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Build this single chip MESSAGE RECORDER By DARREN YATES Throw away those old messages on the fridge. This pro­ject records up to 16 seconds of audio using a new sound chip that retains the recording even when the power is turned off. It happens in just about every household every day. One family member has to rush out and go somewhere but needs to leave a message for someone else in the family to take the chops out of the fridge or bring the clothes in, etc. The tried and true technique is the paper message stuck to the fridge using a rubberised magnet – if you can find paper and pen, that is. How much time has been wasted searching for those two items in your household? And even if you do have a message pad, it’s always completely used up when it’s your turn to write something. If that’s a common scenario in your house, then this solid-state Message Recorder is just what the doctor ordered. It can record up to 16 seconds of speech (or music) and, unlike earlier designs, is based on a single 32  Silicon Chip chip that doesn’t require a back-up battery or external memory devices or controllers. The project fits inside a small plastic case and is operat­ ed using just two pushbutton controls. To record a message, you simply hold down the RECORD button and speak into the microphone. The message can then be replayed at any time by holding down the PLAYBACK button. Unlike a tape recorder, you don’t have to worry about “rewinding” the unit at the end of the message. That’s because the message is stored in memory inside the IC. Each time you press the PLAYBACK button, the message automatically starts from the beginning. When you want to record a new message, you simply record over the top of the old one – just as you would with an ordinary cassette tape. This eliminates the need for an erase control. There’s no need for a power switch on the unit either. When not in use, the IC automatically powers down into a stand-by mode and typically draws less than 1µA. The frequency response of the recorder is about 80-3400Hz, which is about the same as telephone quality. It has a total harmonic distortion of typically 2% at 1kHz and the operating current is 25mA <at> 5V. Of course, there are other applications for the device apart from its obvious role as a message recorder. For example, by using the Playback button as a bell-push, it could be used as a doorbell. Alternatively, it could be built into an answering machine, or into machinery and used to deliver instructions in response to user inputs. Single chip design The new IC used in the Message Recorder is designated ISD1016A and comes from Information Storage Devices in the USA. It uses analog rather than digital technology and includes a microphone preamplifier, 128K cell EEPROM and an audio output amplifier which can directly drive a loudspeaker. Fig.1: the circuit is based on IC1 – an ISD1016A single chip message recorder. When the RECORD button is pressed, signals picked up by the microphone are fed into IC1 & stored in an internal EEPROM. Pressing the PLAY button switches the chip to playback mode. D1 1N4004 OUT 2.2k 0.1 47k 47k 16 0.22 2 MIC 1 D2 1N914 17 0.22 1 18 20 47k RECORD S2 28 VCCA MIC VCCB SP+ 1 C B 21 P/R 8 250mW IC1 ISD1016AP ANA IN SPVSSD ANA OUT VSSA TEST 23 0.1 MIC B C I G O VIEWED FROM BELOW 12 470k 15 12 13 26 A0 A1 A2 A3 A4 A5 A6 A7 CE AGC 47k E 0.1 14 MIC REF 24 Q1 BC548 9-12VDC 300mA PLUG-PACK 10 16VW 10  3 27 E GND IN 10 16VW 10k PLAY S1 10 16VW 0.1 78L05 19 1 2 3 4 5 6 9 10 4.7 16VW 3 SINGLE CHIP MESSAGE RECORDER POWER Let’s now take a look at the circuit diagram of the Message Recorder – see Fig.1. As you can see, there’s not much to it – just the IC, a microphone, a loudspeaker, and a handful of minor parts. As soon as power is applied to the circuit, IC1 goes into “power down” mode. This occurs because the POWER DOWN pin (pin 24) is pulled high by a 47kΩ resistor. The current drawn from the supply is then just the current D1 10uF 78L05 SPEAKER 10uF 10uF 4.7uF 10k 2.2k MIC 470k 1uF 47k 0.22 0.1 1 47k Q1 S2 47k D2 S1 10  47k 0.1 IC1 ISD1016AP 0.22 0.1 Circuit details required to run the 78L05 5V regulator – about 4mA. When the RECORD button is pressed, the PLAY/REC pin (pin 27) is pulled low, while the POWER DOWN pin (pin 24) is pulled low via diode D2. The CHIP ENABLE pin (pin 23) is also pulled low – via a 47kΩ resistor and D2 – so that the IC can now accept an audio input. Finally, pressing the RECORD button also turns the electret microphone on. This now picks up sound and feeds an audio signal into a preamplifier stage inside the IC at pin 17. The 470kΩ resistor and 4.7µF capacitor on pin 19 set the AGC time constant for the microphone preamplifier. The aim here is to achieve the highest level of audio possible without clip­ping, to keep the signal-to-noise ratio as high as possible. The 1µF capacitor between pins 21 & 20 couples the audio signal from the preamplifier to an internal amplifier block. From there, the