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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

PICAXE-controlled slow-down for DCC model trains

Click for larger image

This PICAXE-controlled track switching circuit takes advantage of the “slow on DC” feature that most DCC (Digital Command Control) decoders have these days. When enabled, this feature will slow a train to a stop when the track signal changes from DCC pulses to pure DC. By detecting the presence of a train and then changing the track voltage from the DCC signal to DC, the train can be made to slow down and stop.

Every DCC decoder has CVs (Configuration Variables) which are in the software and accessible by the operator. Most DCC systems allow the operator to program the CVs and as well as this there are a few standalone CV programmers available.

The rate at which the train deceler-
ates and then accelerates away is
controlled by two CVs. CV3 controls the acceleration and CV4 the deceleration. All the user has to do is isolate a block of track at the required stopping point. An optical sensor placed at the start of the block will switch the voltage within the block to DC. After a few seconds delay (user-settable), the track signal is switched back to DCC and the train moves off. The result is quite realistic to watch.

Install the optical sensor so that the block beginning is about two
metres (depending on your layout size) from the required stopping point. A 2-aspect signal should also be placed at this point. Using trial and error, program the decoder’s CVs to stop the train at the signal.

Place the optical detector about 1.5 loco lengths in from the isolated join. Now it is time to set the required stopping/wait time. Place the train a metre or two before the beginning of the block. Press and release the set-up button and then start the train. When the train crosses the isolated join, press and release the set-up button again. The train will now slow and should then stop at the signal. When you are satisfied with the stationary time press and release the set-up button again. The track will now go back to DCC and the train will move off. The time just set is now stored in the PICAXE’s memory and will be loaded next time the system boots up.

The unit will go to normal mode and that is it. Each time the train crosses the sensor it should slow and stop at the signal which is red. After your set time the signal will change to green and the train will move off.

Note the alternative power arrangements for the circuit. As shown, the links at the top righthand corner allow it to be powered from low voltage (eg, 12VAC) from a separate transformer or from the DCC voltage applied to the track.

The software for the PICAXE (DCC_Auto_stop.BAS) will be available on the SILICON CHIP website.

Jeff Monegal,
North Maclean, Qld. ($45)

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