Servo - May 2008

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%