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Automatic Reversing Circuit – Mk IV

Automatic Reversing Circuit – Mk IV

This page shows information for the 4th version of the Automatic Reversing circuit. This includes details regarding the schematic, operation and a printed circuit board.
Automatic Reversing Circuit Mk IV
The following diagram is the complete schematic for the Automatic Reversing Circuit Mk IV.

Operation is controlled by four infrared/visible light sensitive phototransistors (Q1, 2, 3, 4) that are mounted through the roadbed and between the rails. As the SHUTTLE passes over each sensor the corresponding section of the circuit is activated.

Automatic Reversing Circuit Mk IV schematic
Light Emitting Diodes indicate when the phototransistor sensors are covered (LED 1, 2, 3, and 4) and also which direction the SHUTTLE will travel when the wait timer has run out (LED 5 and 6).

A throttle has been incorporated into the circuit and features automatic current limiting and a short circuit indicator (LED 7).

The circuit uses a 74LS00 TTL logic integrated circuit to provide a memory function for the circuit. One half of this device controls the timers that determine how long the SHUTTLE will wait at each end of the track while the second half remembers the SHUTTLE’s direction and controls the reversing relay.

A basic description of how the circuit operates, general notes and information regarding a printed circuit board appear further down this page.

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Automatic Reversing Circuit Mk IV – Basic Circuit Operation
To start: The SHUTTLE is sitting over the EAST end STOP sensor (Q1). The indicator – LED 1 is ON – Timer LM556 B is active.

When B timer completes its cycle its OUTPUT will go LOW and the indicator – LED 6 will turn ON. The direction relay, RELAY 1, will be set for EAST to WEST travel and the SHUTTLE will begin to accelerate.



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The SHUTTLE will continue to accelerate to its maximum as set by R26.

When the SHUTTLE covers the BRAKE sensor Q3 the indicator – LED 3 will turn ON. The LM556 B timer is reset and the LM556 A timer is activated.

At the same time as SHUTTLE covers sensor Q3 the the SLOW brake will be activated and the SHUTTLE will decelerate.

When the SHUTTLE covers the STOP sensor Q4 the indicator – LED 4 will turn ON and the QUICK brake will be activated. The SHUTTLE will stop in a distance of approximately 1/4 of a car length or less.

The SHUTTLE will wait at the WEST end of the track until the LM556 A timer completes its cycle.

When the LM556 A timer completes its cycle its output will go LOW and the indicator – LED 5 will turn ON. The direction relay, RELAY 1, will be set for WEST to EAST travel and the SHUTTLE will begin to accelerate.

When the SHUTTLE covers the BRAKE sensor Q3 the indicator – LED 3 will be ON but there will be no change in the circuit.

The SHUTTLE will continue to accelerate to its maximum.

When the SHUTTLE covers the BRAKE sensor Q2 the indicator – LED 2 will turn ON. The LM556 A timer is reset and the LM556 B timer is activated and begins its timing cycle.

At the same time as SHUTTLE covers sensor Q2 the the SLOW brake will be activated and the SHUTTLE will decelerate.

When the SHUTTLE covers the STOP sensor Q1 the indicator – LED 1 will turn ON and the QUICK brake will be activated. The SHUTTLE will stop in a distance of approximately 1/4 of a car length or less.

The SHUTTLE will wait at the EAST end of the track until the LM556 B timer completes its cycle.

The cycle will now repeat itself with the SHUTTLE traveling between and waiting at each end of the track.

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