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Endless loop tape Fancy a barking doorbell? Or how about a novel pre-recorded message system for a shop display, or for the home? This endless loop tape recorder is the answer. By GREG SWAIN Initially, we didn't quite know what to do with this intriguing little gadget. After all, what possible use is a tape recorder that can only play for 20 seconds before repeating itself? Then we began to think of all sorts of applications. In addition to those already mentioned, it could be used for pre-recorded announcements over a PA system, as an appointments reminder, or as a convenient message system for family members. It could even be used to store telephone numbers, or just as a source of amusement. If you want a really novel doorbell, this unit will do the job. You could record a message telling your guests that the barbeque is around the back, or you could record the spine-chilling sound of a slobbering, salivating dog. Mind you, the effect would be lost when the caller took his finger off the button. (One of our staff members, who shall mercifully remain nameless, decided to do an imitation of a furious canine and performed with great gusto while holding down the record button. It was very effective too although the effect was not quite what he intended. We all broke up whenever it was played back). A pre-assembled tape recorder module from Jaycar Electronics forms the basis of the unit. Instead of the familiar cassette mechanism, it uses a continuous tape loop with only one spool. The tape is wound in a couple of layers on the outside of the spool and is withdrawn from +v---•.il.\!!!.---------------. S1 R-P RECORO/ PLAYBACK an SPEAKER .,. MICROPHONE ELECTRET .,. 22D + 1DVWJ 47Dll + sotw : MOTOR ERASE .,. ENDLESS LOOP TAPE PLAYER SC1-1-488 Fig.1: the circuit for the endless loop tape player. IC1 provides signal amplification while IC2 is for motor speed control. 40 SILICON CHIP player PARTS LIST 1 ED-1 000R endless loop tape unit (available from Jaycar) 1 plastic case, 159 x 96 x 54mm (Altronics Cat No H-0201 or equivalent) 1 Scotchcal front-panel label, 154 x 90mm (optional) 1 50mm diameter 811 loudspeaker 1 4-way AA battery holder 4 AA batteries (preferably alkaline) 1 battery clip to suit holder 1 momentary pushbutton switch 1 pushbutton cap (to suit changeover switch) 1 electret microphone We built the tape module into a plastic zippy case, along with a battery power supply. The speaker, microphone and run switch were fitted to the front panel. Miscellaneous Screened cable, hookup wire, scrap aluminium for battery clamp, self-tapping screws the bottom layer. Apart from that, the rest of the transport mechanism is fairly standard with a capstan, pinch roller and belt-drive from the motor. A spring-loaded multipole pushbutton switch on the side of the module provides the changeover from playback to the record mode. Whenever power is applied the unit is automatically in the playback mode. Putting the module into playback is a good way of finding out what "endless" means; it goes on and on and on. All that is need to make it work is an electret microphone insert, a small 811 loudspeaker and a source of DC which may be anywhere between 4.5 and 6 volts. This power can be derived from batteries or a 4.5V DC mains plugpack. How it works Fig.1 shows the circuit of the ED-1000 endless loop tape recorder. Most of the work is done by ICl which provides all the necessary signal amplification. This view shows the parts layout inside the case. The tape module is secured to one end of the case by two self-tapping screws. In the playback mode , ICl amplifies the signal from the record/playback head and drives the loudspeaker via a 100µ,F capacitor. In the record mode, it amplifies the signal from the electret microphone and drives the record/playback head via a parallel 33k11 resistor and .022µ,F capacitor. This network provides a modest degree of treble boost for the recorded signal. A 5-pole 2-position slide switch, referred to above, is used to switch the circuit between record and replay. In the record position, DC MARCH 1988 41 Above: follow this labelled photograph when hooking up the external connections to the tape player PCB. Note that the electret microphone should be wired using shielded cable. At right is a close-up of the player module showing the endless tape-loop mechanism and the record and replay heads. bias is supplied to the erase head via a 4700 resistor and to the record/playback head via a 22k0 resistor. IC2 (KA2402) and its associated components are used for motor speed control. Construction The above circuit details have been induded so that you'll know r. where to hook the external components. We mounted the tape mechanism in a plastic case measuring 159 x 96 x 54mm. The loudspeaker, electret microphone and switch are all mounted on the lid of the case, while the tape unit is secured to the base using self-tapping screws. Start construction by drilling two holes in the base that align with the • ENDLESS LOOP TAPE PLAYER L: 42 • plastic posts on the bottom of the tape unit. Clean up the holes with an oversize drill bit, then drill and ream a 16mm hole in the end of the case to allow finger access to the record switch. The lid of the case should have holes drilled for the speaker grille, the microphone and the power switch. Attach the Scotchcal art- • • • • ••••• • • • • RUN Fig.2: here is an actual-size reproduction of the front panel artwork. SILICON CHIP .:J