Fullscreen HTML5Med Res (??MB)HTML4

Weak beeper? Build the repeater! Does your computer have a weak beeper? Do you have trouble hearing it? You can solve that problem with this Computer Sound Repeater which turns a weak beep into a much more audible low frequency tone. By LEO SIMPSON & JOHN CLARKE Some computer models do not use a speaker but use a tiny piezoelectric transducer which puts out a high frequency 'beep'. For people who have severe hearing loss, this is just about inaudible. We're thinking specifically of Amstrad computers in this instance but the problem does occur with other brands. Our solution is the Computer Sound Repeater. It is a little box which sits on top of your computer. Whenever the computer beeps, to indicate a wrong keystroke, during a game or whatever, the Sound Repeater emits a low frequency "blurt" which is much ·more audible to those with poor hearing. The Sound Repeater is low in cost and simple to build. It uses an electret microphone, one LM324 quad op amp IC, one transistor, one small loudspeaker and a handful of other bits. How it works The circuitry in the Sound Repeater has been adapted from that in the Discolight, as published in the July and August 1988 issues of SILICON CHIP. The electret microphone is used to pick up the Left: each time your computer beeps, this gadget emits a low frequency "blurt". The circuit is housed in a low-cost plastic case. sound of the computer's internal beeper. The electret microphone has an internal FET source-follower to buffer it and this is powered via the 4.7k0 resistor. The audio signals from the electret microphone are then fed via a .01µ,F capacitor to pin 10 of ICla, which is connected as a noninverting amplifier with a gain of 30 times. ICla feeds a lOOkO trimpot (VRl) which sets the sensitivity of the circuit. The signal then passes to ICl b, which is identical to the previous stage, thus giving a total gain of about 900 at the maximum setting of VR1. Following IClb is a twin-T filter stage which is more or less tuned to the frequency of the computer's beep. Our circuit has values selected to centre it close to ZkHz. The .01µ,F capacitor and the 220k0 resistor at the input of ICla and ICl b also function as a high pass filter , so that the circuit does not respond to low frequency sounds. Any tone signal picked up and amplified by ICla, 1b and le is then rectified by diode Dl. The rectified signal is filtered with a 1µ,F PARTS LIST 1 plastic utility box, 1 30 x 68 x 43mm (Altronics Cat H-0153 or equivalent) 1 PCB, code SC07104891, 66 x 62mm, 1 electret microphone insert 1 50mm 80 loudspeaker 1 1 2V AC plug pack Semiconductors 1 LM324 quad op amp IC 1 BC338 NPN transistor 4 1 N4002 silicon diodes 2 1 N4148 small s ignal diodes 2 1 OV 1 watt zener diodes 68 SILICON CHIP Capacitors 2 470µ,F 25VW PC electrolytics 1 1 00µ,F 16VW PC. electrolytic 1 1µF 1 6VW PC electrolytic 6 .01 µ,F metallised polyester (greencap) Resistors (0.25W, 5%) 4 X 220k0, 1 X 180k0, 2 X 100k0, 1 x 47k0, 1 x 27k0, 4 X 8.2k0, 1 x 4.7k0, 2 X 3.3k0, 2 X 1 kO, 1 x 6800, 2 x 3300 0.5W, 1 x 1 20 1 W, 1 x 1 OOkO miniature trimpot 05 1N4002 04 1N4002 12VAC 1211 1W .,. 06 1N4002 330!l 1k ,----..---'YWr------------...------------------.....---+10V 811 SPEAKER 220 k 220k Li~~l~-11---""I 100k 14 180k 1 ~: c· .,. 3.3k .,. C 8.2k COMPUTER SOUND REPEATER 8.2k 1&vw·T -,. + 27k T .,. B EOc VIEWED FROM BELOW .OlJ .01! C C Fig.1: tone signals picked up by the microphone are amplified by ICla & IClb, filtered by twin-T filter stage ICld, and rectified by D1. When the voltage across the lJ.lF capacitor exceeds lV, pin 7 of IClc switches high and turns on Qt. capacitor to produce a DC voltage at pin 5, the non-inverting input of IClc. This stage operates as a comparator, comparing the DC voltage at pin 5 with a reference voltage at pin 6, which will be about 1 volt DC, as set by the 220k!2 and 27k!2 resistors. If the voltage at pin 5 exceeds 1V, the output of IClc will go high and turn on transistor Ql, via diode D2. Q2 then drives the loudspeaker with half-wave rectified 50Hz AC via diode D4. By using rectified AC we dispense with the need for a low frequency oscillator and an amplifier to drive the loudspeaker. Current through the loudspeaker is limited to a safe value (for the transistor) by the series 120 resistor. Dtode D3 is connected across the transistor to protect it against inductive kickback when it turns off. Power for the circuit comes from a 12VAC plugpack. This not only provides DC for the op amp circuitry but also the power to drive the loudspeaker, as noted above. The plugpack feeds two half-wave rectifiers (D5 and D6) and two The PCB has been designed to clip into the notched ribs on opposite sides of the case. We used super glue to secure the loudspeaker and electret microphone to the lid. Power comes from a 12V AC plugpack. JANUARY 1989 69 12VAC 100 ELECTRET MICROPHONE._.---;m.l + Fig.2: here's how to mount the parts on the PCB. Take care with component orientation and be careful not to confuse the various diodes. At right is a full size reproduction of the PCB. ~0~~~01~2~10~~~0 ,n~ l~ _./iiOl~~~! I '' 00000000000000000 oo~ 00000000000000000 000 00000000000000000 an ~oooo o oo o ~-=~~ SPEAKER {l ' - 0 --00 ooo ooo 0000 0000 il!:02 ~ 0000 000 .. ()~Q + ~~ 1pf :i 0 if 000 ELECTRET ·MICROPHONE If you want to save money, you can build the circuit on a small piece of Veroboard. But be warned - if you use Veroboard, it's very easy to make a mistake. 470µ,F 25VW filter capacitors, to provide positive and negative supply rails. These are regulated with 10V zener diodes D7 and D8 to provide regulated supply rails of ± l0V. Note that you can't use a DC plugpack to power this circuit. It needs 50Hz AC to drive the speaker. Construction Our prototype Computer Sound Repeater was housed in a standard plastic utility box measuring 130 x 68 x 43mm. (Altronics Cat H-0153 or equivalent). We designed a small printed board to accommodate the 70 SILICON CHIP + 11 ! 1 o o o O 00 O 0 ~0000 00 o O 01 o O '0 Fig.3: wiring diagram for the Veroboard version of the Sound Repeater. You can make the cuts in the tracks using an oversize drill. circuitry. It measures 66 x 62mm and is coded SC07104891. Fig.2 shows the parts layout on the PCB. Take care when installing the polarised parts and be careful not to mix up the various diodes. As an alternative, you can build the circuitry on a piece of Veroboard. To help you do this we have shown a Veroboard component layout. This can be checked against our photograph of a Veroboard version of the circuit. After drilling a pattern of holes in the lid of the case (to let the sound out) the loudspeaker can be mounted. We used a quick and dirty method: super glue. The electret can be mounted in a hole in the lid too and held in place with a dob of super glue. When construction is complete check all your work carefully before applying power. Check the supply rails ( ± lOV) with your multimeter. Pin 4 of the LM324 should be at + 10V while pin 11 should be - l0V. Pins 1, 8 and 14 should be close to 0V. Pin 6 should be be close to + 1V and pin 7 should be close to - lOV. You can test the unit, without turning on your computer, by whistling softly, at high pitch, into the microphone. Each time you do so, the speaker should "blurt". ~