The amended objective was to produce a simpler, no-frills DC controller that would meet the original 'anytime anywhere' requirement, working with either a battery, AC or DC power.

Circuit Design

Starting with the emf-sensing feedback circuit used in Part 1, I removed the integrator and schmitt trigger U1 leaving just U1b as a simple non-inverting amplifier. The output driver Q1 - a cheaper and more easily obtainable BD243C - is included in the gain loop to ensure good output voltage regulation. Resistor values were re-calculated so that RV1 would provide a full 12v range at the output. D3 is retained to clamp any back-emf or spikes from the track and/or loco motor. D2 protects the op amp from reverse voltages at power off by discharging C3 via the supply rail.

I also took the opportunity to add a current limiter to protect Q1 from track short-circuits - more a case of 'when' than 'if'. R7 and Q2 work to 'steal' current from Q1 when more than about 500mA flows through R7 - adjust R7 for different limit-current values. Two 3R3 0.25W resistors in parallel worked well in my case - my N locos draw up to about 250mA. Note that Q1 will need a heat sink for higher current motors; use of a metal box might be the best approach.

The unused half of the op amp is wired as a unity gain stage (pins 6 & 7 linked) with its input (pin 5) grounded so as to minimise stray pickup and noise.

To aid testing, typical voltages are shown in green.

Construction

The circuit was built using the same approach as in Part 1 - perforated stripboard - and with the same size plastic box 80x60x40mm. The reduced component count meant an easier fit on the board, but my board layout shown below is rubbish - too many wire links. In case anyone else wants to build it, pinouts for active components are shown below.

All wiring was again checked twice.

Testing

As this is plain old DC a voltmeter should suffice when checking - no wizzy pulses to worry about this time

Power was first applied with no load. The output seemed to waggle happily up and down as RV1 was turned forward and back. With a loco on the track, smooth control was noted - very gratifying! As expected, creep control was not as good as the feedback controller. However, it was possible to move a loco very slowly with about 2.5v applied to clean track.

One observation: "zero output" at the minimum setting of RV1 is actually 2mV. It may be just stray pickup in my test rig, and is unlikely to cause problems, but worth noting.

Top speed using a 12v battery - giving about 10.5v to the track - was more than fast enough for realistic running, so maximum oomph will rarely be required when using a 13-15v power pack.

Conclusion

Sometimes simple (KISS) is best!