Debugging your circuit
A few general tips
No matter how experienced you are, sooner or later you'll
build something that just plain doesn't work. Assuming you
breadboarded things first (so you're SURE you don't have a
design problem), this means there's a problem with how you built up
the circuit.
Debugging is often very design dependent, but a few tips may help in
finding build problems...
- Component placement mistakes are common in home-built
circuits.
Check that all your components are where they belong (it's pretty
easy to get transistors
confused for each other); check things off on the circuit
diagram as you inspect them. Also make sure that
polarity-sensitive components (electrolytic capacitors,
transistors,
diodes,
photodiodes)
are oriented correctly.
If you're freeforming,
inspect every connection to be sure you have soldered things
together correctly -- inspect each physical connection and check
them off on your circuit diagram.
- If you're building a circuit
up on a PCB
or "perfboard," I'd recommend you always clean off the
solder flux after you have things soldered together. This is
absolutely required if you're using solder with water-soluble flux
(it has relatively low resistance, and will mess up Nv
net circuits),
and eases inspection in any case. Make sure the board has had
plenty of time to dry before you (re)apply power to it.
- Inspect every connection to be sure it actually connects. I
always check my solder joints visually (bright light and a
magnifying glass really help here) as I'm building something up,
and do it with a multimeter if the circuit
acts oddly.
- If you're building a circuit
up on a PCB
or "perfboard," check your traces. Make sure there are no "broken
traces" (i.e., traces that are cut or otherwise interrupted), and
no "solder bridges" (bits of solder making undesired connections
between board traces. I check everything visually, then check for
problems with a multimeter where there's any question of
trace health (a scratch that looks superficial could be masking a
cut trace).
If you're freeforming,
make sure you don't have any accidental connections (things
touching that shouldn't be).
- It's hard to do, but you can damage components with
heat in the process of soldering them into place.
Counterintuitively, this is often caused by using a soldering iron
that's not hot enough, or underpowered (since you then have to
apply the iron for a longer period of time). You can also reduce
the likelihood of this happening by never soldering a component 's
leads consecutively (i.e., solder one lead, solder leads for one
or two components elsewhere, then come back).
- In some cases, it's possible to check components when they're
in-circuit
(i.e., after they're soldered in place). This is almost entirely a
function of the circuit
design, and where a given component is within it.
There's a wonderful site with tips on testing various
semiconductors (LEDs,
transistors,
diodes,
etc.) with a multimeter here.
