Connect the Mini RDS to a FM25B

**Wayne J** · 08-23-2008, 09:17 PM

Here are some instructions for connecting the Pira Mini RDS unit to a Ramsey FM25B. These instructions are the setup I used and work for me.

First the BOM from Mouser. I had most of the parts at hand, and Radio Shack had the rest, so you may can find the parts locally.

BOM - capacitors (22pF,220pF) & 62K ohm resistor
140-50N5-220J-RC 1
140-50S5-221J-RC 1
291-62K-RC 1

BOM for COM port connection cable (bidirectional):

78-1N4148 1
512-1N4733A_Q 2
291-680-RC 3
299-2K-RC 1
156-1309 1

I also used some salvage parts from some old PCs, the 3 pin female connectors and two pin connectors.

I made the COM port cable first, using the diagram found here halfway down the page in the "How to connect the unit to a PC computer" section. There are four different configurations. I chose the COM port bidirectional, as I have plenty of COM ports and wanted two way communication for the added functionality.
If you use this configuration, you must change out R9 on the RDS to a 2k ohm resistor. A picture of which resistor R9 is here.

This is the finished product.

The three wires is a harness salvaged from an old PC with a 3 pin female connector on the end. Same for the two pin cables for power and RDS input. The three pin header designation is here in the "unit layout section".

Next was to assemble the RDS injection point components. My first try did not work. I assembled them in the RDS unit enclosure and just ran the wire to the FM25B pcb. This DOES NOT work!
The components MUST be soldered directly to the transmitter pcb. Once I did that, it worked perfect.

Follow this schematic. As for the connector, I just plugged it directly to the RSD unit with a 2 pin female connector from an old PC.

Here is some pictures for the connections.

I am powering the RDS from the FM25B power. I tapped onto the switch, so when I power the transmitter, the RDS comes on also. Here is the connection points on the FM25B. Follow the connections to the RDS from here in the "unit layout section".

I used a small enclosure for the RDS and used velcro to attach it to the FM25B case. The part# for the case from Mouser is 546-1551HBK if you would like to do the same.

And some pics of the final product and a shot of the radio in the car.