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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.

Using the Enhanced Voice Recorder in a model railway

Since publication of the Enhanced 45-Second Voice Recorder in the December 2007 issue of SILICON CHIP, we’ve had requests for more information on using the recorder module in a model railway layout. Presumably this is for providing station announcements, loco whistle sounds etc, whenever a train passes over reed switch sensors placed under the track in various places.

This circuit (Fig.1) shows how this can be done. The basic idea is to connect the reed switch sensors (as shown in the diagram on page 70 of the December 2007 issue), so that each one pulls one of the module’s MSG-bar lines down to ground, whenever the magnet on a train loco passes overhead. It’s a good idea to connect a 4.7μF tantalum or electrolytic capacitor across each reed switch as well, to ensure that even if the loco magnet passes over the switch very briefly, the module input line is still effectively pulled down to logic low (0V) for long enough to trigger message playback.

This works because even a brief reed switch closure discharges the capacitor across it but when the switch opens the capacitor must recharge (through the 22kΩ pull-up resistor inside the module) before the voltage across that input rises to logic high level again. So the effective logic low/switch closure time is extended long enough to ensure reliable message triggering.

The rest of this new circuit is provided to show you how the replayed announcements or sounds from the recorder module (fed through a small audio amplifier) can be switched automatically to any of eight different speakers, using the same reed switch closures. This should give greater realism, because the sounds concerned will emanate from the specific location of the train rather than from a single centralised speaker.

The way the circuit works is by using eight D-type flipflops (in IC4 & IC5) to store the status of all eight message trigger lines, whenever any one of the trigger lines is pulled low by a closure of its reed switch. The storage is produced by a positive-going edge produced by 8-input gate IC3, used here as a negative input logic OR gate. Eight of the inverters in IC1 and IC2 are used to invert the message trigger line levels, so only the line which has been pulled low will be stored as a ‘1’ (the rest, being high, will be stored as a ‘0’).

The eight outputs from the storage flipflops inside IC4 & IC5 are connected to eight relay driver circuits using transistors Q1-Q8. As a result, the relay (RLY1 - RLY8) corresponding to the message line which was pulled down by its reed switch will be energised, switching the output of the audio amplifier through to that particular speaker.

For example, if reed switch 8 was closed, a ‘1’ will be stored in flipflop 5 of IC4 and O5 (pin 15) of IC4 will go high, causing Q8 to be turned on and energising RLY8 to switch the audio through to speaker 8.

Whichever speaker is turned on to replay the sound at that location also needs to be turned off again, when the replayed sound or message ends. This automatic turn-off is achieved by the simple circuitry involving diode D1 and inverter IC1f, which senses the BUSY-bar logic signal available from pin 10 of the HK828 recorder chip inside the recorder module. This logic signal goes low during playback (or recording) of any of the messages and returns high only when the message ends.

The ‘return to high’ edge of the signal is differentiated by a 100nF capacitor and 100kΩ resistor and clipped by diode D1 at the input of IC1f, to produce a brief positive-going pulse. IC1f inverts this pulse to produce a brief active-low reset pulse for both IC4 & IC5. As a result, all the storage flipflops are reset at the end of any message playback, turning off all relays at the same time.

The relays used in this circuit are not critical but should be capable of operating from 12V with a coil current of no more than about 30-40mA. The low cost SY-4066 relays sold by Jaycar would be quite suitable.

Jim Rowe,

SILICON CHIP.

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