Update Home Brew Sound Card Interface Kit Building Session!

Walt KF5AQ reports:

Greetings All;


As referenced in my previous email (10/21/2017, attached below) I have developed and tested an ‘Engineering Change’ to correct the wiring error on the wiring side pictorial diagram and to correct the low voltage to activate the relay.  I have attached updated copies of the schematic and the wiring side pictorial.  The markup of the pictorial is confusing, but I’ll try to explain.

To correct the relay wiring error on the pictorial (and on your board if you have followed the pictorial) is fairly simple.  Looking at the wiring side pictorial there are 4 red circled areas at the relay ‘common’ and ‘NC’ pins.  On the ground bus on row ‘N’ cut it away from the ‘NC’ pins and attach it to the ‘Common’ pins.  On the other side of the relay (row ‘I’) move the return wire from the ‘RX’ led from the relay ‘Common’ to the relay ‘NC’ pins.  This corrects the relay wiring.

To increase the relay driver voltage a transistor and resistor are added to create a ‘Darlington Pair’ to greatly increase the transistor gain.  The resistor is probably not necessary, but is to prevent false activation of the relay in case of excessive leakage of the input transistor.  Both transistors are 2N3904s.  These changes are shown on the pictorial within the red circle around the upper right transistor area of the pictorial. See the schematic for clarification of the connections.  This change increases the relay coil voltage to about 4.2 volts, very safely above the 3.75v needed for reliable operation.  The original circuit provided approximately  3.2 volts to the relay coil which did work MOST of the time.  I will provide the additional transistor and resistor at the build day on Nov. 04th.  Or at lunch next Thursday. (Guys who were there today already have theirs — see what you missed!)

I’m not sure you can tell much from them, but I have also attached pictures of both sides of a completed card with these changes implemented and working (at least on my test bed without a radio).

I have spares for most of the parts should you need them.  I will have them with me next Thursday and on Nov 4th.

I promise (fingers crossed) that I will have the front and back panels for the enclosure drilled  and available by Nov 4th.


Walt  AF5KQ

PS:  To those who have not picked up your kit.  I still have two available and will have them on Thursday Nov. 2 and Saturday Nov. 4.


On 10/21/2017 8:49 PM, Walter H. Wilson wrote:

Hello Crew,

I have been doing a bit of research on the low voltage to the relay coil, sometimes too low to transfer the relay.  I found a datasheet for the relay which specifies the maximum voltage for the relay to activate as 3.75 volts. Our current circuit (2 samples) is a bit below this level.  I am continuing to look at possible changes to increase this voltage to improve the reliability of the circuit.

In looking at the datasheet, I found an error in Gary’s pictorial diagram of the wiring side of the circuit board.  The schematic is correct, but the pictorial has the relay common and normally closed pins interchanged so that when the relay is energized an incorrect  (or no) signal is sent to the din connector PTT pin.  Correction of this is fairly simple — just swap the wires between the two relay points.  Below is a copy of the relay wiring diagram showing the correct relay pin-out.  The picture is a bit small, but the relay commons are the pins next to the wide space near the relay coil pins.  Starting at the coil end, the pins are coil, extra space, common, normally closed,  then normally open.

In my experimenting, I have increased the relay coil voltage some (to about 3.6v) by increasing the value of the capacitors at the input to the relay driver transistor.  My next idea is to swap the transistor from a 2N3904 to a 2N2222 which has a higher current rating.  I hope to try that tomorrowor Monday.


Walt  AF5KQ

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