signal passes via an anti-aliasing filter and is clocked into the 128K cell analog storage array. All clock and timing functions are carried out automatical­ly inside the chip, so no external clock components are required. The chip continues to record the 0.1 During recording, this device samples the incoming audio signal and stores these samples as analog voltages in the EEPROM. This technique is more efficient than digital storage and provides the added bonus of 10-year zero-power data retention. If necessary, individual devices can be cascaded to obtain longer recording times. The chip also has a message ad­dressing facility so that individual messages can be repeated or different messages played back. We haven’t used these features here though, to keep the circuit as simple as possible. For detailed information on the ISD1016AP IC, take a look at the feature article on this chip elsewhere in this issue. TO SPEAKER Fig.2: note the orientation of switches S1 & S2 when installing them on the PC board. The flat side of each switch body faces towards IC1. signal on its pin 17 input until either the RECORD button is released or the device runs out of memory. July 1993  33 When the PLAYBACK button (S1) is pressed, Q1 turns on and pulls the POWER DOWN pin (pin 24) low to bring the chip back “on line”. At the same time, D2 prevents the PLAY/ REC pin from being pulled low again since this diode is now reversed bias­ PARTS LIST 1 PC board, code 01104931, 100 x 55mm 1 green snap action pushbutton switch (S1) 1 red snap action pushbutton switch (S2) 1 plastic zippy case, 130 x 67 x 42mm 1 57mm 8Ω loudspeaker 1 electret microphone insert 4 15mm-long x 3mm tapped spacers 8 6mm-long x 3mm machine screws 4 3mm nuts 8 PC stakes Semiconductors 1 ISD1016AP sound recorder IC (IC1) 1 78L05 5V regulator 1 BC548 NPN transistor (Q1) 1 1N4004 silicon diode (D1) 1 1N914 signal diode (D2) Capacitors 3 10µF 16VW electrolytic 1 4.7µF 25VW electrolytic 1 1.0µF 63VW MKT polyester 2 0.22µF 63VW MKT polyester 4 0.1µF 63VW MKT polyester Resistors (1%, 0.25W) 1 470kΩ 1 2.2kΩ 4 47kΩ 1 10Ω 1 10kΩ Miscellaneous Light-duty hook-up wire, tinned copper wire (for links), epoxy resin. ed. This means that the internal recording circuitry remains dis­abled. The recorder now replays the message stored in its memory. If the PLAYBACK button is released during playback, the mess­ age stops. If the button is then pressed again, the message restarts from the beginning. The audio output signal appears across pins 14 & 15 (SP+ and SP-). These are complementary outputs which provide 50mW of power into a 16Ω load. Since we are using an 8Ω loudspeaker, a 10Ω resistor is installed in series with the output to provide the correct load. In order to keep digital “noise” to a minimum, the analog and digital sections of the circuitry have been isolated by providing separate ground return rails on the PC layout. This helps prevent digital noise from finding its way into the low-level audio sections, such as the preamplifier and the AGC cir­cuitry. The circuit requires a 5V supply and this is derived via reverse polarity protection diode D1 and a 78L05 3-terminal regulator. Power is derived from the mains via either a 9V or 12V DC plug­pack. MIC MESSAGE RECORDER HOLD KEYS DOWN RECORD PLAYBACK Fig.3: this full-size artwork can be used as a drilling template for the front panel. Construction All the parts for the Message Recorder – including the loudspeaker –are installed on a small PC board. This board is coded 01104931 and measures 100 x 55mm. Before starting construction, check the PC board for etch­ing defects by comparing it with the published pattern. If you find any, correct the problem immediately. Fortunately, etching defects are fairly uncommon but it’s always wise to make sure. When you’re sure that everything is OK, you can begin by installing the five wire links – see Fig.2. Make sure that the link wires are straight so that they don’t short against other components and note that one link runs under IC1. CAPACITOR CODES ❏ ❏ ❏ ❏ Value 1.0µF 0.22µF 0.1µF IEC Code 1u0 220n 100n EIA Code 105 224 104 RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ No. 1 4 1 1 1 34  Silicon Chip Value 470kΩ 47kΩ 10kΩ 2.2kΩ 10Ω 4-Band Code (1%) yellow violet yellow brown yellow violet orange brown brown black orange brown red red red brown brown black black brown 5-Band Code (1%) yellow violet black orange brown yellow violet black red brown brown black black red brown red red black brown brown brown black black gold brown Once the links are in, install PC pins at each of the switch mounting pads and at the microphone mounting pad nearest the edge of the board (note: not needed for a 2-terminal micro­phone). This done, the resistors, capacitors and semiconductors can all be installed on the board. The accompanying table shows the resistor colour codes but it’s also a good idea to check each resistor with your multimeter before installing it, as it can be difficult to distinguish the colours on some