Nuts and Volts - February 2013

thru-hole parts as the power planes are heatsinking.

One of the more difficult parts of any homemade equipment is creating a good looking front panel. I chose a minimalist approach, i.e., just enough lettering to identify the various controls and switches, and no more.

The front panel is drilled before the lettering is applied, eliminating a lot of planning and coordination of the exact location of the switches and potentiometers. That is, the holes for the switches do not have to be placed in precise locations to match the lettering; the lettering is placed after the holes are drilled.

Using self-adhesive paper (Avery labels), I print the lettering with an inkjet printer and then seal the print with a spray-on fixative. The lettering is then cut to size and applied to the front panel. This technique creates a functional and good looking front panel with minimal effort. (Or, you can use services from companies such as FrontPanelExpress.com.) Feel free to rearrange the front panel to suit your needs.

Any single pole rotary switch with nine or more positions can be used. A common variety found in the hobby/surplus market is the one pole/12 position type. The specified switch (Mammoth Electronics #820- 1P2-12T) has a stop feature that allows rotation to be limited to the nine positions needed for this project. (Mammoth Electronics will give a 15% discount if you mention Nuts & Volts.)

If you choose to use a switch without a "position stop," connect the extra positions together with the first or last position so the circuit outputs a signal when one of the extra positions is selected.

Note: When wiring the frequency range switch (SW1), increasing frequency with clockwise rotation requires the switch to be wired in the opposite direction of the schematic numbering: f1 goes to switch position 9; f2 goes to switch position 8; etc.

PARTS LIST QTY DESIGNATOR DESCRIPTION

7 C1- C3, C5, C7, C9, C10 0.01, radial ceramic, 50V, 10%, XR7 32 C4, C6, C8, C11-C39 0.1, radial ceramic, 50V, 20%, Z5U 4 C40-C43 22 µF, 10V 1 C44 22 pF, radial ceramic, 100V, 5%, C0G (NP0) 1 Ctf 82 pF, radial ceramic, 100V, 5%, C0G (NP0) 1 Ctp 47 pF, radial ceramic, 100V, 5%, C0G (NP0) 1 D1 1N4148 1 OSC 10 MHz 1 R1 200, 5%, 1/4 watt 1 R2 220, 5%, 1/4 watt 3 R3, R4, R11 1K, 5%, 1/4 watt 1 R5 100K, 5%, 1/4 watt 1 R6 200, 5%, 1/4 watt 1 R7 220, 5%, 1/4 watt 1 R8 10K, 5%, 1/4 watt 2 R9, R10 51, 5%, 1/4 watt 4 RN1-RN4 10K Eight-pin resistor network-bussed 2 U1, U5 Schmitt trigger inverter - SOT- 23 4 U2, U3, U7, U10 Configurable multi-function gate - SOT- 23 2 U4, U6 Dual FET bus switch - SOIC 2 U8, U9 Dual J-K flip-flop - SOIC 8 U11-U15, U17-U20 Dual decade counters - DIP 4 U15, U16, U21,U22 Quad bus buffer gate - DIP 1 U23 Hex inverter - DIP 1 U24 Dual peripheral drivers - DIP Panel Mount Components 1 LED1 1 LED2 2 POT1, POT2 2 SW1, SW2 1 SW3 1 SW4 3 SW5-SW7 3 2 2 2 1 1

Green Red

10K audio Rotary switch, SP- 12 pos Toggle switch, SPDT on-none-on Momentary toggle switch, SPDT on-(on) Toggle switch, SPDT on-on BNC or RCA phono jacks Grounding lugs Knob/range Knob/adj Instrument case Power supply, 5V/0.5A regulated

A more detailed Parts List with both manufacturer and distributor part numbers is available at the article link.

The potentiometer housings need to be grounded to stabilize stray capacitance in the oscillator adjustment circuits. Start by scraping the paint on the inside of the front panel around the potentiometer's mounting hole. Then, use a grounding lug with a star washer when mounting the pot. Connect the lug to ground using the corresponding hole provided on the PCB (GFp and GPp). If you don't have a grounding lug, you can wrap a piece of bus wire around the bushing and use a regular star washer.

Note: Capacitor C44 is mounted at the pulse width ADJ potentiometer (POT2) between the potentiometer and the ground lug (wire); see

Figure 5. A grounding lug is specified in the Parts List. Wiring to the switches is not critical. Just hook them up neatly and you will be fine. (Refer back to Figure 4.) The LEDs are held in place with glue.

Either RCA phono jacks or BNC connectors can be used for the outputs. The phono jacks are much less expensive, and sufficient for most uses. Wiring to the outputs should be short and neat twisted pairs.