Microprocessor output strength:
Most microprocessors have outputs that are not very strong. Many
can sink (drive to ground) less than 20ma. That's about what you
need to drive an LED but not a motor.
Protecting the Microprocessor:
You always want to protect the micro's inputs and output pins.
The easyist way is to simply add a gate between the micro and the
power device driver. Any logic gate that is compatible with
your micro can be used. In most cases this gate can be zapped and
spare the micro - keeping your little rug-runner from becoming brain
dead. Gates are cheap, micros are expensive.
Another way is to use the following circuit which clamps out-of-range voltages:
+5v
|
-------- _|_ 1N4000-4
| /_\ Diode
| I/O pin 220 ohm |
Micro |<----------\/\/\/\---o-------- to circuit
| _|_
| /_\ 1N4000-4
-------- | Diode
GND
Opto-isolation can be used to protect circuitry but it's ususlly
not necessary unless you're driving plasma cutter and other
high-frequency/high-voltage devices. If you're still interested,
look up data on the 4N25.
For loads < 200ma:
Many loads are small but just too big for a micro to drive.
This includes small lights and small motors. Make sure to measure
the current draw of your device before deciding on a drive
scheme. Make sure to stall motors to see what their maximum
current draw is.
My favorite parts for driving small loads < 200ma is the popular 2N2222 transistor. These can often be had for less than 10 cents each. They come in small plastic cases (TO92). When looking at the front of the package (flat side) with the legs going down, the pinout is E B C (Emitter, Base, Collector). It doesn't take a EE degree to use these great parts - here's an example. Notice that the transistor drives the load to ground.
+5
|
/
\ R3
------- / 2N2222 ------
| \ _____| |___/\/\/\___ +5v
| | R1 B |/ C | lamp | R2
Micro |<----/\/\/\---------| ------
| |\ E
------- |
GND
R1 is not needed if the output of the micro is simply a
week pullup-open collector/drain like the those found on the 8051.
Otherwise, R1 is needed to limit the current through the base
of the transistor. Try 1k ohm for R1 if your micro has a
strong output.R2 may be needed to limit the current through the load. Use Ohm's law to calculate.
R3 may be needed to pull up the input if the micro outputs nothing and has no internal pull-up's such as PO of the 8051. A 1k to 10k works good here.
Again, you can put a gate between the micro and transistor to add an extra layer of protection.
For loads > 200ma:
Ok, now for the big stuff you've been waiting for.
MOSFETs are powerfull transistors that are great for doing this
sort of thing. It's not a problem to drive loads > 10 amps
using a MOSFET. Parts like the IRF540 are good for this job.
Normally you can get them for $1-$3 depending on specs.
Standard MOSFETs need 10volts at their Gate pin to turn them
on completely. This can be done with a 7406 level converter or
a 2N2222 transistor. This level conversion usually inverts the
signal so make provisions in your software. There are logic-level
MOSFETs now that turn on with only 5v.
Here's an example:
+12v
|
\
/ 10k
\
------- / IRF540 D --------
| |\ | |----| vacuum |----- +12v
Micro |---| \O--o--------| --------
| |/ G |-- S
------- 7406 |
GND
Looking at the MOSFET with the legs pointing down the pinout
is G D S (Gate, Drain, Source).
Voltage ratings:
These circuits are dealing with low voltages (<12v). For higher
voltages, make sure to check the transistor specs.
Driving inductive loads:
Inductive loads have coils in them. Examples include motors,
relays, and solenoid valves. When you turn an inductive load
ON then OFF, it creates a voltage spike. Normally this spike
will distroy a transistor. Sometimes a MOSFET will handle this
condition but it's best to protect the transistor with a simple
diode across the load. Use 1N4000 diodes for 1 amp loads and
1N5400 diodes for 3 amp loads. Simply put the diode across
the load with the cathode at the + power supply and the anode at
the other side that connects to the transistor.
Software and variable duty cycles:
In software you can simply turn ON or OFF the device by sending
a 1 or 0 to the port pin. Remember that your drive circuitry
will determine which will turn the device ON.
Another not-so-obvious possiblity is that quickly turning ON
and OFF the device can result in variable speed for motors
and variable intensity for lamps. You can do this by setting
up a timer or simply do a software timing loop.
Normally a frequency of 100 to 1000hz will do the job just fine.
Experiment, experiment, experiment.
Power up condition:
One often overlooked issue is that of power up conditions.
When your micro is turned on, the output pins assume a default
condition. Often this condition is HIGH (5v). If this is
driven directly into the 2N2222 circuit shown above, then
the device with turn on as soon as power is applied. Even
if the micro's program quickly turns off the pin by sending
a LOW (0v), you could still get a quick jolt on a motor or
other device which could be bad. Use a gate such as a
74HC04 to invert the signal if needed.
That's a rap.