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How Electronic Gates Work


The Power Supply

You will definitely need a regulated 5-volt power supply to work with TTL chips. As mentioned previously, neither Radio Shack nor Jameco seem to offer a standard, inexpensive 5-volt regulated power supply. One option you have is to buy from Jameco something like part number 116089. This is a 5-volt power supply from an old Atari video game. If you look in the Jameco catalog, you will find that they have about 20 different surplus power supplies like this, producing all sorts of voltages and amperages. You need 5 volts at at least 0.3 amps (300 milliamps) -- you need no more than 2 amps, so do not purchase more power supply than you need. What you can do is buy the power supply, then cut off the connector and get access to the 5-volt and ground wires. That will work fine, and is probably the easiest path. You can use your volt meter (see below) to make sure the power supply produces the voltage you need.

Your alternative is to build a 5-volt supply from a little power-cube transformer. What you need is a transformer that produces 7 to 12 DC volts at 100 milliamps or more. Note that:

  • The transformer MUST produce DC voltage.
  • It MUST produce 7 to 12 volts.
  • It MUST produce 100 milliamps (0.1 amps) or more.

­ You may have an old one lying around that you can use -- read the imprint on the cover and make sure it meets all three requirements. If not, you can purchase a transformer from Radio Shack or Jameco.

Radio Shack sells a 9-volt 300-milliamp transformer (part number 273-1455). Jameco has a 7.5-volt 300-milliamp model (part number 149964). Clip the connector off the transformer and separate the two wires. Strip about a centimeter of insulation off both wires. Now plug the transformer in (once it is plugged in, NEVER let the two wires from the transformer touch one another or you are likely to burn out the transformer and ruin it). Use your volt meter (see below) to measure the voltage. You want to make sure that the transformer is producing approximately the stated voltage (it may be high by as much as a factor of two -- that is okay). Your transformer is acting like a battery for you, so you also want to determine which wire is the negative and which is the positive. Hook the black and red leads of the volt meter up to the transformer's wires randomly and see if the voltage measured is positive or negative. If it is negative, reverse the leads. Now you know that the wire to which the black lead is attached is the negative (ground) wire, while the other is the positive wire.


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