Nuts & Volts - April 2005

Project

Figure 3. The rates and widths are marked on the front panel.

of the components were installed on the board with the exception of C5 through C15, which I will address shortly.

Q2 was chosen for its high-speed operation and power handling ability. Most high-speed switching transistors will work fine, and even the lowly 2N3904 performed satisfactorily in this circuit. C16 should be mounted close to this collector. No circuit layout problems were encountered in this construction, even with the front panel wires grouped and laced. J1, J2, and J3 are all accessible through a cutout in the back wall of the housing.

As for the timing capacitors, C5 through C15, I elected to mount these between the decks of rotary switches S6 and S7. When the associated variable controls (P2 and P3) are in their calibrated position (fully counterclockwise), the rates and widths will be as marked on the front panel switch positions. These are as follows:

200 mA.

I built the actual circuit on a perf board with traces that would accept dual inline package (DIP) sockets. I always use sockets in my projects for future modifications or troubleshooting. This board was 3. 25 x 5. 5 inches and was laid out as shown in Figure 2. Almost all

RATE:

2 µsec, 10 µsec, 100 µsec, 1 msec, 10 msec, 100 msec

WIDTH:

1 µsec, 10 µsec, 100 µsec, 1 msec, 10 msec

F o r

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Circle #128 on the Reader Service Card.

The variable controls will take you from one range up to the next, with a 10 percent overlap to ensure complete coverage. In the calibrate position, accuracy will depend on how close the capacitor values are to the required values.

On some ranges, I nailed it on the first capacitor picked. On others, I had to sneak up on it with two capacitors (one large, one small). I aimed for one-percent accuracy on all ranges, and it was not difficult to obtain this. There is no point in trying for greater accuracy than this, as capacitor stability will not guarantee the timing will remain that stable. If you want greater stability and accuracy, you will have to use expensive polystyrene or similar capacitors.

These extreme accuracies are really not necessary, as you will usually be using this unit in conjunction with other test equipment (scopes, etc.) for cross-checking. The parts list shows the target values of capacitors C5 through C15. These values may vary in your particular circuit, but offer a close starting point.

Also, the potentiometers (P2 and P3) can be shunted with high-value resistors to slightly alter their values and bring the low-end timing (P2 and P3, fully clockwise) more in line with the expected rates and widths at these points. The important function in these two circuits is not so much that you have front panel accuracy, but rather that you have complete coverage across the switched bands.