Wednesday, 27 January 2010

Using six outputs on Zimo MX620

I have a 2mm scale Class 24 diesel, fitted with a Zimo MX620. I wish to use all six of the outputs on the chip to drive various features; independent lights, working couplings, and I might use the last output for a cab light.

The standard Farish 24 comes with a circuit board for a six-pin (2 function) decoder, and took up all the top of the chassis. This has the lights paired front and back. So the board was replaced. I'll post a sketch of those new boards later, their main feature is a 2.2Kohm resistor on each function output to control the current to the LED lights at each end.

The Zimo MX620 has six function outputs. Four are normal 12v full current, and two are "logic level". Two of the normal outputs are on the wiring harness (or pins), and all the others are on solder pads at the other end of the chip (along with a "ground" or 0v, and another 12v (or blue) pad). The outputs Fo3 and F04 are enabled by a CV change; they have a lot of capabilities; Susi sound data output, Servo driver or normal function output.

The logic outputs are about 5v, and could drive a very small current LED. But, the wiring of the lamps in the class 24 make it difficult to connect; they are wired for a common positive supply.
So, it was necessary to make F03 and F04 drive a normal output. This can be done several ways, but this simple transistor circuit does the job nicely:

The only additions to the normal connection to a function output is the transistor, the 50Kohm resistor, and the connection back to the Ground Pad on the decoder.

In my case, I used the track pickup (decoder red wire) to provide the positive supply. I had to attach wires to the solder pads for ground and the Function output.

I built the circuit twice; once with big parts and using long leads from loco to a test board. The second circuit (used in the loco for real) is a few square mm of 0.3mm PCB with surface mount resistors, and transistor laying on its side with legs cut very short. Check all the resistances, etc. before hooking up to the decoder !