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...

  1. 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.

  2. 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.

  3. 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.

  4. 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).

  5. 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).

  6. 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.

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