Need a PCB?
The PCBs and/or a complete kit for this project can
be purchased through the SERVO Magazine
Webstore at www.servomagazine.com.
either purchase a higher tolerance X7R
or temperature compensating NPO or
COG types.
Since the output of IC2 is a pulse
train, components C4, D3, and R4 form a
differentiator to get the negative-going
edge required to trigger IC3 in one-shot
mode. The output of IC3 goes to the FIGURE 9
three-pin header, so R8 is there in case the
signal pin gets shorted to ground or +5V. The output pulse
width of IC3 is set by R5, R6, R7, and C6. R6 is a single-turn
pot with a shaft for easy adjustment (see Figure 9). The pot
is a no-name brand I bought from Electronix Express (part
#18STS100K). It’s similar to a Bourns type 3386T.
If you need more precise control of servo position, use
a multi-turn pot. R5 in series with the pot and R7 across the
pot allow you to set the minimum and maximum pulse
widths. The values used here give a range of 0.8 to 2.5 ms.
The pulse width can be observed at test point TP.
Construction
Construction is straightforward. You will need a
breakaway (“snappable”) type male header with at least
five pins. For the jumper JP, you need two pins and for the
servo connector you need three pins. Such header strips
are available from distributors such as Jameco, Electronix
Express, and others. The circuit is simple enough to build on
a pre-drilled proto board like the RadioShack 276-150. SV
ITEM
R1
R2
R3A
R3B
R4
R5
R6
R7
R8
Parts List
ITEM DESCRIPTION
C1 0.1 μF, 50V monolithic
C2 100 μF, 10V electrolytic
C3 0.1 μF, 50V monolithic
C4 0.001 μF, 50V monolithic
C5 0.1 μF, 50V monolithic
C6 0.022 μF, 50V monolithic
C7 0.1 μF, 50V monolithic
C8 0.1 μF, 50V monolithic
DESCRIPTION
1K, 1/4W, 5%
22.0K, 1/4W, 1%
20K, 1/4W, 5%
133K, 1/4W, 1%
10K, 1/4W, 5%
33K, 1/4W, 5%
100K pot (seetext)
220K, 1/4W, 5%
100 Ω, 1/4W, 5%
IC1
IC2,IC3
LM7805
LMC555
D1
D2
D3
1N4001
Green LED
1N914
• PCB or proto board
• Removable jumper (Jameco p/n 112416; Electronix Express
p/n 24SHRTBAR, or equivalent)
• Male header strip on .100 inch centers, break-away type
(AMP type 103185 or equivalent)
• Two-position terminal block (RadioShack p/n 276-1388 or equivalent)
SERVO 05.2008 35