols for boht, note the difference in arrow direction - it always points to N type material (or away from P type)
You have a couple of options:
- Use a TIP41 or similar NPN power transistor and use the current circuit with base resistor suggested.
- Wire the circuit correctly to use your TIP42 PNP, which would look something like this:
Depending on how much current your motor is rated for, you may be better with a darlington or MOSFET, as the power transistors gain will drop at saturation, loading the 555 output too much so it may not have enough current to turn it on fully (which will cause it od disspate more power, etc) If it's under 1A you should be okay.
The resistance depends on the motor's maximum current consumption and the transistor's h(FE) - the ratio between collector and base current. As I can see in the datasheet (http://www.datasheetcatalog.com/datasheets_pdf/T/I/P/1/TIP132.shtml), TIP132's h(FE) is at least 1000. Say your motor consumes at most 2 A (which you have to find out yourself, either from its datasheet or block the rotor and measure the current consumed), so the microcontroller has to source about 2 mA to drive the transistor.
The transistor's base-emitter voltage should be about 1.5 V (it's a Darlington transistor, that is, two base-emitter junctions are connected in series, dropping about 0.7 V each), so the resistor should drop about 3.5 V with 5 V supply. Having that we can calculate that its resistance should be: 3.5 V / 2 mA = 1,75 kOhm, so 1.5 kOhm would be suitable in my case.
Again, check your motor's maximum current consumption and re-calculate the resistance. You may also find an interesting option in replacing a BJT with a suitable MOSFET (an IRL3202 could be a good choice). A MOSFET has lower "open-state" voltage drop than a BJT, so it allows to deliver nearly full supply voltage to the motor and dissipates less heat.
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Best Answer
OK rather than continuing comments I'll answer here.
For kids only five years old this will be a bit much I think.
I'd start with just batteries and light bulbs... How do you make the light turn on?
You can then do some series and parallel stuff. But even that may be over the top for some.. maybe best to just let them play around.. (How many different can you get a light bulb to turn on.) If you wanted you could add some LED's to the mix. (Maybe with the current limiting resistor soldered onto on leg and covered with heat shrink.) I can think of lots of fun things... but again may not when they are only 5. (A red and a green/blue LED in parallel would be interesting.)