brands. Pay particular attention to the orientation of the polar­ised components. These include the electrolytic capacitors and the semiconductors. Pin 1 of the IC is adjacent to a small notch in one end of the plastic body. The two pushbutton switches (red for RECORD, green for PLAYBACK) can now be soldered to the tops of the PC stakes (see photo). To do this, first lightly tin the PC stakes and switch pins, then position the switches on the PC stakes and heat the contact points with a soldering iron to re-melt the solder. Note that the switches must be oriented exactly as shown on Fig.2 – ie, with the flat side of each switch body towards IC1. The electret microphone insert is mounted with its top surface about 16mm above the PC board. If it is a 3-terminal device, it should be oriented so that its outer shield connection is soldered to the PC stake previously installed. If it is a 2-terminal device, ignore the outer shield connection. In both cases, the positive terminal goes Make sure that all polarised parts are correctly oriented when installing them on the PC board. The loudspeaker is mounted using double-sided tape or epoxy resin, while the microphone is mounted with its top surface about 16mm above the PC board – see photo below. July 1993  35 to the centre of the three pads. Finally, complete the PC board by installing four mounting spacers (each consisting of a 15mm spacer and a nut) and then mounting the loudspeaker in position. The loudspeaker can be affixed to the board using double-side tape or epoxy resin. Use light-duty hook-up wire to connect the output terminals on the PC board to the loudspeaker terminals. Similarly, connect two 120mm-long flying leads to the power supply terminals – these will later be wired to the DC power socket. doesn’t foul the PC board. After that, it’s simply a matter of attaching the board to the lid and connecting the supply leads to the DC input socket. Before doing this though, connect the plugpack supply and use your multimeter to identify the positive and negative terminals on the back of the socket. The supply can then be disconnected and the leads soldered to their respective terminals. Testing Final assembly The completed board assembly can now be installed in the specified plastic case. To do this, first attach the adhesive label and use it as a drilling template for the front panel. You will have drill four mounting holes for the PC board, two clearance holes for the switches and access holes for the loud­speaker and microphone. In addition, you will have to drill mounting holes in one end of the case to accept the DC power socket. This hole should be positioned near the bottom of the case, so that the socket Fig.4: check your PC board for etching defects by comparing it against this full-size pattern before mounting any of the parts. In particular, check the tracks that run between IC pads. To test the unit, apply power and hold down the RECORD button while you speak into the microphone. Now check that the message replays when you press the PLAYBACK button. If it doesn’t work, first check for +5V at the output of the 3-terminal regulator. Check also that this voltage appears on pins 16 & 28 of the IC. If these checks prove OK, check that pin 24 switches from +5V to almost 0V when the PLAYBACK button is pressed. If it doesn’t, check the circuit around Q1. Finally, if your microphone is a 2-terminal device, check that it is correctly oriented, with the positive terminal going to the centre pad. SC CEBus AUSTRALIA KITS CEBus Australia has opened the Circuit Cellar door to bring you a range of high quality, educational electronics kits. There are three types of kit available: an Experimenter’s Kit which includes the PCBs, manuals, any key components that are hard to find and the basic software required by the finished product. Then there is the Complete Kit which includes everything above plus the additional components required to complete the kit. Finally, there is the complete kit with Case & Power Supply. Regardless of which kit you purchase you get the same high quality solder masked and silk screened PCB and the same prime grade components. Our range of kits includes: HAL-4 4 Ch, EEG Monitor, Complete kit only ................... $356.00 Experimenter’s Kits: SmartSpooler, 256K print spooler ..................................... $214.00 IC Tester, Tests 74xx00 family ICs .................................... $233.00 Serial EPROM Programmer, For 27xxx devices ............... $214.00 Ultrasonic Ranger Board with Transducer.......................... $194.00 NB: The above prices DO NOT include sales tax. Don’t forget we also have the HCS II, Home Control System, available, Its features include: Expandible Network, Digital & Analog 1/O, X-10 Interface, Trainable IR Interface and Remote Displays. Call fax or write to us today for more information. Bankcard, Mastercard & Visa accepted. CEBus AUSTRALIA. Ph (03) 467 7194. Fax (03) 467 8422. PO Box 178, Greensborough, Vic 3087. 36  Silicon Chip