Minimalist Throttle ~ Saturday, February 26, 2011

The rest of the model railroading hobby may be embracing DCC, but I work on software for a living, and the last thing I want to do is to get computers mixed up in my hobby. It's still fun to dabble in analog electronics, though.

Lately Nathan & I have been running trolleys and trains independently of our regular layout, just snapping together extra pieces of Tomix Fine Track on the dining room table. For quick set-up and disassembly, I thought a simple, battery powered controller would be nice. Tomix actually makes such a thing--it looks like a miniature vintage streetcar controller (cool!). But I thought it would be fun to put together a simple transistor throttle, basically a 9v battery combined with a minimalized version of the "NTrak Basic Throttle".

Putting together some basic components (most from Radio Shack, some from Fry's) I came up with this:



Schematically, as below. (Here I'm adding two switches that I don't have in my breadboard mock-up version, but which are really necessary for a usable throttle--an on/off switch and a DPDT direction switch.)



The potentiometer is 1.0k linear, the fixed resistor is 100 ohms, the transistor is a TIP 120 Darlington. (A Darlington Transistor is actually a bundling of two transistors in one package, for double amplification).

The basic idea is that a small amount of current flowing from base (B) to emitter (E) controls/scales the amplitude of a larger current flowing from collector (C) to emitter (E), and from there through the motor. The controlling B->E current depends on the setting of the potentiometer; sliding that arrow "up" and "down" determines how much resistance there is in the B->E path vs the "vertical" path through the potentiometer that bypasses the transistor and motor.

It works!



I'm going to add one last tip: the package of the transistor (at least the one I got) explains which lead is which, but there's no markings on the actual thing--so put a dab of paint on each, in different colors.