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Digital Audio Signal Generator, Pt.2

Last month, we described how the S/PDIF Digital Audio Signal Generator works. This month, we describe how to assemble the PC boards, mount them in the case and check that they are working correctly.

By Nicholas Vinen

Choosing The Trickle Charge Resistor

As mentioned in Pt.1, Nickel Metal Hydride (NiMH) rechargeable cells can be used to power the unit and the circuit includes a 10Ω 1W resistor to trickle charge them whenever the plugpack is connected. This resistor value is suitable for 2000mAh cells and provides just under 100mA to the cells once they are fully charged

This equates to a charge rate of C/20 for 2000mAh cells, although it will be appreciably higher than this when the cells are flat.

If you use lower capacity cells, then you need to increase the value of the resistor accordingly. For example, 800mAh cells require a 27Ω 1W resistor, while 600mAh cells require a 33Ω 1W resistor.

Note that you should install this resistor only if you intend using NiMH or Nicad cells in the device. Do not install it if you intend using alkaline (or any other non-rechargeable) cells.

The Digital Audio Signal Generator is built on two PC boards: a main board and a control board. Construction can begin with the assembly of the main PC board. There are two versions, one to suit the Jaycar case (Fig.10) and the other to suit the Altronics case (Fig.11). The Jaycar main board is coded 04203101 while the Altronics board is coded 04203103.

Before starting, examine the copper side of the PC board for any defects. It’s also a good idea to place it inside the case, up against the end, in order to check that it fits properly. Verify that the mounting holes line up with the posts in the base of the enclosure.

Once you are satisfied that it will fit, start the assembly by installing the seven wire links. You can use 0Ω resistors for the shorter links and 0.71mm tinned copper wire for the longer ones (or you can use tinned copper wire for the lot).

Click for larger image
Fig.10: follow this diagram to build the main PC board for the Jaycar case. Make sure that all polarised parts are correctly oriented and install the 10Ω 1W resistor only if you intend using rechargeable cells (see panel).

Next, install the 0.25W resistors. It’s best to check the value of each with a DMM before installation, as the colour codes can be hard to read.

Follow these with the four diodes (D1-D4) and zener diode ZD1. Pay careful attention to the orientation of these parts. You will have to bend the leads of the 1N5819s close to their bodies for them to fit.

The five IC sockets can now be installed. Be sure to line the notches up with those shown on the overlay. Solder two diagonally opposite pins on each to begin with, then check that they are sitting flat on the PC board before soldering the rest.

Crystals X1 and X2 are next on the list. Be careful not to get them mixed up. The markings on their cases should match the corresponding frequency values on the PC board overlay.

Once these are in place you can install the 10Ω 1W resistor (see panel). If you install it, you can only use rechargeable cells or the plugpack to power the device – you cannot use alkaline or other non-rechargeable batteries. If you do want to use alkaline batteries (or the plugpack), then leave this resistor out.

Since the physical size of this resistor can vary, you will need to make sure that it doesn’t interfere with the battery connector. If necessary, install it slightly proud of the PC board so that it sits above the adjacent 1N5819 diode (D2).

Follow with the two TO-220 voltage regulators (REG1 & REG2), taking care not to get them mixed up. In each case, bend the leads down through 90° about 5mm from the body using a pair of needle-nose pliers. That done, mount the device on the board, line up the tab mounting hole and secure it using an M3 x 6mm machine screw, nut and star washer (the latter goes under the head of the bolt).

Finally, solder the leads to their respective pads and trim away the excess. Do not solder the leads before you have bolted the devices down, otherwise you could crack the copper tracks as the screw is tightened.

The next step is to install the IDC (insulation displacement connector) socket (CON4). It should be installed with its notched side towards the bottom (see layout diagram). Solder pins 1 & 16 first and make sure the socket is sitting flush against the board before soldering the rest.

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