The simple five-volt DC circuit
Whether you’re working with a complex integrated circuit such as a PICAXE, or something simpler like the LM555 timing circuit, you need to have regulated, direct current.
While the LM555 is flexible about its voltage (it can operate with anywhere from five to 15 volts), the PICAXE requires five-volt current. For most of my illumination projects outlined on this web site, this circuit isn’t necessary. The lamp posts and the illuminated buildings all just use bridge rectifiers, resistors and sometimes zener diodes; no circuit boards need to be built (you just solder a couple of components together). But if you want to move beyond, to flashing lights or crossing lights, the five-volt circuit is necessary. And, if you’re new to electronics and soldering, building a couple of them can help give you confidence. As I’ve done more and more electronics experiments, it has turned out to be useful to have a 5-volt, DC circuit lying around, waiting for a project.
You can Google the phrase “5 volt DC circuit” and come up with 10-dozen circuit diagrams, and they all pretty much look like the one below. In fact, mine is an adaptation of Ken Bigelow’s, which you can find at http://www.play-hookey.com/analog/experiments/plus_5_volt_ps.html. The big difference between what he has done and what I did is the addition of the components to change AC to DC; those thoughts came from the writings at http://www.allaboutcircuits.com/vol_6/chpt_5/6.html.
How does it work? You start with 12 volts, AC, which comes (in my case) from a Malibu garden lighting transformer. The AC is sent through a bridge rectifier, which changes the alternating current to direct current; it’s still 12 volts. At this point we introduce a fairly large capacitor, (1000µf), which will take most of the peaks-and-valleys of voltage and current that are our U.S. electrical system and smooth them out. (From here, we can optionally tap off 12 volts DC, which sometimes is needed in circuits; the crossing light flasher taps from here.) Then we introduce a small integrated circuit, the 7805 voltage regulator. From the output of this device we now have five volts; we throw in a couple of other capacitors (10µf and .01µf) to smooth out the result and we’re ready to supply an IC its juice.
(Optionally, you can put an LED and a resistor — 1k ohm works for most LEDs — into the circuit at this point so that you know when the circuit is hot; with all the capacitors in this circuit, there’s bound to be some left-over current lingering even after the supply has been turned off. The LED here also helps discharge the capacitors, which is a Good Thing.)
In testing, I’ve found this circuit to work up to about one full amp; higher-duty capacitors and a bigger bridge rectifier are needed to get more than that.
You can easily build this circuit on one-half of a Radio Shack Dual PC Board (Part No. 276-148) and the adjacent photos and drawings show the part layout on that board.
Bill of materials
| • 10µF 35V Axial-Lead Electrolytic Capacitor | R.S. No. 272-1013 | $ 0.99 | |
| • 1000µF 35V Electrolytic Capacitor | R.S. No. 272-1019 | $ 1.69 | |
| • 0.1µF 50V Hi-Q Ceramic Disc Capacitor Pk/2 | R.S. No. 272-135 | $ 1.29 | |
| • 1.4A, 100V Full-Wave Bridge Rectifier | R.S. No. 276-1185 | $ 1.49 | |
| • +5V Fixed-Voltage Regulator 7805 | R.S. No. 276-1770 | $ 1.59 | |
| • Dual PC Board | R.S. No. 276-148 | $ 1.99 | |
| Subtotal | $ 8.05 | ||
| Optional |
|||
| • 1K Ohm 1/4-Watt Resistor5 Pack | R.S. No. 271-1321 | $ 0.99 | |
| • 3mm Red LED | R.S. No. 276-026 | $ 1.29 | |
| Total | $ 10.33 |
Tools
• Wire cutters.
• Needle-nose pliers.
• Soldering iron.
• Flux.
• Electronics solder.
• 24-gauge hookup wire.
Instructions
• Break the Dual PC Board into two pieces; store one piece for another project.
• Cut six lengths of the hookup wire to match the green wires in the layout diagram (make the AC leads about six inches long each). Cut one black wire and one red wire about six inches long (optionally, cut a second red wire if you want access to the regulated 12-volt DC).
• Place the hookup wire into the PC Board as seen on the adjacent layout diagram, counting holes to make sure they’re laid out correctly. I bend the wire on the underside of the board to make sure it stays in place.
• Place the components on the PC Board, again counting holes to make sure they line up right according to the adjacent diagram. The two axial-lead capacitors, the 10µf and the 1000µf, are polarity sensitive: make sure that you have the arrows on these pointing toward the left (negative) side of the board. I bend the leads on the components to make sure they stay in place.
• On the underside of the board, solder the connections, as seen in the adjacent diagram.
• Trim the excess leads on the underside of the board.
• Apply 12-volt, AC current to the AC leads of the board; using a multimeter, check the output of the red and black wires; it should be a nice, steady five volts. Optionally, test the 12-volt, DC output.
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