Chips, Ahoy! 151Shopping List: Experiments 16 Through 24Latching relays
You’re going to need a 5-volt latching relay that has two coils, instead of
one. The first coil flips the relay one way; the second coil flips it back. The
relay consumes no additional power while remaining passively in each
state. I suggest the Panasonic DS2E-SL2-DC5V relay. If you buy a different
relay, it must be dual-coil latching to run off 5 volts DC, switching at least
1 amp, in a “2 form C” package, to fit your breadboard.
Potentiometers
You’ll need 5K, 10K, and 100K linear potentiometers (one of each). Also, a
10K trimmer potentiometer (which you may find described just as a “trim-
mer”). The manufacturer is unimportant.
Voltage regulators
Because many logic chips require precisely 5 volts DC, you need a voltage
regulator to deliver this. The LM7805 does the job. Here again, the chip
number will be preceded or followed with an abbreviation identifying the
manufacturer and package style, as in the LM7805CT from Fairchild. Any
manufacturer will do, but the package should look like the one in Figure
4-8, and if you have a choice, buy a regulator that can deliver at least 1
amp.
Tactile switches
These are SPST pushbuttons (momentary switches), usually with four legs.
Look for the ALPS part number SKHHAKA010 or any similar item that has
pins to fit your breadboard or perforated board. See Figure 4-9.
12-key numeric keypad
Velleman “12 keys keyboard with common output” (no part number, but
has been available through All Electronics under catalog code KP-12).
Quantity: 1.
This type of keypad has the same layout as an old-fashioned touchtone
phone. It should have at least 13 pins or contacts, 12 of which connect
with individual pushbuttons, the thirteenth connecting with the other
side of all the pushbuttons. In other words, the last pin is “common” to all
of them, and this type of keypad is often described as having a “common
output.” The type of keypad that you don’t want is “matrix-encoded,” with
fewer than 13 contacts, requiring additional external circuitry. See Figures
4-10 and 4-11. If you can’t find the Velleman keypad that I suggest, look
carefully at keypad descriptions and photographs to make sure that the
one you buy is not matrix-encoded and has a common terminal.
Alternatively, you may substitute 12 cheap SPST NO pushbuttons and
mount them in a small project box.
Figure 4-8. Many integrated circuit chips
require a controlled power supply of
5 volts, which can be delivered by this
regulator when you apply 7.5 to 9 volts to
it. The lefthand pin is for positive input,
the center pin is a common ground, and
the righthand pin is the 5V output. For cur-
rents exceeding 250mA, you should bolt
the regulator to a metal heat sink using
the hole at the top.Figure 4-9. A tactile switch delivers tactile
feedback through your fingertip when you
press it. They are almost always SPST
pushbuttons designed for mounting in
circuit boards with standard 1/10-inch
hole spacing.